ELEMENTARY SURVEY AND MAP
What is a land survey?
Survey: Is the science of measuring and recording distances, angles and heights on the earth’s surface to obtain data from which accurate plans and maps are made. Is the making of measurements in the field from which maps are drawn.
Survey: is the science of measuring and recording distances, angles, heights and areas on the Earth’s surface so as to acquire the geographical informations from where maps and plans are made.
Surveying: Is the process of measuring and recording distances, angles, heights and areas on the Earth’s surface so as to acquire the geographical informations from where maps and plans are made.
FORMS OF MEASUREMENT IN LAND SURVEY
There are four main forms of measurement in land survey which are:-
1. Measurement of distance: This is the form of measurement in land survey which usually deal with measuring the distance from a given reference between two points on the earth surface. However sometimes this form of measurement is called linear measurement.
Instruments used for measuring distance
i. Chain may be used.
ii. Tape measure may be used.
iii. Surveyor band may be used.
2. Measurement of angles: This refers to the forms of land survey which deal with measurement of angles on land survey. Normally angular measurement are taking clockwise from the north direction. This measurement is applied on both chain survey and in leveling survey.
Instruments used for measuring angles
i. Cross staff may be used.
ii. An optical square may be used.
iii. Survey telescope may be used.
iv. Prismatic compass may be used.
3. Measurement of heights: This is the measurement of land survey which deal with measuring height of the place between points on the earth surface in relation to the sea level. This form of measurement is commonly in leveling survey.
Instruments used for measuring heights
i. Leveling staff may be used.
ii. Tripod stand may be used.
iii. The spirit level may be used.
4. Measurement of areas size: This is the form of measurement which based on determination of areas on the given piece of land/ plot on the earth surface.
Plot: Is aa piece of land which usually both linear height measurement may conducted. Instruments used for measuring area size
i. Chain may be used.
ii. Tape measure may be used.
iii. Surveyor band may be used.
RATIONALE/ PURPOSE OF CONDUCTING GEOGRAPHYICAL/LAND SURVEY
i. To prepare topographic maps.
ii. To carry out land use planning.
iii. To determine the area on the piece of land.
iv. To determine the height of the place above the sea level.
v. To determine the distance between two points on the earth surface.
vi. To determine the distribution of geographical features on the earth surface.
vii. To meet the construction activities for the country's development.
viii. To solve the land use conflicts in the society.
IMPORTANCE OF LAND/GEOGRAPHICAL SURVEY
1. It helps to establish boundaries on the earth surface. E.g Cadastral survey.
2. It provides knowledge for employment opportunities. E.g. Engineering survey.
3. It help to know the distribution of geographical phenomena on the earth surface. E.g hydrological survey, Geological survey etc.
4. It helps in solvig the land use conflicts. E.g chain survey.
5. It helps to carry out land use planning. Eg. Landuse planning for establishmentbof settlements, etc.
6. It helps in production of topographical maps.
7. It helps in construction activities. E g engineering survey.
DISADVANTAGES/WEAKNESS/LIMITATION OF SURVEY
1. Time consuming, Some types of survey method are to old and slowest on taking for measurement hence resulted in to time consuming.
2. It is very expensive: some methods of land survey use equipments which are at high expensive e.g prismatic compasses.
3. It is risk to the surveyors: This can occurs when the survey is conducted in areas with dangerous animals, or areas with bad geographical location like at very elevated mountain.
4. It is selective: Normaly some methods of land survey is limited to geographical areas hence ere too selective. E.g chain survey.
5. Some mistakes/errors are inevitable: These are discrepancies which occurs during the measurement, recoding and data computation.
6. Sometimes survey can led to increase in land use conflicts: Especially when the proper care is not.
STAGES/STEPS OF CONDUCTING GEOGRAPHICAL SURVEY
Normally any kind of survey in order to be conducted successifully should pass through the following stages or steps:-
1. RECONNAISSANCE/PILOT/PRE-VISIT/PRIMARY SURVEY
Reconnaissance: is the first stage of survey which deal with critically visiting of the area to be surveyed before actual survey. It is normally conducted to collect primary information which will be used in the excursion of the survey work. However primary/reconnaissance survey enable surveyor to be familiar with the area yo be survived. However reconnaissance survey involves the examination of the area to be surveyed without taking any measurements, but investigating the area and deciding on features and lines of sight to be used as stations on the map. The surveyor also must investigate any problems or difficulties that may arise during the survey process and consider their possible solutions.
Activities conducted during the pilot survey
In reconnaissance the surveyors make themselves familiar with the area to be surveyed by doing the following activities:-
i. Walking around the area: by noting the best lines of sight, corners, and intersections of points that can be seen from each other.
ii. Fixing the best position of survey lines and stations: that can be used to establish the main survey triangles in the area.
iii. Estimating distances between ponts in the area: that will be used during the surveying process.
iv. Preparing a rough sketch map of the area to be surveyed the survey lines and important features such as buildings, roads, paths, rock outcrops and streams in the area that would be used as control or reference points.
v. Consideration of all necessary measurement: The of the required and necessary measurements and how to book them and other notes in the field book should also be included in the plan.
IMPORTANCE OF PRELIMINARY/ RECONNAISSANCE/PILOT SURVEY
i. It helps surveyor to be familiar with an area to be survey.
ii. It helps surveyor to suggests the type of survey which will be used.
iii. It helps surveyor to choose the appropriately survey instruments which will be suitable for making measurement.
iv. It helps to save time and simplify survey processes.
v. It help surveyor to identify the possible obstacles on the given areas.
vi. It help surveyor to prepare enough survey team.
vii. It help surveyor to choose the suitable scale for map production.
2. FIELD OPERATION/ACTUAL SURVEY
Field operations: is the stage of implementation. This stage normally involves taking measurement of distances, angles, elevations and locating points and off-sets in the area as it has been suggested by the sketch map prepared during the preliminary survey.
In field excursion data are collected and recorded in the field notebooks. All measurements whether linear or angular, must be recoded for the purpose of preparing the map of the surveyed area after the field work. Therfore all measurements must be taken with greater care and accuracy to minimise the possibility of returning to the field for supplementary data for the same purpose.
Activities conducted during the field operation
i. Measuring disyance.
ii. Measuring angles.
iii. Measuring ateas.
iv. Measuring height.
v. Booking.
vi. Overvoming obstacles.
3. PRESENTATION
Presentation: is the stage where survey the implementation is already conducted. It normally deal with presenting the whole informations obtained from the fields. Normally the data may be presented through maps, diagrams, charts by using graphs.
TYPES/CLASSIFICATION OF THE LAND SURVEY
There are several types of survey which based on various criteria such as follow:-
A. ACCORDING TO THE SHAPE OF THE EARTH AND COVERAGE
There are two main types of land survey which are:-
1. Geodetic survey: is the type of survey which cover large area in making measurement on the earth surface like the whole continent, country or region by the consideration of the earth shape. The geodetic survey it normally consider the earth's shape as the spherical body.
2. Plane survey: is the type of survey which cover small areas on making measurements without consider the earth shape. Example the school compound, villages or district wide. However the plane survey cannot consider the earth shape. It normally assume that, the earth shape is flat and smooth like geod or Globe.
DIFFERENCES BETWEEN GEODETIC AND PLANE SURVEY
1. In plane surveying the earth curvature is ignored while in geodetic survey the earth shape is considered.
2 Plane surveying cover smaller areas less more than about 260 km2 while geodetic surveying cover larger areas than about 260 km2.
3. Plane surveying has lower degree of accuracy while geodetic surveying ensure higher degree of accuracy.
4. Plane surveying is mostly done locally by the concerned state or government department while geodetic surveying it is mostly done by an individual organization.
5. In a plane survey aline joining any two points is considered straight while under yhe geodetic survey the line join the two points is considered curved.
B. BRANCHES/ TYPES OF SURVEY ACCORDING TO THEIR PURPOSE/FUNTIONS
The types of survey according to purpose sometimea is generally referred to branches of the geograohical survey. Therefore these include:-
1. Topographical survey: This kind of survey is carried out for the purpose of preparing topographical maps. Is the type of survey which deal with studying both natural and man made features on the earth surface. Topographical maps are those maps whose contents include both man-made features such as linear features (roads, railways, telephone lines, water systems, and electricity poles) and natural features such as rivers, oceans, mountains and valleys. The topographical survey has the following ways or methods of conducting the.
Types of topographical survey
i. Thematic survey.
ii. Layout survey.
iii. Project survey.
2. Hydrological survey: Is the branch or type of land survey which is conducted for the purpose of studying the distribution of water bodies on the earth surface. It is the branch if survey which deal with studying the distribution of water bodies on the earth surface like lakes, oceans and seas, rivers and swamps.
3. Cadastral survey:This kind of survey is which deal with establishmentbof legal boundaries on the earth surface. It normally conducted with the aim of preparing a legal document such as house plans, town or city boundaries. It is mainly used for ownership purposes.
4. Engineering surveys:This refers to the type survey which is conducted for carying out constructions on the earth surface. Normally this branch of survey is conducted for the building and construction activities like railways, bridges, roads and building foundation.
5. Geological/Mining survey:This is the branch of survey which is conducted for the aim of knowing the distribution of rocks and minerals on the Earth ’ s crust. Normally this branch of survey sometimes is known as mining survey.
C. METHODS OF SURVEY/TYPES OF SURVEY ACCORDING TO THEIR TOOLS USED
1. The chain or tape survey.
2. The prismatic compass survey.
3. The plane table survey.
4. The technique of levelling.
1. CHAIN/TAPE SURVEY
Chain/tape survey: Is a method of land survey which deal with taking linear measurement on the flat area between two points on the earth surface especially in a short distance of not exceed thirty (30) meters. no angles are measured but only linear measurement is taken in the field by using a chain or tape measure.It measures a series of straight lines on the ground with a chain or tape measure and all fixed points relative to theline of traverse either by right angles (offsets) or tie lines.
CHARACTERISTICS OF CHAIN SURVEY
Chain survey has the following characteristics:-
1. It based on making linear measurement on the earth surface.
2. It cover small geographical areas.
3. It is more suitable on the areas with no obstacles.
4. It is simplest method of survey.
5. It is oldest method of land survey.
6. Its equipments can be replaced each other example chain and tape.
PRINCIPLES OF A GOOD CHAIN SURVEY
QN: Outline five (5) principles of conducting a good chain survey.
1. It should be conducted on the areas with no obstacles like hills, pond, river, building or forest.
2. There should be conducted on the small areas example the school compound.
3. There should be presence of survey team.
4. There should be availability of correct and enough equipments.
5. It should be conducted on the flat area.
6. There should be proper measurement, booking and presentation.
PRINCIPLES/CONDITIONS OF A GOOD SITE TO BE CONDUCTED A GOOD CHAIN SURVEY
1. The area should be free from obstacles like hills, pond, river, building or forest.
2. The areas should be physical accessible.
3. The area should be free from dangerous animals like lion, snakes and other threats.
4. The area should be flat as much as a possible.
5. The area should be small to accomplish the requirements of chain survey like school compound.
6. The areas should have fair weather conditions for successful and avoiding possible errors.
TOOLS/EQUIPMENTS USED IN CHAIN SURVEY
1. A Chain: Is made up of steel wire which is divided in to links and togs (rings) to facilitate folding. It has brass handles at both ends for easy handling. The link is 0.2m or 200mm in diameter. Chain is used for making linear measurement like measuring distance and area on the flat area between two points on the earth surface. The length of the suggested suitable chain line is between 20m and 30m.
