Methods of plane table survey

The four methods are:

1. Radiation - rays are drawn from one station to the points to be plotted and distances are measured and scaled along them.
2. Intersection (triangulation) - a point is fixed by drawing rays to it from two stations at the ends of a plotted base line.
3. Traversing - the table is set up at successive stations of a traverse; each new station is plotted by radiation and the table is oriented by back-sighting.
4. Resection - the table station itself is located by drawing rays back from known plotted points to it (two-point and three-point problems).

Radiation method

The table is set up at one station O, from which all the points to be plotted are visible.

1. Level, centre and fix the sheet; mark the point o on the sheet for O.
2. Pivot the alidade on o and sight each station A, B, C, D in turn; draw a ray towards each.
3. Measure the distances OA, OB, OC, OD with a tape and mark them to scale along the rays to get a, b, c, d.
4. Join a, b, c, d to obtain the plan.

          b               c
           \             /
            \    ray    /
   a ------- o (table) -------- d

Suitability: small areas with open ground, where all points can be seen from one station and distances can be measured easily. It is the base of detailing by plane table.

Intersection method

Two stations A and B (the ends of a base line) are chosen so that the point P to be located is visible from both.

1. Set the table at A, orient it by compass, and plot a. Sight B and draw the ray; measure AB and plot b to scale.
2. Sight P from a and draw the ray aP (also rays to the other points).
3. Shift to B, centre b over B, and orient the table by back-sighting A along the line ba.
4. Sight P again from b and draw the ray bP. The intersection of the two rays is the plotted point p.

                 p
               / |  \
             /   |    \
           /     |      \
         a ------------- b
       (stn A)  base   (stn B)

Suitability / advantages:

• Used for locating inaccessible points (across rivers, hills, buildings, a far-away church spire, etc.) where distance cannot be measured.
• Only one linear measurement (the base line) is needed; many points can be fixed quickly.
• A third ray from another station checks the plotted point.
• Good intersection angle (about 60 to 120 degrees) is needed for accuracy.