Methods of Orientation

Commonly the first two methods are used whereas the third method is used occasionally. The third method is described under the article methods of plane tabling by resection.

(a) Orientation Using Trough Compass: the plane table is set on first station and by rough judgement the table is oriented when the survey work starts, such that the plotted position of the area falls in the middle portion of the paper. Then table is clamped and north direction is marked on right hand side top corner of drawing sheet. Trough compass is utilized to identify north direction. At all subsequent stations this orientation is to be maintained. After levelling and centering the table trough compass is kept along with the marked north direction and to get freely suspended magnetic needle centred the table is rotated. The table is clamped after achieving it.

As the local attraction to magnetic needle affects the orientation, this method of orientation is considered rough. This method is used as preliminary orientation and by observing the already plotted points finer tuning is made.

(b) Orientation by Back Sighting: commonly it is the used method in plane table surveying., if table is to be shifted to next station B, After completing surveying from plane table set at A, a line is drawn from the plotted position of station A towards B station. Then distance AB is measured scaled down and plotted position of B station is obtained. Then table is shifted to B station, centred, levelled. Then keeping alidade along BA, A station is sighted and table is clamped. Therefore by back sighting the orientation of the table is achieved. Orientation can be checked by observing already plotted objects.