2. Tape measure: Is made up from fiberglass or a steel strip and is 10m, 20m or 30m in length graduated in 10mm divisions and numbered at each 100mm (10) divisions. It is used for measuring short distances
3. Arrows: Are made up of steel wire of diameter 4mm and their ends are bent into a circle where red cloth is tied to facilitate visibility. They are used for showing points on the ground. They are also used for counting the number of chains while measuring a chain line. Moreover arrows they used for making temporary stations between two points on the earth surface.
3. Ranging poles: Are made up of wood or light metal and measure about 2m long at the top. The equipment has steel shades on its legs so it can be stuck stable into the ground. Ranging poles are painted red and white so that they can be easily seen even from a distance. They are used for making temporary stations on the ground between two ponts.
4. Pegs: They are made of wood 40mm square by 50cm long and they are used for making permanent positions during surveying.
6. Surveyors' band: Is made up of a steel strip which is rolled into a metal frame with a winding handle. It is 30m, 50m or 100m long. Is used for linear measurement especially on measuring distance on the earth surface between two points.
7. Cross staff: Is made of metal or wood with eye slipsat right angles. Cross staff is used for measuring right angles from the line of traverseright angle from the line.
8. An optical square: It is mad up of steel and wood materials. It is used for setting right angle.
9. The prumb Bob: It is used for measuring vertical points or hanging direction over station point to fix position exactly.
FIELD WORK OF CHAIN SURVEY
Field work in Chain surveying includes:-
i. Reconnaissance.
ii. Selection of the station
iii. Measurement of lines and taking offsets of different objects in the field.
A. RECONNAISSANCE
This is the preliminary survey in which to the survey party will examine the plot to be surveyed in order to know as to how the works can be executed in the best possible ways.
i. The party will note all details like roads, buildings, canals, ditches, culverts and the difficulties and obstacles that may arise during the carrying out of the work.
ii. The party should locate the suitable points for stations by driving pegs. Sometimes a small triangle or a circle is made around the stations and the pegs are inserted into the centers.
iii. The party should then make a rough sketch of the plot showing the possible stations and from there the arrangement of different lines.
iv. It is important to give a north line on the rough sketch and though the sketch is not prepared according to the scale, it should represent the approximate positions of the different things in the plot and hence to be a good guide for further work.
B. SELECTION OF THE STATION
These are points on the ground fixed by driving pegs. Every station should be located with respect to three permanent objects i.e. the distances from these objects to the stations should be measured very accurately and recorded in the field book. The advantage of taking this measurement is that if in future the peg at the station is lost, then it can be located again by knowing descriptions and distances of these objects.
PRINCIPLES OF SELECTING GOOD CHAIN SURVEY STATION/LINE
The selected survey stations and chain lines should be intervisible from each other. The stations should form well coordinated triangles that will form the framework of the area. The selection of the stations and chain lines should base on the following principles:-
i. Select one major line (base line) along which all the triangles can be established.
ii. Avoid steep slope and major obstacles in the chain lines.
iii. Use a few chain lines as possible during chain surveying.
iv. Keep chain line short and measure them accurately.
v. Maintain all angles of the triangles between 30o and 120o not less than that.
vi. Keep the survey lines closer to the details and avoid long offsets and ties which are more than 10metres.
vii. Make check lines whenever necessary to detect errors when plotting.
Types of Survey Stations
1. Main Stations: Main stations are the stations taken along the boundary of an area as controlling stations, Subsidiary and Tie stations. They are taken at the end of the lines which command the boundaries of the survey. The lines joining the main stations are called the Main Survey line or the Chain Lines. The main survey lines should cover the whole area to be surveyed.
Types of Chain Lines
i. Base Line: This is the line where the framework of the survey is established. Is the main and longest line in a chain survey which passes approximately through the centre of the field. All the other measurements to show the details of the work are taken with respect to this line.
ii. Check line: Is the line joining the apex point of a triangle to some fixed points on its base. It is laid by joining the apex of the triangle to any point on the opposite side or by joining two points on any two sides of the triangle to check the accuracy of the field work. Check lines is also known as a proof lines as it runs in the field to check the accuracy of the triangle. The length of a check line sometimes helps to locate the interior details.
iii. Tie lines: Is a line which joins subsidiary or tie stations on the main base line. The main object of running a tie line is to take the details of nearly objects but it also serves the purpose of a check line.
2. Subsidiary or the tie stations: Subsidiary stations are the point selected on the main survey lines or any other line. These stations are taken to run subsidiary lines for dividing the area into triangles for checking the accuracy of triangles and for locating interior details where it is necessary to locate the interior detail such as fences, hedges, building etc.
3. Tie stations: These are also subsidiary stations taken on the main survey lines. Lines joining tie stations are known as tie lines. A tie line joins two fixed points on the main survey lines. It helps to checking the accuracy of surveying and to locate the interior details. The position of each tie line should be close to some features, such as paths, and building.
TAKING OFFSETS
The main station A is located with respect to three permanent objects and a ranging rod is fixed to the station. One ranging rod is fixed at main station B and another at an intermediate point in between A & B. The three rods will be in a straight line when only the intermediate rod is visible if a man looks from A to B. Now measurement of line AB is taken by the chain. The chain should be properly stretched so that there is no sag in it. As the measurement proceeds, offsets are taken on both sides of the line AB and recorded in the field book. In this way, all the lines including tie and check lines are measured and offset taken and recorded in the field book.
OFFSETS
Offsets: are lateral distances measured from survey line to locate interior details with reference to the chain line. They are the lateral measurements from the base line to fix the positions of different ground features with respect to base line. They are generally set at right angles and can be drawn with the help of a tape.
Types of Offsets
There are two kinds of offsets which are:-
a. Perpendicular/right angled Offsets: These are the lateral measurements taken perpendicularly or at right | A i a0 angle to the survey line. They are also called Right angled Offsets. Perpendicular offsets may be taken by:-
i. By setting a perpendicular by swinging
ii. By setting a right angle in the ratio of 3:4
iii. By setting a right angle with the help of builder’s square or tri-square.
iv. By setting right angles with the help of cross Staff or optical square. Advantages of perpendicular offsets
Perpendicular offsets are preferred because of the following advantages:-
i. They can be taken very quickly.
ii. The progress of survey is not hampered.
iii. The entries in the field book are simplified.
iv. The plotting of the offsets also becomes easier.
b. Oblique Offsets: Any offsets not perpendicular to the chain line are said to be oblique offsets. They are the measurements which are not made at right angles to the survey line. They are also called tie line offsets. Oblique offsets are taken when the objects are at a long distance from a chain line or when it is not possible to set up a right angle due to some difficulties.
Number of offsets
There is no hard and fast rule regarding the number of offsets. The offsets should be taken according to the nature of the object and should be decided on the field. Sufficient number of offsets should be taken so that, the object can be plotted clearly and accurately. However, in a good survey work, number of offsets is kept as small as possible.
GUIDELINES OF TAKING PERPENDICULAR OFF-STS
Some guidelines in relation to the number of offsets are given below:-
i. When boundary of the object is approximately parallel to the chain line, perpendicular offsets are taken at regular interval.
ii. When the boundary is straight, perpendicular offsets are taken at both ends of it.
iii. When the boundary line is zigzag, perpendicular offsets are taken at every point of bend to represent the shape of the boundary accurately. In such a case, the intervals of the offsets may be irregular.
iv. When a road crosses the chain line perpendicularly, the chainage of the intersection point is to be noted.
v. When a road crosses a chain line obliquely, the chainage of intersection A and B are noted. Then at least, one offset is taken on both sides’ of the intersection points. More offsets may be taken depending on the nature of the road. Here, perpendicular offsets are taken at c and d.
vi. When the building is small, its corners are in fixed by perpendicular of an oblique offsets, and the 3m other dimensions are taken a directly on the field and noted in the field book.
vii. When the building is large, zig-zag in shape and oblique to the chain line, the corners are fixed by perpendicular or oblique offsets. Then the building’s full plan is drawn on a separate page along with all the dimensions. This page should be attached with the field book at the proper place.
viii. When the object is circular or is a curved boundary, large number of perpendicular offsets is necessary and is taken at short and regular intervals.
PROCEDURES/PROCESSES/STEPS/STAGES OF DOING/CONDUCTING CHAIN SURVEY
Qn: Describe six procedures involving in conducting a good chain survey. Normally the procedures of conducting successifully chain Survey include:-
1. Reconnaissance: Before the commencement of any survey work, the area to be surveyed is thoroughly examined by the surveyor. This is termed as Reconnaissance and is defined as “the exhaustive preliminary investigation of the area to be surveyed”. Reconnaissance may be either ground reconnaissance or aerial reconnaissance survey.
Objectives of reconnaissance
i. To helps the surveyor to note various objects which are to be located and prepared a hand sketch referred to as “index sketch or key plan”.
ii. To assess the possibility of building or constructing route or track, through the area.
iii. To choose the best one or more routes and record on a map.
iv. To estimate probable cost and draft a report.
v. To take care that the triangles to be formed are well conditioned.
Advantages of Reconnaissance
i. It helps to build map terrains, which is important in land survey.
ii. It helps the surveyor to become familiar with the area to be surveyed.
iii. It helps the surveyor to know weather conditions of the area.
iv. It saves time during survey as it makes the surveyor to remember the stations.
v. It enables the surveyor to identify the cost and equipment needed in the survey.
vi. It helps the surveyor identify the possible obstacles and how to deal with them.
2. Preparation: This stage involves the formation of survey team for doing the chain survey. Normally the survey team include three persons (the Leader, the Follower and the Booker). The leader takes ten arrows and a ranging rod, and the follower takes ranging rods.
i. preparation of survey team.
ii. Preparation of equipments needed.
iii. Preparation time budget. This might be one week, month etc.
iv. Preparation of transport means and cost.
v. Preparation of techniques of overcoming obstacles.
3. Marking Stations: A surveyor fixes up the required stations at places to where maximum possible stations are possible. The main stations should so be selected that they enclose the whole area. As the surveyor walk over the area during reconnaissance survey, he notes the various obstacles and whether or not the selected stations are intervisible. After marking them, the station should be referenced by locating measurement called ties. Ties are taken from three permanent points which are easily identified such as corners of buildings. Some of the methods used for marking are: Fixing ranging poles, Driving pegs, Marking a cross if ground is hard or Digging and fixing a stone.
4. Setting main chain line: Here the surveyor selects the way for passing the base line, which should be horizontal and clear as possible and should pass approximately through the centre of the field.
5. Inserting ranging poles: Before starting the chaining Operations, two ranging poles should be fixed on the chain line at the end stations. The other ranging poles should be fixed near the end of each chain length during ranging operation. The follower erects his ranging pole at the first base point and places a brass handle of the chain against the ranging pole. The leader drags the chain so that the follower’s end is on the leader’s arrow; the follower moves to another point and places his ranging pole behind the arrow.
6. Chaining: After fixing the stations, chaining could be started. Processes involving during chaining
This stage involves the number of processes such as follows:-
i. The surveyor unfolds the chain and throws it carefully holding one of the two handles strongly in his hand. The chain is thrown to extend it and disentangle any knots.
ii. The leader moves forwards by dragging the chain taking with him a ranging rod and ten arrows.
iii. The follower stands at the starting station by holding the other end of the chain.
iv. When the chain is full extended, the leader holds the ranging rod vertically at arm’s length.
v. The follower directs the leader to move his rod to the left or right until the ranging rod is exactly in line. Then the follower holds the zero end of the chain by touching the station peg.
vi. The leader stretches the chain by moving it up and down with both hands and finally places it on the line. He then insert an arrow on the ground at the end of the chain and marks with a cross (‘X’)
vii. The leader moves forward by dragging the chain with nine arrows and the ranging rod. At the end of the chain he fixes another arrow as before.
viii. The follower picks up the arrows which were inserted by the leader. During chaining, the surveyor or an assistant should conduct the ranging operation. In this way the chaining operation is continued.
