Plane Table Surveying is a graphical method of survey in which the field observations and plotting are done simultaneously. It is simple and cheaper than theodolite survey. It is most suitable for small scale maps.
The plan is drawn by the surveyor in the field, while the area to be surveyed is before his eyes. Therefore, there is no possibility of omitting the necessary measurements.
Equipments and Accessories for Plane Tabling
The following instruments are used in plane table surveying.
- Plane Table
- Trough Compass
- Spirit level
- U-Fork with Plumb bob
- Water proof cover
- Drawing paper
- Drawing accessories
- Plane Table: The drawing board for plane tabling is made from well-seasoned wood with its upper surface exactly plane.
- It is normally rectangular in shape with size 75 cm x 60 cm
- It is mounted on a tripod and clamps are provided to fix it in any direction. The table can revolved about its vertical axis and can be clamped in any position, when necessary.
1) Plane Table
- The plane table is mounted on a tripod
- The tripod is generally of open frame type, combined rigidity with lightness. The tripod may be made to fold for convenience of transportation.
- Tripod is provided with three foot screws at its top for leveling of the plane table.
- The alidade is useful for establishing a line of sight.
- Two Types of alidade are used.
- Simple alidade
- Telescopic alidade
- Simple alidade:
- It is used for ordinary work
- It is generally consists of a gun metal or wooden rule with two vertical vanes at the ends.
- The eye-vane is provided with a narrow slit while the object vane is open and carries a horse hair. Both the slits, thus provide a definite line of sight which can be made to pass through the object to be sighted
- To draw the rays, one of the edge of alidade is beveled and this perfectly smooth working edge is known as the fiducially edge.
- The fiducial edge is graduated to facilitate the plotting of distances to a scale.
- Telescopic alidade: The telescopic alidade is used when it is required to take inclined sights.
- It essentially consists of a small telescope with a level tube and graduated arc mounted on horizontal axis.
- It gives higher accuracy and more range of sights.
1) Trough Compass
- The trough compass is required for drawing the line showing magnetic meridian on the paper. It is used to orient the table to the magnetic meridian.
- When the freely suspended needle shows 00 at each end, a line is drawn on the drawing paper which represents the magnetic north.
2) Spirit Level
- A Spirit Level is used for ascertaining If the table is properly level.
- The Table is leveled by placing the level on the board in two positions at right angles and getting the bubble central in both positions.
3) U-Fork With Plumb bob
- U-fork with plumb bob is used for centering the table over the point or station occupied by the plane table when the plotted position of that point is already on the sheet.
- Also, in the beginning of the work, it is used for transferring the ground point on the sheet.
4) Water Proof Cover
- An umbrella is used to protect the drawing paper from rain.
5) Drawing Paper
- Drawing paper is used for plotting the ground details.
Advantages and Disadvantages of Plane Table Surveying
- The plan is drawn by the surveyor himself while the area to be surveyed is before his eyes. Therefore, there is no possibility of omitting the necessary measurements.
- The surveyor Can compare the plotted work with the actual features of the area.
- It is simple and cheaper than the theodolite survey.
- It is most suitable for small scale maps.
- No great skill is required to produce a satisfactory map and work may be entrusted to a subordinate.
- It is useful in magnetic areas where compass may not be used.
- The mistakes in writing field books are eliminated.
- It is not intended for very accurate work.
- It is not suitable in monsoon.
- It is essentially a tropical instrument.
- Due to heaviness, it is inconvenient to transport.
- Since there are so many accessories, there is likelihood of them being lost.
Principle Of Plane Table Survey
- Principle: “All the rays drawn through various details should pass through the survey station.”
- The Position of plane table at each station must be identical, i.e. at each survey station the table must be oriented in the direction of magnetic north.
- Fix the table on the tripod and level it. Transfer the survey station on the drawing paper as “o”.
- Now, place the alidade on the paper such that zero mark of alidade coincide with point “o” on the paper. This process is called pivoting.
- Pivoting the alidade about “O” rotate it to sight “A” on the ground. Draw ray OA.
- Measure distance OA on the ground and plot it along the ray to get a.
- Normally, points O, A, B, C…. etc. represents the points on the ground while the points o, a, b, c… etc. represents the points on the drawing paper.
Method Of Setting Up The Plane Table
- Three processes are involved in setting up the plane table over the station.
- The Table should be set up at convenient height for working on the board, say about 1 m. The legs of Tripod should be spread well apart and firmly into the ground.
- The table should be so placed over the station on the ground that the point plotted on the sheet corresponding to the station occupied should be exactly over the station on the ground. The operation is known as centering the plane table. It is done by U-fork and plumb bob.
- For leveling the table ordinary spirit level may be used. The table is leveled by placing the level on the board in two positions at right angles and getting the bubble central in both directions.
- The Process by which the positions occupied by the board at various survey stations are kept parallel is known as the orientation. Thus, when a plane table is properly oriented, the lines on the board are parallel to the lines on ground which they represent.
There are two methods of orientation:
- By magnetic needle
- By back sighting
1) By magnetic needle:
- In this method, the magnetic north is drawn on paper at a particular station.
- At the next station, the trough compass is placed along the line of magnetic north and the table is turned in such a way that the ends of magnetic needle are opposite to zeros of the scale.
- The board is then fixed in position by clamps. This method is inaccurate in the since that the results are likely to be affected by the local attraction.
