The setting up of the plane table includes three operations viz: 1. Levelling 2. Orientation 3. Centering.
Operation # 1. Levelling:
Place the table over the station-point and spread out legs to make it level and at a convenient height, preferably not above the elbow.
Then oriented and centre the table approximately as explained in the following two operations and complete the final levelling. The levelling is done by means of levelling-screws or the ball and socket arrangement or simply by adjusting the legs if there is no levelling head, with reference to a circular level or a level tube placed in two positions at right angles to each other.
Operation # 2. Orientation:
The table is said to be oriented when it is so placed that all lines on the paper are parallel to the corresponding lines on the ground. This is necessary when more than one stations have to be occupied by the table as otherwise the board would not be kept parallel to itself at the various stations, and the effect would be similar to using a different meridian at each station.
ADVERTISEMENTS:
There are two methods of orienting the table viz:
(i) By the use of a magnetic needle and
(ii) By back-sighting.
Orientation is also possible by solving two point problem and their-point problem.
ADVERTISEMENTS:
(i) Orientation by the Magnetic Needle:
To orient the table at any station other than the first, place the trough compass along the line representing the meridian drawn already on the paper at the first station and then turn the table until the ends of the needle are opposite the zeros of the scale or points towards north-south direction, then clamp the table in position. This is a quick method of orientation but is unsuitable in areas affected by local attraction.
(ii) Orientation by Back-Sighting:
Suppose the table is to be set over a station B, represented on the paper by a point b which has been plotted by means of a line ab drawn from a previous station A (Fig. 6.4), the orientation consists in bringing ba on the paper over BA on the ground.
ADVERTISEMENTS:
The edge of the alidade is therefore placed along ba, and the table is turned until the point A is bisected by the line of sight. Then clamp the table in position. This is more accurate of the two methods and should be preferred.
Operation # 3. Centering:
The point on the paper representing the station being occupied should be vertically over the point on the ground. This operation is known as centering of the table. This may be done by means of plumbing fork or U-frame. To do this, place the pointed end of the fork touching with the point on the paper and suspend a plum-bob from the other end.
Shift the table bodily until the plumb bob hangs vertically over the centre of the station peg. If plumb-bob is not available, centering may be done by dropping a piece of stone from a point on the underside of the board which is directly under the point on the paper. Accurate centering is necessary for large scale maps and when the scale is small, accurate centering becomes a needless task.
ADVERTISEMENTS:
Orientation and centering are inter-related because when one is done, the other gets disturbed, therefore, both of them have to be done simultaneously.