7. Setting the Offset: A leader straightens the chain and inserts an arrow at end of the brass handle where offsets and tie lines can now be taken. The surveyor will give the instructions or alignment to make it straight. Then offsets are taken by using the tape and each offset should have its value and chain length recorded. The offset are taken to locate the nearby details.
8. Booking: Is the process of entering the field data or informations in the field book. Normally the field data inclufing both diagrams, charts, maps and other numerical information. As surveying procedures are conducted the measurements and readings namely offsets and chain lengths to the important stations are entered in the Field Notebook usually with a double red line ruled up at the middle of each page.
A FIED NOTEBOOK
Field notebok: Is a special notebook which usually used in recording the field data during the surveying.
Components of the Field Notebook
i. The name, location and the date of the survey.
ii. The description and reference number of the tapes and other instruments.
iii. The names of the survey team members.
iv. A sketch of the layout of the survey lines made during the reconnaissance.
Importance of the notebook in chain surveying
i. Field book is used by the surveyor to record all the observations and measurements. With the help this book he plots the complete work in the office.
ii. It helps the surveyor to keep thorough memory of observation in the field under the principle that, “Leave Nothing to Memory, including explanatory notes on details such as street names, house numbers and names, kind of tree (girth and height), types of pavement boxes.
iii. It helps the surveyor to prepare an Index Sketch or Key Plan: by sketching the features and show their relationship for easy map making. The index sketch indicates the main survey stations, sub-stations, ties stations, base lines, arrangement for framework of triangles and the approximate position of different objects.
BOOKING PROCESSES IN CHAIN SURVEY
Booking: is the process of entering or recording data during survey process.
Types of booking techniques
1. Single line booking: is the type of booking technique which deal with recording the information along the single line during the survey process. In this type of field book, a single red line is drawn through the middle of each page. This line represents a chain line. All chainages are written on it. The space on either side of the line is used for sketching the object and for noting offset distances. Thus, the offsets are recorded with sketches to the left or right of the chain line.
2. Double line booking: is the booking technique whereby the informations/details along the line measurement are recorded. In double line field book the two blue or red lines with a space of 1.5 cm to 20 cm are drawn in the middle of each page. The column represents the chain line and the space between the two lines is used for noting the chainages. The offsets are recorded with sketches of the left or right of this column.
PRINCIPLES OF A GOOD BOOKING
Good booking techniques should put in to consideration the following principles:-
i. All features should appeared in the correct position: This means that All linear measurement should be appear in the centre of the colum, all details that is on the right- hand side of the line is booked on the right-hand side of the page and vice versa.
ii. Field notes in terms of sketches and offsets should accurate and clear: If modification is necessary, the old entry should be struck off and the new one written over it. If a whole page is not required, it should be marked ‘cancelled’ and struck across.
iii. All forward and back bearing should be circled.
iv. Booking should start and proceeds from the bottom of the page upwards.
v. All features should be recorded step by step from the nearest to the furthest.
vi. All measurements should be recorded as they are taken or seen in the field.
vii. A booker should use pencil of good quality to prevent the notes from being washed out by rain.
viii. All off-sets and ties should appear in the relevant side of the column.
ix. In windy areas, it is convenient to have a Strong rubber band to hold the leaves of the field book together.
x. The complete record should include general sketch of the framework of lines, the date of survey, page index of the lines, the members of the Survey, and the detailed records of the lines.
Sketches in the Field Notebook
Index sketch is an important document for the surveyor and for the person who will plot the map. The sketches in the field notebook are not generally drawn to scale. Do not sketch details ahead of measurements and exaggerate the size of complicated features.
Components of the sketch
Hence a good field scketch has the following components:-
i. The names or letters designating stations.
ii. The line numbers.
iii. Arrow indicating the direction of the survey.
iv. The witnessing and description of station marks.
v. An index of lines and stations.
vi. The weather at the time of the survey and any other feature likely to offset the accuracy of the work.
The field Sketches must not be drawn to scale because of the following reasons:-
i. The sketches are not the map: They are just preparations for actual map making plan which will be drawn to scale.
ii. The sketches are just temporary on the note book: They are mere records for further use in the process of map making.
iii. To saves time: it saves time to sketch the field without using scale.
iv. It is easier to record information: With sketches it is dimple to record information because it does not need instruments other than a note book and a pencil and all are done in hand writing.
9. Office Work: This is the post field work stage through which plotting is done to get plan of the area surveyed. At this Stage, data collected and recorded in the field notebook are decoded and used to prepare the chain plans and maps for presentation to the clients and the target audience.
PLOTTING
plotting: is the process of transforming the field information in to the relevant place by the consideration of their respect places.
Steps involved in plotting
i. Scaling: Depending upon the area in the field and area of drawing sheet, scale is decided. It is normally decided before even commencement of survey itself.
ii. Orientation: Skeleton of the network of triangles should be drawn to scale on a tracing sheet and the orientation of the plan on drawing sheet be decided. The area plotted should come in the middle of the sheet and the north line appears towards the top.
iii. Drawing networks of triangles: First, baseline is drawn to the scale. By intersection, other stations are fixed and main triangles are drawn. After checking the accuracy of the triangles, next step in plotting is taken up.
iv. Plotting offsets: This is done using two methods, these are:-
a. Mark points along the main chain line and draw perpendicular with set square. If oblique offsets are taken, arcs are drawn from respective positions on the chain line.
b. Main scale and offset scales may be used: Main scale is kept along direction of chain line and offset scales gives the perpendiculars to it.
v. Other details: If readings are taken to only one or two faces of buildings, using overall dimension, outline of the building may be completed.
vi. Marginal information: Graphical scale should be plotted so that, even if the paper shrinks, correct measurements may be obtained. The title of the plan and scale should be given at the bottom of the right hand corner. North direction is shown on the top portion of the sheet.
DISCREPANCIES/ERRORS IN CHAIN SURVEY
An errors: Are: mistakes or shortcomings that happens during the Survey process leading to wrong survey information. They usually called discrepancies. There are different types of errors or discrepancies in chain surveying which are divided in to divided into three groups:-
1. Cumulative/systematic errors: Are errors that occur in the same direction and keep on accumulating as the survey progresses. If not checked they have serious implications to the accuracy of the survey. Errors in this class are caused by the expansion of steel, frequently changes in electromagnetic distance and the use of the instruments with similar errors. Example the use of half chain or tape measure. measuring instrument for example incorrect length of the tape or the tape not being in line.
Types of cumulative errors
Cumulative errors can either be positive or negative errors.
a. Positive Errors: Positive errors occur when the measured length is more than the actual length. They usually cause the increase in survey data. They shorten the measurement.
b. Negative Cumulative errors: In this type of errors, the observed distances are shorter than the true or actual ground distance which can be a result of using a longer chain.
The chain is said to be longer due to the following reasons:-
i. It might be that the chain has become longer due to expansion caused by high temperature.
ii. It can also be due to addition of connecting rings accidentally while repairing the chain.
iii. Flattering of the rings due to being hit by heavy and hard objects, which cause them to extend in length.
iv. Application of a greater pull on the tape than its standardised pull which increases the length of the tape or chain
Causes of cumulative errors
i. The tape or chain being shorter than what it should be.
ii. The opening of the ring joints.
iii. The applied pull being much greater than the standard pull.
iv. High temperature above the standard during taking measurements.
v. Wearing of connecting rings.
vi. Elongation of the rings.
Way to avoid cumulative errors
Since the sources of these errors are known, they can be eliminated by:-
i. Frequently checking the equipment and make measurement corrections e.g. the effect of tension and temperature on steel tape.
ii. To calibrate the observing equipment and quantify the error allowing corrections to be made to further observations.
2. Gross Errors: Are the mistakes that can be be caused by either the surveyor or his assistants due to carelessness or incompetence. They result from the inexperience of the team leaders or his assistants. These are very serious errors which although are random in accordance may lead to faulty plans and maps if not checked.
Causes of gross errors
i. Miscounting the number of tapes or chain length: A full chain length may be omitted or added when arrows are lost or wrongly counted.
ii. Measuring anti-clockwise reading: A reading may be taken from the wrong end of the chain. This happens when the tooth of the tally is 1out observing the central tally. Here the tally may be noted.
iii. Misreading the tape: Some numbers may be called wrongly. For example, ay may be called fifty-two or 502 without the decimal point being mentioned.
iv. Reading the tape upside down: The number may be read from the ane direction as for example a ‘6’ may be read as a ‘9’.
v. Displacement of arrows or stations marks: once an arrow is one, bra slide ground during chaining, it may not be replaced in proper position if required due to some reasons.
vi. Wrong booking: while making entries in the field book the figure ae anged due to carelessness. For instance 245 may entered instead of 254.
vii. Wrong measurements: other mistakes are turning instruments incorrectly and sighting.
Ways to avoid gross errors
Gross errors can occur at any stage of survey when observing, booking, computing or plotting and they would have a damaging effect on the results if left uncorrected. They can be eliminated only by necessary precautions and careful methods of observing booking and constantly checking both operations. Reading must be made loudly and repeated with single digit as for example, two-one-three meaning 213.
3. Compensating or Accidental errors: Are errors with little effects on chain survey. They are unavoidable errors which always exist in any measurement. They usually caused by human limitation in reading/handling of instruments. These errors arise as a result of not being perfect in the use of the equipment or in the whole survey process.
Causes of compensating errors
i. Incorrect holding of the chain.
ii. Horizontality and verticality of steps not properly maintained during the stepping operation.
iii. Fractional parts of the chain or tape not being uniform throughout its length.
iv. Inaccurate measurements of right angles with the chain or tape. Ways of avoiding compesanting errors
Elimination of Compensating Errors Random errors cannot be removed from observation but methods can be adopted to ensure that they are kept within acceptable limits. These errors are cancelled out and do not pose serious problem to the accuracy of the survey.
PRECAUTIONS AGAINST ERRORS AND MISTAKES IN CHAIN SURVEY
1. The point where the arrow is inserted should be marked with cross (X).
2. The zero end of the chain or tape should be properly held.
3. During chaining the number of arrows carried by the follower and the leader should always tally with the total number of arrows taken.
4. While noting the measurements from the chain, the teeth of the tally should be verified with respect to the correct end.
5. The chairman should call the measurements loudly and distinctly and the surveyor should repeat them while booking.
6. Measurements should not be taken with the tape during strong winds.
7. In stepping operations, horizontality and verticality should: be maintained.
8. Ranging should be done accurately.
9. No measurement should be taken with the chain in suspension. 10. Care should be taken so that the chain is properly extended.
Entering Field Notes in the Field Book Example1:
While measuring a chain line, AB the following offsets were taken. A telegraph post is ten meters perpendicularly from chainage 2.5m to the right of the chain line. A road crosses obliquely from left to right at chainage 10m and 14m. Perpendicular offsets are 2m and 3m to the side of the road from chainage, 5m and 20m respectively. A well is) 5m perpendicularly from chainage 30m to the left of the chain line. Total chainage of AB is 45m. Now, make the necessary entries in the field book.
Example2:
Enter the field book according to the following records: Chainage of line AB is
95.5m.The offsets of the pond at the left of the chain are as follows: Chainage: 10, 15, 20, 25 and 30m. Offsets: 16, 12, 14, 10, and 20m. The offsets to the road at the right of the chain line arc. Chainage: 5, 25, 40, and 80m. Offsets: 13, 17, 19 and 19.5m.
OBSTACLES IN CHAIN SURVEY
Obstacles: a Are any objects which can hinder the success of chain survey. Are occasionally happens that a survey has to be made on a field where obstacles are encountered like a pond, sand, crops of a small wood in the middle and river. In such cases, it is not possible to employ direct chaining; other methods are used for solving the problems.