2) By back sighting:
- A= First survey station
- B= Second survey station
- Suppose a line is drawn from station A on paper as ab, representing line AB on ground
- The table is turned till the line of sight bisects the ranging rod at A. The board is then clamped in this position.
- This method is better than the previous one and it gives perfect orientation.
Methods Of Plane Tabling
There are four distinct methods of plane tabling:
- Method of Radiation
- Method of Intersection
- Method of Traversing
- Method of Resection
1) Radiation Method
- In the radiation method of plane table surveying, the direction of the objects or points to be located are obtained by drawing radial lines along fiducial edge of alidade after getting the objects or points bisected along the line of sight of the alidade. The horizontal distances are then measured and scaled off on the corresponding radial lines to mark their positions on the drawing.
2) Method Of Intersection
- In intersection method of plane table surveying, the objects or points to be located are obtained at the point of intersection of radial lines drawn from two different stations.
- Let us consider plane table stations Z and Y from which details got plotted by method of intersection. In this, first the plane table is set over the station Z, clamped and its position is plotted on paper as z. If the position of the station is not already plotted, then the station Y is bisected and plotted by method of radiation. Now, with alidade pivoted at z, salient object points on the surface of earth such as 1, 2, 3 etc of a building, 4 a telephone pillar, 5 a tree etc around the plane table got bisected and radial lines are drawn showing their directions. The table is then shifted to the station Y and get it set and after orienting by back sighting to station Z get it clamped. With alidade pivoted at y, same objects i.e., 1, 2, 3, 4, 5 etc. get bisected and rays are drawn. The intersection of the respective rays provides the plotted positions of the respective details. The plotted positions of 1,2, 3 are then joined to represent the building corner. Telephone pillar and tree are represented by their conventional symbols.
- In this method, the plotting of plane table stations are to be carried out accurately. Checking is important and thus done by taking third sight from another station.
- The intersection method is suitable when distances of objects are large or cannot be measured properly. Thus, this method is preferred in small scale survey and for mountainous regions.
3) Method Of Traversing
- This method of plane table surveying is used to plot a traverse in cases stations have not been previously plotted by some other methods. In this method, traverse stations are first selected. The stations are plotted by method of radiation by taking back sight on the preceding station and a fore sight to the following station. Here distances are generally measured by tachometric method and surveying work has to be performed with great care.
- Let us consider the stations P, Q, R and S which are to plotted by method of traversing. Stations are to be chosen in such a way that adjoining stations are visible. First, the plane table is to be set at 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. With the alidade pivoted at p, draw the rays to Q and S. Distances PQ and PS are measured and plotted on the respective rays, pq and ps respectively.
- 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 qr. In this way, plane table is shifted to stations R and S and corresponding rays are drawn to obtain the plotting of the traverse pqrs.
- To check the accuracy of the plane table traverse, a few check lines are taken by sighting back to some preceding station. In this example, a check line rp is drawn from the station R to P when the plane table is occupying the station R.
- If the traverse is correct, the check line rp would pass through p, the plotted position of station P.
- Likewise, a check line sq is drawn from S to Q. In case there is no suitable preceding station visible, any well-defined point, such as a corner of a building or a tree, which has been previously plotted, can be used for checking.
Error of closure
- If the traverse to be plotted is a closed traverse, the foresight from the terminating station should pass through the first station.
- Otherwise the amount by which plotted position of the first station on the foresight fails to close is designated as the error of closure.
- It is adjusted graphically, if the error is within permissible limits, before any further plotting works are done.
4) Method of Resection
- Resection is the process of determining the plotted position of the station occupied by the plane table, by means of sights taken towards known points, locations of which have been plotted.
- There are four methods of resection.
- By Compass
- By back sighting
- By two point problem
- By three point problem
- This method is used only for small scale or rough mapping.
- Let A and B be two visible stations which have been plotted on the sheet as a and b. Let C be the instrument station to be located on the plan.
- Set the table at C and orient it with compass. Clamp the table.
- Pivoting the alidade about a, draw a ray towards A, as Similarly, pivoting the alidade about b, draw a ray towards B, as bb’, The intersection of aa’ and bb’ will give point c on the paper.
Points to be kept in mind in plane tabling
The following points should be kept in mind while doing plane table survey.
- Ground points shall be marked as A, B, C… etc. and plan. Points (on Paper) shall be marked as a, b, c etc.
- The rays from survey stations to the objects shall be drawn by dashed line.
- The alidade should be properly pivoted while sighting the objects
- The first survey station and the scale of the map shall be so chosen that the entire area can be plotted on the paper.
- While establishing magnetic north on the paper using trough compass, things causing local attraction shall be kept away of the table.
- The Plane table should be clamped after centering and leveling. The table should be rotated at the time of orientation.
Error In Plane Tabling
- The various sources of error may be classified as :
- Instrumental errors
- Errors in manipulation and sighting
- Errors in plotting
- The surface of drawing board is not plane
- The edge of alidade is not straight.
- The object vane and sight vane are not perpendicular to the alidade.
- The edge of alidade is not is not parallel to the line of sight.
- The fixing clamp is not proper.
Errors In Manipulation And Sighting
- Defective Leveling
- Defective sighting
- Defective orientation
- Defective centering
- Movement of Board between sights
Errors in Plotting
- Defective scale of map
- Wrongly intersecting the rays drawn from two different stations.