To draw the position in plan of the given points by intersection method.
Equipment and Accessories:
Plane table and its accessories (tripod, alidate, trough compass, plumbing fork, spirit level, drawing sheet, cello- tape, pencil, eraser and dusting cloth), chain, tape, ranging rods, pegs, hammer and field-book.
Plane table is a surveying instrument that can be used to prepare a map or plan of an area directly in the field without the direct measurement of any angles. Intersection is one of the methods employed in plane table survey. This method is generally employed for locating the details.
In this method the location of an object is determined by sighting at the object from two plane table stations and drawing the rays. The intersection of these rays will give the position of the object. Therefore in this method it is essential to have at least two plane table stations. The distance between the two plane table stations is measured and plotted on the sheet to some scale. The line joining the two plane table stations is known as the base line. No linear measurement other than that of the base line is made in this method of surveying.
This method is preferred when the distance between the point and the plane table station is either too large or cannot be measured accurately due to some field conditions.
It is required to plot the position of ground points PQRS (shown in figure given below) on the drawing sheet. The line AB is a base line measured on the ground. It is represented by the line ab on the board drawn to scale. The position of the base line AB is chosen such that it is in the middle of the boundary formed by PQRS.
- Set up the plane table over A and orient the plane table by laying the alidade along the drawn line ab and rotate the board until B is sighted from A through the alidade (Now the line ab is aligned with line AB on the ground).
- Pivot the alidade at ‘a’ and sight to the points P, Q, R & S and draw the rays. These rays represent the lines of sight to these features.
- Shift the table to B. Plumb point b on the board over B on the ground.
- By laying the alidade along the drawn line ba rotate the board until A is sighted and clamp the board (Now the line ab is aligned with line AB on the ground)
- Mark the direction of the magnetic meridian on the drawing sheet by means of trough compass.
- Pivot the alidade at ‘b’ and sight to the points P, Q, R & S and draw the rays (The rays from B will intersect those drawn from A, thus establishing the positions p, q, r and s on the board).
- Join the points p, q, r & s on the drawing sheet.
Fig.17 Intersection method – plane table
Observations and Calculations:
- Measure the distance PQ, QR, RS and SP on the ground.
- Scale the distance pq, qr, rs and sp on the drawing sheet.
Compare the ground and plan distances between the stations P, Q, R and S.