TYPES OF OBSTACLES
1. Visual Obstacles: These are the obstacles which hinder visual penetration but chaining is free. They are the obstacles that obstruct vision or ranging only but not chaining. Thus, the chain remains free. An example of visual obstacle is a forest.
2. Chain Obstacles: These are the obstacles where chaining is obstructed but vision is free. They are the obstacles that obstruct chaining but not ranging. These are for examples Rivers, lakes and ponds.
3. Visual and chain obstacles: These are the obstacles where both chaining and vision are obstructed. They are the obstacles that obstruct both vision (ranging) and changing as for example buildings or mountain.
TECHNIQUES (WAYS) TO OVERCOME THE OBSTACLES
There are seven ways on how to overcome the obstacles. The methods depend on the nature of obstacles. The different types of obstacles encountered a chain surveying can be treated by any of the three methods:-
1. By rectangular method.
2. By using similar triangle method: Let LM be the line of traverse or the chain line which is supposed to cross the river. On LM, select three convenient points A, B and C. B on one side and A and C on the accessible side of the river.
Methods used to overcome visual obstacles
This problems arises when a rising ground or a jungle area interrupts the chain line. As a result, the end stations are not intervisible. Therefore these obstacles include forest, hill and tall building. However thee obstacles are treated as a follows:-
i. By rectangular method.
ii. By using Pythagoras theorem: This technique is applied to those obstacles which prevent visual penetretion example hill or building.
iii. By using parallel offsets and right angle method.
1. Hill: is a rounded upland it is a considerable obstacle if lies on the path of the chain line. The obstacle of hill can be avoided by the following consideration techniques.
i. Simple system of offsets: This technique, a surveyor sets out equal perpendicular far long enough distances to clear the obstacle as follow
In the above application BD distance= AC distance This it obvious, as BD= AC the measured DC distance is equal to BA distance
ii. Similar triangle method
It is applied as follows
AOC = BOD
Them
As Aoc=Bod The measured DC distance is equal to AB distance
iii. The method of equal triangle: With the technique of equal triangle a applied above C is the mid point of AO and D is the mid point of BO it thus AB=2Xcd
iv. Trigonometric method/Pythagoras theorem: From point A measure the length of random line chosen to avoid the hill to C. Then chain a line at right angle from AB to C Calculate the length of AB by Pythagoras theorem
2. POND
The obstacle of pond can be avoided by the following application.
i. Parallel off-sets: Measure the perpendicular the same length from AB (AX and BY) Measure the length of XY.
Then AB = AY
3. RIVER
Make chain line from one side to the opposite side i.e from B to D
ii. Similar triangle: Set up ranging pole at A and develop chain line along the river to C and it should be assured that AO=Oc from C develop chain line of right angle to D
Then
AO = OC
BO=OD
CD=AB
4. BUILDING
Set up ranging pole at A and B along the line traverse
Parallel off-sets: Measure an equal distance along the line of traverse AQ and BR check corner of the rectangles so farmed are right angles
Extend the line of traverse through C and D
IMPORTANCE/USES OF CHAIN SURVEY
1. It is suitable for small areas of fairly open ground.
2. It is used to shows the distribution of geographical features on the earth surface.
3. It is used in mapping the small areas of flat ground. Especially on areas with no obstacles except small objects like foot paths, roads and railways.
4. It is used in land use planning.
5. It help in setting boundaries in a small geographical areas like school compound.
6. It helps in solving land use conflicts in the societies.
STRENGTHS/ADVANTAGES OF CHAIN SURVEY
1. It is a simplest method of the land survey.
2. It need only few people/survey team members.
3. Its equipments are easy to be replaced each other. Example chain, tape measure and surveyor band.
4. Its equipments are not more expensive.
5. Its equipments are easy to be handled.
6. Map can produced immediately.
7. It can involves less skilled people.
WEAKNESS/DISADVANTAGES OF CHAIN SURVEY
1. It is a slowest method of land survey.
2. It is the oldest method of land survey.
3. It is conducted only in a small areas.
4. It cannot be conducted in areas with possible obstacles like pond, rivers, hills, etc.
5. Time consuming, since the methos of land survey is too slow.
6. It is encountered by the posdible errors.
7. The technique is limited to weather condition. E.g high temperature can led to expansion of the chan.
2. PRISMATIC COMPASS SURVEY
Prismatic cmpass survey: is the method of land survey which deals conducting measurents on the earth surface by taking bearing from the given object. Normally is the brach or method of survey in which directions of survey line are determined with a compass and the tape or chain. Chain or Tape is used for linear measurement is used for fixing direction.
EQUIPMENTS USED IN PRISMATIC COMPASS SURVEY
The instruments needed in compass traverse include:-
1. The prismatic compass: this is a portable form of magnetic compass used in measuring angles and determining bearings and directions of points in relation to other points on the earth’s surface.
2. Ranging poles: Three or more ranging poles are needed for marking positions of points on the ground from which angles and distances will be measured.
3. Tape measures or chains: these are required for determining distances between points while the surveying work is taking place.
4. A chain: It is used for carry out linear measurement on the plat area between two points on the earth surface. The length of the stable chain is between 20m and 30m.
4. Metal markers or arrows: are required for temporary marking points of the surveyed points.
5. A notebook and pencil: for recording all the measurements and sketches made during the surveying process in the field.
6. Tripod Stand: This is a portable three-legged frame used as a platform for supporting the weight and maintains the stability of some other object. It provides stability against the downward forces and horizontal forces and movements about horizontal axes and it has a screw for mounting the prismatic compass.
7. Stationers: These are writing sheets, ordinary pencil and other related equipments. They are used for recording or booking all necessary field work information or good quality notebook
8. Chalk: for marking temporary locations when the survey is taking place along tarmac surfaces and in built up areas.
USES/IMPORTANCE OF PRISMATIC COMPASS SURVEY
1. It helps in map production.
2. It is used in construction activities.
3. It helps to locate the position or direction of different objects on the earth surface.
4. It helps in landuse planning.
5. It helps in locating the new settlement.
ADVANTAGES/ OF COMPASS SURVEY
1. It is very quickly to take bearing with the instrument.
2. It is accurate method of land survey.
3. Some errors are can be ignored.
4. It is very suitable to retrace old surveys.
5. The mechanism of the instrument is very easy.
DISADVANTAGES OF COMPASS SURVEY
1. Some errors are inevitable. The method is less precise and very often subject to errors due to the needle being not perfectly straight or needle being not sensitive. There may be errors in compass surveying due to local attraction, variation of magnetic meridian and magnetic changes in the atmosphere.
2. The method use equipments which are very expensive.
3. It can be easier affected by the environments.
4. It needs people with high skills.
5. Its equipements are less replaced.
6. It is rigid/not flexible method of survey.
PROCEDURES OF CONDUCTING PRISMATIC SURVEY
1. Collect a Prismatic Compass, a Sighting Pole and possibly a Chain for the Fieldwork. Try not to wear too many jewellery or rings as the metals can interfere with the compass readings.
2. Remember that Compass readings are made along straight segments of a boundary. Irregular paths (or boundaries) should therefore be first divided into straight segments before readings are taken.
3. To begin, pick the prismatic compass and locate the Starting Point (station 1). Let your partner move to station 2 with the sighting pole. Your partner must then hold the pole upright from the position marked station 2. Take a reading from your location (marked station 1) onto the sighting pole at station 2 and record the azimuth (angles) you get.
4. To verify whether the forward azimuth reading you made is correct, exchange positions with your partner (or preferably let your partner take a back azimuth onto the sighting pole now located at station 1). As a rule, if the forward azimuth is greater than 180 , you should subtract 180 from the forward azimuth to get the back azimuth but if the forward azimuth is less than 180 you should add 180 to it to get the back azimuth. With the rule, make a quick check of the forward azimuth you made and record it if it is right. If it is wrong, redo the reading all over.
5. Record the forward azimuth you read earlier.
6. Measure the segment of the boundary between station 1 and station 2 and record your answer beside the azimuth reading for this segment. You may use a chain or a tape and remember to take the measurement in feet. In the absence of a chain or a tape, you may take the measurements by pacing along the boundary and counting the number of paces you make. Generally, a pace taken in a relaxed mood (not running) is about a yard (three feet) for many people. If you will use this method, you should first determine the length of your pace by marking three feet segments on the floor and walk along them for some time.
7. Walk along the boundary segment between station 1 and station 2 and make any other required readings such as resection or intersection then record such measurements on the page you have already opened. Make some sketches if necessary, to portray the features and positions you find in the field.
8. Now go to station 2 and let your partner move with the sighting pole to station 3. Take the forward and backward azimuths as explained above and record only the forward azimuth in your survey book. Check to make any required chain and compass readings along the segment between stations 2 and 3 and then move on to the next segment. Continue with the process in the same manner as described until all stations (or segments) are measured and the measurements recorded in your notebook.
9. Keep your note book entries for you shall use it to plot the shape of land you Earinure in the field. You will also hand in your note book entries for grading.
TRAVERSING
Traversing: refers to the surveying process which involves a series of connected lines. The sides of the traverse are no n as ‘traverse legs’.In traversing, the lengths of the lines are measured by chain and the directions are fixed by compass or by forming angles with chain and tape. Traversing is the process whereby the surveyors travel by walking in a series of connected lines while recording traversing, is the main principle of compass surveying. of Traversing angles or bearings and side detail in a root way. It is a surveying which involves creation of a series of connected lines with distance (magnitude) and direction (bearing) Traversing is a method in surveying in which a number of connected survey lines form a framework. The lengths of the survey lines are measured with the help of a chain or tape while the directions are measured with the help of angle measuring instrument. The angle measuring instrument used is the Compass; hence the process is frown as Compass Traversing.
TYPES OF TRANVERSE
Traverse consists of a series of straight lines connected together to form an open or closed polygons which conform to various mathematical and geometrical rules. Thus, there are two types of traverse; Closed and Open. Normally traverse classified into two types namely:-
i. Closed traverse.
ii. Open traverse
1. OPEN TRAVERSE
Open Traverse: Open traverse is a series of straight lines which occur when the circuit ends elsewhere and not at the starting point. In other words, Open Compass Traversing is a process where surveyors team travel by walking in a series of connected lines and finally do not return to the starting point, for example road measurement. When a sequence of connected lines extends along a general direction and does not return to the startin point it is no n as ‘ open traverse ’ or ‘ unclosed traverse. Normally Open Traverse. Open traverse is suitable for the linear survey of roads, rivers and coastlines.
2. CLOSED TRAVERSE
Closed Compass Traverse: This is a polygon which occurs when the lines form a circuit which ends at the starting point. When a series of A connected lines forms a closed circuit, i.e. when the finishing point coincides with the starting point coincides with the starting point of a survey, it is called a ‘closed traverse’. Closed traverse is suitable for the survey of boundaries of ponds, forests estates and so on. Closed compass Traversing is the process where surveyors team walk in a series of connected chain and finally return to or close to the starting point. The pieces of line connecting the traverse are known as traverse legs. Normally the closed traverse is suitable for the survey of boundaries of ponds, forests and estates. It starts at a known points and ends at a same known point.
