Plane Table Surveying by Intersection method, All equipment used in plane table Survey.

Plane Table Surveying

* Plane table consists of a board made up of a well seasoned wood mounted on a light tripod with

suitable mounting and clamping devices. The table is to be levelled by adjusting tripod.

* Accessories for plane table required are:

1. Alidade or sight rule—plain or telescopic

2. Plumbing fork with plumb bob

3. Spirit level

4. Trough compass

5. Drawing sheet and accessories for drawing.

* Working operations are centring, levelling and orientation. Orientation may be achieved by using

trough compass, back sighting or by solving two-point or three-point problems.

* Method of plane tabling are radiation, intersection, traversing and resection.

* The problem of finding plotted position of the station point occupied by plane table with the help of

two well defined points, the plotted position of which are known, is called two-point problems.

Similarly, three-point problem is the problem of locating station point with the help of three

plotted positions is known as three-point problem. For this tracing paper method, graphical

method or trial and error (Lehman’s) method may be employed.

* Strength of fix: It is defined as the accuracy with which the point sought p can be fixed with respect

to the plotted positions of the three points.

Strength of fix is excellent when:

1. Point P is close to the orthocentre of the great circle

2. The middle station is much nearer

3. Of the two angles subtended at P, one is small and the other is large

The strength of fix is poor when:

1. P is near the circumference of the great circle

2. Both angles subtended at P are small.

• Main advantage of plane table survey is there is no chance of omitting any measurement and plotted

plan can be checked in the fieldwork itself. Main disadvantage is that it cannot be conducted in

wet weather and on rainy days.