METHODS OF TRAVERSING
Traverse survey may be conducted by the following methods:-
i. Chain Traverse (By Chain): it is mainly conducted when it is not possible to adopt triangulation (to form triangles) as for example in a pond. In this method, the angles between adjacent sides are fixed by chain angles. The entire survey is conducted by chain and tape only and no angular measurements are taken. However, the method is unsuitable for accurate works and is generally not used if angle measuring instruments such a compass, sextent, or theodolites are available.
ii. Compass Traversing (By Free Needle): the direction of the traverse lines is determined with a magnetic compass. In this, fore bearings and back bearings between the traverse legs are measured by prismatic compass and the sides of the traverse by chain or tape. Then the observed bearings are verified and necessary corrections for
local attraction are applied. In this method closing errors may occurred when traverse is plotted and can be adjusted by using Bowditch’s rule.
iii. Plane Table Traversing (By Plane Table): a plane table is set at every traverse station in the clockwise or anti-clockwise direction, and circuit is finally closed. During traversing, the sides of the traverse are plotted according to any suitable scale. At the end of the work, any closing error which may occur is adjusted graphically.
iv. Theodolite traversing (By Fast Needle): the horizontal angles between the traverse legs are measured by theodolite. The lengths of the legs are measured by chain or by employing the stadia method. The magnetic bearings of the other sides are calculated. The independent coordinates of all the traverse stations are then found out. This method is the most accurate.
ERRORS IN TRAVERSING
The closing traverse error: is a gap in either open or close traverse which occurs when the traverse fail to end at the expected point.
Types of traverse errors
i. Open Traverse Error: Open traverse is not used in engineering control application because there is no way to check for errors. You will have to accept whatever coordinates computed. However, to check on open traverse error, we take cut-off lines. Measure the bearings and lengths of cut off line after plotting and tally with actual values.
ii. Closing Traverse Error: Closed traverse is plotted; the finishing and starting point may not coincide. The distance by which the traverse fails to close is said to be a closing error. Closing error is the amount by which a closed traverse fails to satisfy the requirements of a true mathematical figure, as the length of lines joining the true and computed position of the same point do not coincide.
Types of Closing Errors
Closing traverse error can result from Error in Angles or Error in Distance as the last point may not end at the starting point. The problem is that, we do not know where the errors are and how much each error. Measurements are never exact. We always assume that we have errors in angles and distance. Because of errors in both angles and distance, there are two types of closing errors:-
i. Angles Disclosure closing error
ii. Linear Disclosure closing error. Causes of Closing Error
A closing may occur due to mistakes made in the measurements of lengths and bearings of the lines or because of an error in plotting.
SOURCES/CAUSES OF ERRORS IN COMPASS SURVEY
The errors may be classified as Instrumental errors, Personal errors and Natural errors or environmental arising from natural causes.
1. Instrumental Errors: These are errors which arise due to the faulty adjustments of the instruments.
Sources of instrumental errors
They may be due to the following reasons:
i. The needle not being perfectly straight.
ii. Improper balancing of the compass.
iii. Pivot being bent.
iv. Sluggish needle.
v. Blunt pivot point.
vi. Plane of sight not being vertical.
vii. Line of sight not passing through the center of graduated ring.
2. Personal Errors: These may be due to the following reasons:
i. Inaccurate leveling of the compass box
ii. Inaccurate centering
iii. Inaccurate bisection of signals and
iv. Carelessness in reading and recording when making addition or conversion,
v. Wearing of metal materials.
3. Natural errors: These may be due to following reasons:
i. Local attraction due to proximity of metal ores.
ii. Magnetic changes in the atmosphere due to clouds and storms.
iii. Irregular variations due to magnetic storms and position of metal materials.
iv. Variation in declination.
WAYS TO AVOID ERRORS IN COMPASS SURVEY
i. Check the accuracy of the compass by comparing with an accurate compass or with location of magnetic North for the years of survey.
ii. The prismatic compass should centre accurately with plumb bob and level instrument. That is, the centering should be done correctly.
iii. A surveyor must ensure that the compass is held as steady as possible taken.
iv. Make sure the compass is on non-metallic tripod stand.
v. The observer should not carry magnetic substances. He must not wear metal watch and metal element when surveying by compass.
vi. Avoid surveying by compass in area which comprises iron ore rocks. That is, the stations should not be selected near magnetic substances.
vii. Readings should be taken correctly from the line of sight and not from any side and must be recorded carefully.
viii. Only one person should be trusted to use the compass to avoid external influences.
3. PLANE TABLE SURVEY
Plane table survey: is a graphical method of surveying in which the field works and the plotting is done simultaneously. It is particularly adopting in small mapping.
Plane Table: A plane table is a device used in surveying and related disciplines to provide a solid and level surface on which to make field drawings, charts and maps. The early use of the name plane or plain table reflected its simplicity and plainness rather than its flatness.
PRINCIPLES OF PLANE TABLE SURVEY
The principle of plane tabling is ‘ PARALLELISM ’ , meaning that: “ All the rays drawn through various details should pass through the survey stations.
i. The rays drawn from the stations to objects on the paper are parallel to the lines from stations to the objects on the ground”.
ii. The field work and plotting are done simultaneously and such survey does not involve the use of a field book.
iii. Field book is not necessary.
iv. Suitability of Plane Table Survey
Plane table survey is mainly suitable for filling interior details when traversing is done by theodolite. Sometimes, traversing by plane table may also be done. This survey is recommended for the work where great accuracy is not required as the fitting and fixing arrangement of the instruments is not perfect. Thus, most accurate work cannot be expected.
PROCEDURES OF CONDUCTING PLANE TABLE SURVEY
1. Fixing the table on a tripod stand: This involves fixing the plane table to the tripod stand. The tripod stand is placed over a require station with its legs well apart. Then the table is fixed on it by a wing nut at the bottom.
2. Setting the Table: The position of plane table at each station must be identical. That is, at each survey station the table must be oriented in the direction of magnetic north. Normally thehe setting up and a temporary adjustment of the plane table is done by three stages:
i. Leveling the table: Leveling the plane table is done with the help of spirit level. The spirit level is placed at different corners and various positions of the table. Leveling up of the table is done by shifting the legs of the tripod where the bubble is brought to the center of its run at every position of the table.
ii. Centering the table: Centering is the process of keeping the table over the station so that the point on the paper representing the station being occupied is vertically over the point on the ground. It is done by forked plumb bob. Care should be taken not to disturb the levelling, where at last the table is clamped.
iii. Orientating the table: Plotting of detail points cannot begin until the plane table is properly oriented.
ORIENTATION
Orientation: is the process of keeping the table to the position which is occupied at the
first station. It involves rotating the leveled table around its vertical axis until the plotted information is in exactly the same relationship as the data on the ground.
Methods of Orientation
i. Orientation by the magnetic needle: In this method, the magnetic north is drawn on a paper at a particular station. To orient the table at the next or any subsequent station, trough compass is placed along the line representing the magnetic north which has been drawn on the paper at the first station, and the board is then turned until the ends of the needle are opposite to the zeros of the scale. The board is then clamped in position. This is suitable for roughly small scale mapping. However, the method is inaccurate since the results are likely to be affected by the local attraction.
ii. Orientation by Back Sighting: This is the most accurate and gives perfect orientation and is always preferred than the previous one Suppose a table is set up over station Q on the line PQ which has been previously drawn as PQ from station P. The alidade is placed along the line QP and board then turned until the line of sight bisects the ranging rod at P. The board is then properly clamped.
3. Marking the North. The trough compass is placed on the right hand corner with its north end approximately towards the north. Then the compass is turned clockwise or anticlockwise so that the needle. coincides exactly with the 0-0 mark. Now a line representing direction is drawn through the presenting the north ensured that the table is not turned.
4. Sighting: This process involves sighting the points with the help of alidade.
INSTRUMENTS USED IN PLANE TABLE SURVEY
The main instrument in plane table surveying is a table top, similar to drawing board fitted on to a tripod stand. With a number of accessories. The accessories consist of alidade, spirit level, trough compass, plumbing fork, plumb bobble, drawing sheet.
i. Plane table/Drawing Board
ii. Alidade
iii. Spirit level
iv. Trough Compass
v. Plumbing Fork
vi. Water Proof Cover
vii. Paper or Drawing Sheet
viii. Tripod
1. Plane table/Drawing board: It is made of well-seasoned wood. It varies in size; the common sizes are from 40 cm x 30 cm to 75 cm x 60 cm or 45 cm square, 60 cm square, etc. The board may be mounted on a tripod with a leveling head or a ball-and- socket arrangement in such a fashion that it can be leveled and revolved about a vertical axis and may be clamped in any position. There are various types of plane tables, depending upon the arrangement of fixing the boards to the tripod, leveling of the table and rotating arrangement in a horizontal plane.
2. The lidade: is a ruler with a sight line attached and is used on the plane table for bisecting the object, drawing rays, direction lines, etc. It consists of a metal (brass or gunmetal) or boxwood straightedge or ruler of about 45 cm lon . The beveled ed e is called the “ruling edge” or “the edge” or “ the Fiducial edge.”The alidade may be plain fitted with sight vanes at both the ends, or it may be equipped with a telescope. One of the sight vanes is provided with a narrow rectangular slit. While other is provided with a central vertical hair or wire. If the alidade is telescopic, the telescope is provided with a vertical circle, and a level tube is fitted with cross-hairs.
Types of alidades
The types of alidade depending on the type of line of sight there are two types of alidade which are:-
i. Plain Alidade: In this type, a sight vane is provided at each end of the ruler. The vane with narrow slit serves as eye vane and the other with wide slit and having a thin wire at its centre serves as object vane. The two vanes are provided with hinges at the ends of ruler so that when not in use they can be folded on the ruler. Plain alidade: is not suitable Fiducial edge/Beveled edge in surveying hilly.
ii. Telescopic Alidade: This consists of a telescope mounted on 4 column fixed to the ruler. The line of sight through the telescope is kept parallel to the bevelled edge of the ruler. The telescope is provided with a level tube and vertical graduation arc. If horizontal sight is required bubble in the level tube is kept at the centre. If inclined sights are required vertical graduation helps in noting the inclination of the line of sight. By providing telescope the range and the accuracy of line of sight is increased.
3. The spirit level: This is used to level the plane table and check the level of table by placing it on the board in two positions at right angles to each other. To get perfect level, spirit level should show central position for bubble tube when checked with its positions in any two mutually perpendicular directions. When the bubble tube remains in the centre at any point on the plane table is considered to be properly leveled. If the spirit level which may be very sensitive is not fitted to the alidade, the table can be leveled by placing the spirit level in two positions at right angles to each other and setting the plane table such that the bubble is central in both positions.
4. Prismatic compass: The compass is used for marking the magnetic north line on the drawing sheet of the plane table. The magnetic compass is also used to orient the plane table with respect to the magnetic meridian.
Type of prismatic compass
i. Trough compass: A trough compass is a long, narrow rectangular box, covered with glass with its centre having a magnetic needle resting on the pivot. There are also graduated scales with zero at the centre and marking up to 5° on either side of the zero line. The trough compass is used for marking the magnetic north line on the drawing sheet of the plane table. A trough compass is also used to orient the plane table with respect to the magnetic meridian. Trough Compass with two bubble tubes at right angles to each other mounted on a square brass plate is used for indicating the direction of the magnetic meridian on the paper.
ii. Circular Compass: Circular compass carries a pivoted magnetic needle at the center. The circular box is fitted on a square plate base. Sometimes two bubble tubes are fixed at right angles to each other on the base plate. The compass is meant for marking the north direction of the map
5. A Plumb Fork: is a metal frame bent in 2 U-shape with equal length upper horizontal arm and a lower inclined arm. The upper arm is provided with a pointer (it is pointed) at the end while the lower arm is provided with a hook to suspend plumb bob. The plumb bob helps to center the table over the station for transferring the ground point to the drawing sheet and vice versa. A plumbing fork with a plumb bob attached to one end is used for centering the table at a particular station. This is used in large-scale work for setting the table such that the point on the paper (representing the instrument station being occupied) may be brought vertically over the station marked on the ground.
6. Water proof cover: The Waterproof cover is used to protect the sheet of paper on the plane table from the rain.
7. The drawing paper/Drawing sheet: This is a sheet of drawing paper, called plane table sheet which is fastened to the board. It should be a good quality drawing sheet. For important works fibre glass sheets or paper backed with thin aluminium sheets are used. The drawing sheet may be rolled when not in use, but should never be folded. Other drawing accessories are hard pencil, good eraser, pencil cutter and sand paper to keep pencil point sharp. The paper used for plotting the survey on the plane table should be of excellent quality because it is very sensitive and liable to changes due to the variation of humidity of the atmosphere.
The paper expands and contracts in different directions with different amounts which consequently alters the scale and distorts the map. The sheet should be well seasoned for about a week by exposing it alternately to damp and dry atmosphere. This treatment reduces its tendency to distort. The sheet should never be folded. Zinc and celluloid sheet is used for plane table surveying in damp climates. The paper may be fixed on the drawing board by using drawing pins or by pasting the edges. Besides these, the other drawing instruments like a pencil, rubber, scales, etc., are also required for plane table survey.
8. The Tripod stand: is also required for plain table survey. It is in one of the important plane table survey instruments without it is impossible to do plain table survey. Tripod is used to fix the drawing board. It consists of a screw at the bottom which is used to set and level the drawing board on it comfortably.
ERRORS IN PLANE TABLE SURVEY
In plane table surveying, errors may occur during field observation as well as during plotting. The errors are categorized into two: Instrumental errors and Personal errors or Errors due to manipulation and sighting. Precautions need to be taken to minimize the amount of errors so as to improve the accuracy of surveying work.
1. Instrumental Errors: The primary sources of instrumental errors in plane table surveying arise from the lack in temporary adjustment. The causes for instrumental errors are:-
i. Undulated plane table surface.
ii. Loose fittings in plane table.
iii. Improper magnetic compass.
iv. Non-perpendicularity of the sight vanes.
v. Defect in level tube.
vi. Unseasoned wood and poor quality drawing paper.
2. Personal Errors
i. Improper leveling of plane table.
ii. Inaccurate centering.
iii. Improper orientation.
iv. Improper clamping of plane table.
v. Inexact bisection of object.
vi. Improper plotting.
vii. Instability of tripod stand.
METHODS OF PLANE TABLE SURVEYING
There are mainly four methods of plane table surveying which are:-
i. Radiation method
ii. Intersection or triangulation method
iii. Traversing method and
iv. Resection method
1. RADIATION METHOD
Radiation Method: This is the simplest method which is useful in surveying small areas which can be commanded from one station. A ray is drawn towards the point with the help of the alidade. With the help of chain or tape, the horizontal distance is measured from the plane table to the point and this point is located on the sheet by plotting it into the scale of the map. In this method of plain table survey, the plane table is set up at only one station, and various points are located by radiating (drawing) a ray from the instrument station to each of the points, and plotting to scale along the way the distance measured from the station to the point is sighted.
The radiation method is suitable for surveys of small areas which are likely to be commanded from a single station. It is useful in large-scale works if used in combination with other methods. If the distances are obtained tacheometrically with the help of telescopic alidade (provided with stadia-hairs), the work can be finished very rapidly and thus, in this case; the method will have a wider scope.
Procedures of Radiation method
i. Select a point P so that all the corners of the traverse ABCD are seen.
ii. Carry out the temporary adjustments of centering and leveling, then mark the north line on paper.
iii. Put the alidade on point P and draw a line of sight for station A.
iv. Measure the distance PA on the ground and put this length to a suitable scale on paper which will give point a.
v. Similarly, obtain points b, c and d on paper by drawing lines of sight for stations B, C and D and measuring the distances PB, PC and PD on the ground respectively.
vi. Join points a, b, c and d on a paper, as shown in the figure. vii. For checking the we scam accuracy of work, measure the distances AB, BC, CD oar and DA on the ground and compare them with the lengths ab, bc, cd and da respectively on paper.
2. INTERSECTION OR TRIANGULATION METHOD
Intersection Method: This method is useful where it is not possible to distances on ground as in case of a mountainous country. Hence, this method is employed for locating inaccessible points, the broken boundaries, rivers, fixing survey stations. In this method the positions of the object on the plan are fixed by the intersection of rays drawn from two instrument stations but no chain or tape is required. Intersecting rays are drawn from these two stations whose location is already plotted by measuring the distance between them, the point of intersection is the location of the point. The line joining these instrument stations are called baseline.
This method is widely employed for plotting the details on the maps. It can also be used for plotting the position of points to be used at subsequent stations. The various points can be located by the intersection of rays drawn from two different stations (A and B) forming a baseline. The only linear measurement required is that of the baseline AB on the ground. The method may also be employed for locating the distant and inaccessible objects, the rivers, in a survey of the hilly areas (where distances cannot be measured easily), and for checking the remote objects.
Procedures of Intersection Method
i. Select two stations are easily seen from them.
ii. Plot the line pq, be done in one of the two ways:
iii. The table can orient at station Q, the put up to some scale on the paper.
iv. The line pq can be drawn board can be adjusted from station P by P and Q so that the points to be located on paper which is known as the base line, on paper. This can be centered and leveled at station P and then after distance PQ can be accurately measured and to some scale on the paper and then the back sighting at station Q.
v. From station P, draw rays for stations A, B.
vi. Shift the table to station Q and after proper orientation, take rays of stations A, B etc.
vii. The intersection of rays from stations P and Q will give points a, b etc. on ' paper.
viii. For checking the accuracy of work, measure the distance AB on the ground and compare it with its corresponding length ab on the paper.
3. TRAVERSING METHOD
Traversing Method: This is the main method of plane table surveying and is similar to compass or theodolite traversing. This method is used for running Survey lines for close or open traverse and it involves running a traverse with a Plane table and locating details by taking offsets in usual manner. This method of plane table surveying is used to plot a traverse in cases stations have not been previously plotted by some other methods.
Traversing by the plane table is similar to compass or the odolite traversing. The method, therefore, can be used for laying down the survey lines of a closed or unclosed traverse. The details may be plotted in the usual way by taking offsets. The plane table traversing is also useful for the survey of roads, rivers, etc.
Procedures of Traversing
i. Let us consider the stations P, Q, R and S which are to be plotted by method of traversing.
ii. Stations are to be chosen in such a way that adjoining stations are visible. First, the plane table is to be station P and then plotted as p. The orientation of the table and scale of plotting should be such that all other stations will be accommodated within the boundary of the sheet.
iii. With the alidade pivoted at p drew the rays to Q and S. Distances PQ and PS are measured and plotted on the respective rays, pq and ps respectively. "
iv. The plane table is then shifted to station Q, get it set and then Oriented by back sighting to station P. With the alidade pivoted at q, draw a ray to R. Distance QR is measured and plotted on the ray as gr. this way, plane table is shifted to stations R and S and corresponding rays are drawn to obtain the plotting of the traverse pars.
4. RESECTION METHOD
Resection Method: Resection is the process of determining the plotted position of the station, occupied by the plane table, by means of sight taken towards known points, locations of which have been plotted. It involves finding the position of a station where plane table is placed, with the help of sights taken towards known and visible points whose location has already been plotted. The method of resection is employed for the location of station points only. After establishing the stations, the details are located by either radiation or intersection. The main feature of this method is that the point plotted on the sheet is the station occupied by the table. Only one linear measurement is required as in the intersection method of plane tabling.
Orienting a plane table through resection: Orienting a plane table by back sighting or by compass needle requires occupying a station whose position has been plotted.
Resection however, enables you to orient the plane table without setting it up at a previously plotted station. This technique uses O points whose positions are plotted on the plane table.
Methods of table orientation through resection
i. Orientation by Two Point Problem: The two-point method of resection is used to orient the plane table and establish the position of a station when two previously plotted points cannot be occupied. In this method, two well defined points whose positions have already been plotted on the plan are selected. Then by perfectly bisecting these points, a new station is established at the required station.
ii Orientation by three points problem: In this problem, three well defined points whose position have already been plotted on the map. Then by perfectly bisecting these three well defined points, on new station is established at the required position. The three- point method involves orienting the plane table and plotting a station when three known plotted stations can be seen but not conveniently occupied. Set up the plane table at the unknown point P and approximately orient the table by eye or compass.
ADVANTAGES OF PLANE TABLE SURVEY
1. It is very effective method of surveying for preparing small or medium size topographical maps.
2. It is suitable for location of details as well as contouring for large scale maps directly in the field.
3. As surveying and plotting are done simultaneously in the field, chances of getting omission of any detail is avoided.
4. The plotting details can immediately get compared with the actual objects present in the field. Thus errors as well as accuracy of the plot can be ascertained as the work progresses in the field.
5. Contour, specific and irregular object and features may be represented accurately and checked conveniently as the whole area is - in view at the time of plotting.
6. Only relevant details are located because the map is drawn as the survey progresses. Irrelevant details get omitted in the field itself.
7. The plane table survey is generally most rapid method. It is more - rapid and less costly than most other types of survey.
8. No great skill is required for satisfactory map as the instruments used are simple. No much skill for operation of instruments is required.
9. In this method of survey, field book is not necessary, it requires no field book. It is less costly method.
10. The method is particularly suitable for magnetic area where prismatic compass is not reliable.
DISADVANTAGES OF PLANE TABLE SURVEY
1. The technique is highly affected by weather conditions: The plane table survey is essentially a tropical areas. Because it is not possible in un-Plane tabling is not suitable for work in wet climates, in places where high winds predominate, and in wooded country.
2. It is limited to geographical areas: This method of survey is not very accurate as the other survey methods and results, It is unsuitable for highly accurate work.
3. The technique tend to ignore field sheets: As no field book is maintained, plotting at different scale require full exercise.
4. Time consuming: The method requires large amount of time to be spent in setting the table.
5. It cover small areas: It is not very accurate for large-scale surveys as compared to compass and theodolite surveys. If the area to be surveyed is large, frequent changes in the size of drawing sheets are required.
6. It is suitable much in open area: This method is effective in relatively open country where stations can be sighted easily.
7. Its equipments are not easy to be handled: The instruments and its accessories are heavy and cumber-some, and they are likely to be lost in the field.
4. LEVELLING SURVEY
Leveling is the science and art of measuring heights in relation to above the sea level. It offers an Inexpensive, simple and accurate methods for measuring heights, and it is widely used in construction sites.
Leveling is a branch of land surveying that is defined as an art of determining the relative height of different points on, above or below the surface. Leveling is the procedure by which the heights of points on the earth’ s surface are determined. The height of one point can only be given with reference to the height of another point. Objectives of Levelling.
The aim of leveling is to determine the relative heights of different objects on or below the surface of the earth and to determine the undulation of the ground surface. Thus, the objectives of levelling include:-
1. To find elevation of given point with respect to some assumed reference line called datum.
2. To establish point at required elevation with respect to datum.3. To locate contours, spot heights, trigonometrical heights, and bench marks on maps. “ All heights are relative to a datum, a basic line usually at mean sea level. For east Africa, the datum line is situated in Mombasa, for Nigeria It is in Lagos, and for South Africa it is in Cape Town” (Pritchard 1984).
Principle of Levelling
The principle of leveling is to obtain horizontal tine of sight with respect to which vertical distances of the points above or below this line of sight are found. For short distances, this method is accurate but for longer distances more accurate instruments are required.
USES/IMPORTANCE/USEFULNESS OF LEVELING SURVEY
1. It is used to calculate the volume of earthworks and other materials.
2. To helps to investigate drainage characteristics on the earth surface.
3. It helps to determine the height of the place above the sea level.
4. It is also used in drawing the contour lines.
5. It helps to prepare a longitudinal section and traverse sections section of a given project.
6. It is used for construction activities like road and railway construction.
7. It helps to determine housing foundation. E.g city planning.
INSTRUMENTS USED IN LEVELING
Levelling instruments are employed for determining the relative heights of different points on the earth’s surface.
TYPES OF LEVELLING INSTRUMENTS
According to the general arrangement of various parts, the levelling instruments may be classified as:-
i. Dumpy level,
ii. Wye level,
iii. Reversible level such as Coo e’s reversible level and Cushin ’s level and
iv. Tilting level.
INSTRUMENTS OF LEVELING SURVEY
1. Level: This is a device which gives a truly horizontal level. A level is basically a telescope attached to an accurate levelling device, set upon a tripod so that it can rotate horizontally through 360 ° . Normally the levelling device is a bubble, but modern ones incorporate a pendulum.
i. Tilting level: Tilting levels use a spirit level instead of a compensator to establish a horizontal line of sight. Tilting level is adjustment of level bubble needed before every measurement not already set up.For each observation the main bubble is viewed through an eyepiece and the telescope tilted by a fine screw to bring the two ends of the bubble into coincidence.
ii. Dumpy level: This is more basic level often used in construction work. It is adjustment of level bubble needed only once after level set up. The telescope staff is rigidly attached to a single bubble and the assembly is adjusted either by means of a screwed ball-joint or by foots crews which are adjusted first in one direction, then at 90.
iii. Automatic Level: This is 4 more modern type of level which is now in general use. It is a self leveled instrument. Automatic level has a compensator which consist of an arrangement of three prisms that ensures that the line of sight remains horizontal once the operator.The advantages of digital levels are that observations are take need to reads staff or record anything by hand. Introducing t removes two of the most common errors when levelling, incorrectly and writing down the wrong value in the field book.
2. Levelling staff: This is a suitably graduated staff for reading vertical heights. The levelling staff is a box section of aluminium or wood, which extends to 3 or 5 meters in height by telescoping, hinging or addition of sections. One face has a graduated scale attached for reading-with the cross- hairs of the level telescope. These faces can vary in pattern and graduation. 2mm graduations should be the maximum for accurate levelling of gauging stations.
Levelling Staff: is known as the staff rod, Levelling rod, and self- reading staff, as the staff readings may be taken directly by the instrument man from the levelling instrument.The width and thickness of the Level staff are 75 mm and 18 mm respectively. The staff is made of well-seasoned cypress, blue pine or deodar wood free from defects such as knots, cracks, shakes, insect attacks, etc.
3. Staff bubble: This is generally a small circular bubble on an angle plate which is held against one corner of the staff to ensure that the staff is held in-a vertical position. If the staff is not held vertical, the reading will be too large and may be significantly in error.
Other instruments are chain survey instruments
i. A Chain: Chains are used to measure distances on the field. A chain is made up of connected steel segments, or links, which each measure 20 cm. Sometimes a special joint or a tally marker is attached every 5 metres. Usually, a chain has a total length of 20 metres, including one handle at each end.
ii. Tape measure: Tapes are used to measure distances on the field. Measuring tapes are made of steel, coated linen, or synthetic material. They are available in lengths of 20, 30 and 50 m. Centimetres, decimetres and metres are usually indicated on the tape.
iii. Notebooks: are used during field work to record data obtained. The notebook should be of good quality and 150mm x100mm in size.iv. Hard pencil and rubber: are used for drawing in the field and a rubber is used to erase mistakes or errors which are made. A pencil should be hard enough to avoid tearing during survey.v. Arrows: Survey Arrows are hardened galvanized steel arrows in red and white with a 35mm red ringed top. Each arrow is 400mm long with a diameter of 4.5mm. Used in conjunction with Land chains for measuring in land Surveying.
vi. Pegs: are used when certain points on the field require more permanent marking. Pegs are generally made of wood; sometimes pieces of tree-branches, properly sharpened, are good enough. The size of the pegs (40 to 60 cm) depends on the type of survey work they are used for and the type of soil they have to be driven in. The pegs should be driven vertically into the soil and the top should be clearly visible.
METHODS OF LEVELLING
There are two methods of leveling, namely:-
i. Height of Collimation Method
ii. Rise and Fall Method
1. COLLIMATION METHOD
It consist of finding the elevation of the plane of collimation ( H.I.) for every set up of the instrument, and then obtaining the reduced level of point with reference to the respective plane of collimation.
Procedures
i. Elevation of plane of collimation for the first set of the level determined by adding back side to Reduced Level (R.L) of Bench Marks (B.M).
ii. The Reduced Level (R.L) of intermediate point and first change point are then obtained by starching the staff reading taken on respective point (Intermediate sight (IS) & Foresight (FS) from the elation of the plane collimation. [H.I.]
iii. When the instrument is shifted to the second position a new plane collimation is set up. The elevation of this plane is obtained by adding Back sight (BS) taken on the Change plate (C.P).
From the second position of the level to the Reduced Level (R.L). Change plate (C.P. The Reduced Level (R.L) of successive point and second C.P. are found by subtract these staff reading from the elevation of second plane of collimation.
Arithmetical check Formula
Sum of B.S. – sum of F.S. = last R.L. – First R.L.
2. RISE AND FALL METHOD
It consists of determining the difference of elevation between consecutive points by comparing each point after the first that immediately preceding it. The difference between their staff reading indicates a rise fall according to the staff reading at the point. The R.L is then found adding the rise to, or subtracting the fall from the reduced level of preceding point.
Arithmetic check
Formula: Sum of B.S. – sum of F. S. = sum of rise – sum of fall = last R. L. – first R.L. BASIC TERMINOLOGIES APPLIED IN LEVELING SURVEY
Foresight (FS): Last staff reading taken before moving the instrument to another location.
A Set-Up: A set-up refers the position of 2 Level or other instrument at the time in which a number of observations are made without moving the instrument. This results to back sight, Intermediate and Foresight observations.
Back sight (BS): This is the first observation made to the known point. It is a sight with- level back to a point of known elevation. It refers to the first staff reading taken immediately after setting up the instrument. In other words, it is the very first reading taken from an instrument station.
Foresight (FS): This is the last staff reading taken before moving the instrument to another location. It is a sighting with a level to determine elevation of a point. It refers to
the last observation to the final point or the next to be measured on the run. In other words, it is the very last reading taken from an instrument station.
Intermediate Sight (Inter-Sight) (IS): This refers to all the points or readings taken in between Back Sight and Fore Sight. It is a fore short to a point at which you want to know the elevation but which will not be used as a turning point. So, if the instrument is set up at one location only, there will be just one BS (first reading), one FS (last reading) and several ISs.
Change Point (CP): This is a point on which Fore-Sights and Back- Sight are taken. It is the staff station at which both Fore Sight from the First Instrument station and Back Sight from the Second instrument station have been taken. It is also referred to as a turning Point which Is the point at which you have established an elevation with FS which you will subsequently take a BS.
Balancing short: This is an attempt when doing the levelling survey to keep the lengths of FS and BS at any given instrument as close as possible.
Level surface or Level Line a level surface: is a surface which is everywhere perpendicular to the direction of the force of gravity. For ordinary levelling surfaces at different elevations can be considered to be parallel. An example of level surface is the surface of a completely still lake.
Datum: A datum is an arbitrary level surface to which elevations are referred. In order to calculate the heights of points, a datum is required as a reference level. The most common surveying datum is mean sea- level (MSL). But as hydrological work is usually just concerned with levels in a local area, we often use An Assumed Datum or A Reduced Level.
An Assumed Datum: This is a datum which is established by giving a benchmark an assumed value, say 100.000 meter as a temporary benchmark to which all levels in the local area will be reduced. It is not good practice to assume a level which is close to the actual MSL value, as it creates potential for confusion.
A Reduced Level: is the vertical distance between a survey point and the adopted level datum. The reduced level of a point is its height or elevation above the surface adopted as a datum. The height of any target point is referred to as Reduced Level: (RL), because it is reduced to a known datum.
Reference Datum/Bench Mark(BM): This is defined as a station with a known reduced level. It is a point with known height above mean sea level or other reference datum. They are permanent points unchanged by weather conditions and are provided by the
Department of Land and Surveys. It is at this point the height of other points can be referred.
A Run: A run is the levelling between two or more points measured in one direction only. The outward run is from known to unknown points and the return run is the check levelling in the opposite direction.
A Close: A close is the difference between the starting level of the initial point for the outward run and that determined at the end of t e return run. If the levels have been reduced correctly this value should be the same as the difference between the sum of the rises and falls, also the difference between the sum of the back sights and fore- sight.
Closure Error: This is the difference in elevation determined from the levelling survey and the known elevation of a benchmark.
Orders of Levelling: Orders of levelling refer to the quality of the levelling, usually being defined by the expected maximum closing error.
Peg Test: This is the surveying operation carried out to determine if the levelling bubble and telescope line of sight are parallel.
TYPES OF LEVELING
Main types
1. Differential leveling: is the process of measuring vertical distances from a known elevation point to determine elevations of unknown points.
2. Profile levelling: is the process of determining the elevations of a series of points at measured intervals along a line such as the centerline of a proposed ditch or road or the centerline of a natural feature such as a stream bed. This method is used for taking levels along the centre line of any alignment like road, railway canal etc. The object is to determine the undulations of the ground surface along the alignment.
Other types
i. Simple Levelling: This is the simplest method used when it is required to find the difference in elevation between two points.
ii. Check levelling: This kind of levelling is carried out to check the accuracy of work. It is done at the end of the survey work in the form of fly levelling to connect the finishing point and starting point.
iii. Cross sectioning: This operation is carried out perpendicular to alignment at an interval of 10m, 20m, 30m and 40m. The idea is to make an estimate of earthwork.
iv. Precise levelling: This is used for establishing bench marks for future public use. It is carried out with high degree of accuracy using advanced instruments.
v. Trignometric levelling: In this method vertical distances between points are computed by observing horizontal distances and vertical angle between points.
vii. Barometric levelling: In this method the altitude difference is determined by means of a barometer.
viii. Reciprocal levelling: This method is adopted to accurately determine the difference of level between two points which are far apart. It is used when it is not possible to set up level in mid way between two points.
PROCEDURES OF CONDUCTING LEVELING SURVEY
1. Setting up; Back sight and foresight distances should be approximately equal to avoid any errors due to collimation, refraction or earth’s curvature. Distances must not be so great as to not be able to read the graduations accurately. The points to be observed must be below the level of the instrument, but not lower than the height of the staff.
2. Elimination of Parallax; Parallax is the apparent movement of the image produced by movement of the observer's eye at the eyepiece. It occurs when the focusing screw and the eyepiece is done incorrectly. This condition can be detected by moving the eye to different parts of the eyepiece when reading the staff. If different readings are obtained then parallax is present.
Procedure for Parallax Elimination
Parallax is eliminated by focusing the telescope on infinity and then adjusting the eyepiece until the cross-hairs appear in sharp focus.
i. Remove the lid from the object glass.
ii. Hold a sheet of white paper in front of the object glass.
iii. Move the eyepiece in or out until the cross hairs are distinctly visible.
iv. Then remove the sheet of paper and bring the staff into focus using the focusing screw. Direct the telescope towards the staff.
v. Turn the focusing screw until a clear and sharp image is formed in the plane of the cross hairs.
vi. If parallax is still occurring repeat the adjustment procedure until the setting remains constant for a particular observer’s eye.
3. Reducing The Levels: The Reducing Levels can be computed in two ways: Height of Collimation and Rise and Fall method.
i. Height of Collimation (H.C)
Height of collimation method: It deals with the deductions of all stations from a common reduced level (R.L) of lineof sight and it reduces levels relative to the instrument height. The H.C method is a very simple, tedious method quicker than rise and fall method. It can be used when there are no inter sights. H.C is obtained by the formula: Datum Height + the first reading.
Advantages of height of collimation method
i. Height of Collimation Method is simple and easy, reduction of levels is easy and visualization is not necessary regarding the nature of the ground. There is no check for intermediate sight readings.
ii. This method is generally used where more number of readings can be taken with less number of change points for constructional work and profile levelling.
ii. The Rise and Fall Method
Rise and Fall method: deals with the comparison of R.L.s of successive stations in all site levelling. Reduction is carried out on site before packing up to ensure that the levelling has been done correctly. For the same set up of an instrument, staff reading is more at a lower point and less for a higher point. Thus staff reading provides information regarding relative rise and fall of terrain points. This provides the basic behind the rise and fall method for finding out elevation of unknown points.
Procedures for Reducing the Levels through Rise and Fall Method
i. Calculate the rise and fall between successive points and book them in the appropriate column (one can determine whether each shot is a rise or fall by the following rule of thumb: a higher value on top denotes a rise, a higher value on the bottom denotes a fall).
ii. Add up the back sight and foresight columns for the entire traverse and note the difference between them; this is the close.
iii. Add up the rises and falls for the entire traverse, and compare the difference between them with the difference between the back sights and foresights; they should be the same.
iv. Carry the reduced levels in the R.L. column down the page by adding or subtracting the appropriate rise and fall values to the successive values of R.L. The final value of the original starting point will differ from the original value by the amount of the close.
v. If the levelling has been done correctly and all arithmetic reductions are correct, the differences between total back sights and foresights, total rises and falls, and starting and finishing R.L ’ s should be the same. This difference is the close; and for site inspection purposes it should be within ± 2mm or ± 6mm, depending upon which water- level standard is being followed.
Disadvantages of Rise and fall method
i. Rise and Fall Method is complicated and is not easy to carry out. Reduction ot levels takes more time and visualization is necessary regarding the nature of the ground. Complete check is there for all readings.
ii. This method is preferable for check levelling where numbers of change points are more.
4. Leveling Booking:
i. Level books or loose-leaf levelling sheets shall be numbered and indexed in a register. Details of the site, work, date, observer, chain man, booker, weather, wind, instrument and any other relevant items shall be entered.
ii. Enter the first observation (which is on a known point) in the Back sight column, and sufficient detail in the Remarks column to identify it. Enter the point’s R.L. zero from the site register or plate on the BM, etc.
iii. Enter all other points on subsequent lines as intermediates except the point chosen as the foresight. Identify them in the Remarks column as above. Enter the foresight on a further line in the Foresight column.
iv. Change the instrument to the next setup. Enter the following back sight on the same line as the previous foresight but in the Back sight column.
v. Repeat the above procedure at each setup on the outward run then reverse it to work back to the starting point on the return run. The furthest point out is treated as for all other change points.
5. Permanent Booking: All levelling results shall be booked in either level books or levelling sheets which shall be retained as permanent records. Level books shall be numbered so that they can be referenced on station history and inspection forms.
i. They should be stored in fire proof storage as for original record: They should also include an index. Levelling sheets shall be filed in time-sequential order in site files and also need to be in fire-proof storage as for level books.
ii. Staff not vertical: The staff may not be fully extended or may not be held vertical. Since the staff is used to measure a vertical difference between ground and line of collimation, failure to hold the staff vertical will result in incorrect readings.
iii. Parallax: The parallax may not be eliminated. This occurs when the internal arrangement of focusing tube is not corrected. This effect must be eliminated before any readings are taken.
SOURCES OF ERRORS IN LEVELLING
There are three source of errors in leveling and their importance must be appreciated and precaution taken to reduce their effects. All these errors cannot be corrected unless the work is repeated. These sources include:-
i. Personal errors.
ii. Instrumental error in equipment and 3. Error due to natural causes.
iii. Instrument not correctly leveled.
1. Personal Causes or Sources of Errors
i. Instrument not level: The Instruments may not be leveled. For automatic level this source of error is unusual. It is a common error for a tilting level in which the tilting screws has to be adjusted for each reading. The best procedure here is to ensure that the main bubble; centralized before and after a reading is taken..
ii. The wrong cross-hair reading recorded (e.g. top instead of middle).
iii. Distance: The focusing of eye piece and objective glass may not be perfect. This occurs due to sighting the staff over too long distance which makes it impossible to take accurate reading. It is therefore recommended that sighting distance should be limited to 60m, but where absolutely unavoidable this may be increased to a maximum of 100m.
iv. Position change: The position of staff may have changed. Stable change points must be chosen so that when turning the staff round or when replacing it after removal no alteration of height takes place. Always choose a hard ground and mark the staff position with chalk.
v. Sinking: If the instrument is setup on soft or marshy ground, it may settle and alter the height of collimation. Here also, always choose a hard ground.
vi. Entry and recording: Entry and recording in the field book may be correct. Many mistakes are made during reading and booking the readings and the general rule is that staff sighting must be carefully entered into the leveling table immediately after reading. Extra care must be taken.
2. Instrumental Causes or Sources of Errors
There are various sources of instrumental error which include the following:-
i. Collimation errors: This occurs if the permanent adjustment of the instrument is not perfect. That is, the line of collimation is not truly horizontal when the instrument is level.
ii. Defect of the staff: The graduation of the staff may not be perfect.
Thus, new repaired staff should be checked against the steel tape. Particular attention should be paid to the base of the staff to see if it has become badly worn.
iii. The pod defect: The stability of tripods may be the source of error. Thus, tripods should be checked before any field work commence by testing to see the tripod head is screwed and the shoes at the base of each are not loose.
3. Natural Causes or Sources of Errors
The following are the sources of natural errors in leveling.
i. Earth curvature error: the curvature of the earth may affect the staff reading s when the distance of sight is long. The curvature of the earth makes the line of sight at the instrument deviate from a horizontal line as one move away from the level. The effect of curvature is that, it causes the objects sighted to appear lower than they really are. Curvature correction is always subtractive (-). As a result true staff reading = (observed staff reading -0.0785 D2) where D=distance in km. The effect is eliminated by using equal sight lengths for fore and back sights.
ii. Refraction error: The rays of light pass through layers of air of different densities and become refracted or bent down. This cause a wrong staff reading. The effects of refraction are that, it makes the object appear higher than it really is. Refraction varies considerably with climatic condition and it is always additive. As a result: True Staff Reading = Observed Staff Reading + Refraction Correction. The effect of refraction is almost totally eliminated by using the fore and back sights. This is because atmospheric conditions along small and back sights will not be completely identical, there will be residual error.
iii. Wind: There are some errors in staff readings due to high ne staff wind. The wind causes vibration of the level, tripod and 1 particularly when it is fully extended thereby making accurate sighting impossible, Always shelter the staff and keep short sights of weather. The staff should also be kept short.
iv. The sun: The sun can cause an apparent vibration of the staff wing to irregular refraction. It also affects the bubble by causing unequal expansion of the level and tripod. In hot weather length of sight are reduced to at least 0.5m above the ground through- out the length. The ray-shade in front of the instrument should be extended or shade the instrument with umbrella.
(v) Rain: Rain makes accurate work difficult and unpleasant; rain dropping on the objective glass and condensation on the eye piece make sighting impossible. For precise work it is advisable to wait for better weather condition or ray shade can be used or protecting the Instrument with umbrella.
LEVELING AND CONTOURING
Levelling is defined as “ an art of determining the relative height of different points on, above or below the surface while Contouring is the science of representing the vertical dimension of the terrain on a two dimensional map.
A contour is defined as an imaginary line of constant elevation on the ground surface. It can also be defined as the line of intersection of a level surface with the ground surface. For example, the line of intersection of the water surface of a still lake or pond with the surrounding ground represents a contour line.
CHARACTERISTICS OF CONTOURS
Contours show distinct characteristic features of the terrain as follows:-
i. Closely spaced contour lines indicate steep slope.
ii. All points on a contour line are of the same elevation.
iii. Widely spaced contour lines indicate gentle slope.
iv. Equally spaced contour lines indicate uniform slope.
v. Contour lines cross ridge at right angles.
vi. The SI units of contour interval are meters.
vii. No two contour lines can meet or cross each other
viii. All contour lines must close either within the map boundary or outside.
ix. Closed contour lines with higher elevation towards the centre indicate hills.
x. Closed contour lines with reducing levels towards the centre indicate pond or other depression.
xi. Contour lines of ridge show higher elevation within the loop of the contours.
xii. Contour lines of valley show reducing elevation within the loop of the contours and they cross a valley at right angles.
Terms used in Contouring
1. Contour Line: A Contour line is an imaginary outline of the terrain obtained by joining its points of equal elevation. In our example of the cone, each circle is a contour line joining points of same level.
2. Contour Interval (Cl): Contour interval is the difference between the levels of consecutive contour lines on a map. The contour interval is a constant in a given map. In our example, the contour interval is 100m.
3. Horizontal Equivalent (HE): Horizontal equivalent is the horizontal distance between two consecutive contour lines measured to the scale of the map.
4. Gradient: Gradient represents the ascending o of the terrain between two consecutive
contour lines. The slope or gradient is usually stated in the format 1 in S, where 1 represents the vertical component of the slope and S its corresponding horizontal component measured in the same unit.
Uses of Contours
1. The nature of ground surface of a region can be known.
2. Contour helps in locating proper sites for bridges, dams, reservoirs etc.
3. Capacity of a reservoir can be calculated with the help of contour map.
4. The quantity of cutting and filling can be determined from contour maps.
5. Through contours, routes for roads, railways, canals etc. can be traced.
6. Contours provide general appearance of the surface by height and nature.
Methods of contouring
1. Direct method: Direct method consists in finding vertical and horizontal controls of the points which lie on the selected contour line. The contour to be plotted is actually traced on the ground. Points which happen to fall on a desired contour are only surveyed, plotted and finally joined to obtain the particular contour. This method is slow and tedious and thus used for large scale maps, small contour interval and at high degree of precision. Direct method of contouring can be employed using Level and Staff.
2. Indirect Method: In this method, levels are taken at some selected points and their levels are reduced. Thus in this method horizontal control is established first and then the levels of those points found.
Methods used in selecting points
For selecting points anyone of the following methods may be used:-
i. Grid Squares Method.
ii. Cross Section Method.
iii. Radial Line Method.
1. Grid square method: in this method the area is divided into a number of squares and all grid points are marked. Commonly used size of square varies from 5 m x 5 m to 20m x20m. Levels of all grid points are established by leveling. Then grid square is plotted on the drawing sheet. Reduced levels of grid points marked and contour lines are drawn by interpolation.
2. Cross-section Method: In this method cross-sectional points are taken at regular interval. By levelling the reduced level of all those points are established. The points are marked on the drawing their reduced levels (RL) are marked and contour lines interpolated. The spacing of cross-section depends upon the nature of the ground, scale of the map and the contour interval required. It varies from 20 m to 100 m. Closer intervals are required if ground level varies abruptly. The cross- sectional line need not be always being at right angles to the main line. This method is ideally suited for road and railway projects.
3. Radial Line Method: In this method several radial lines are taken from a point in the area. The direction of each line is noted. On these lines at selected distances, points are marked and levels determined. This method is ideally suited for hilly areas. in this survey, theodolite with tacheomeiry facility is commonly used.
GUIDING QUESTIONS
1. Why the field Sketches do not drawn to scale?
2. Describe six (6) conditions/principles of a good booking techniques.
3. Describe six limitations of using survey on geographical studies.
4. Briefly, give out five attributes of a good site to be conducted any successfully chain survey.
5. Outline five things to consider during the preparation for chain survey.
6. Describe any four (4) components of a good field notebook.
7. Describe six (6) ways to avoid errors in chain survey.
8. Why do we need for geographical survey? Six points.
