Setting Up The Plane Table
The survey details of ground features are normally obtained by sighting the object through sight vane or telescope of the alidade and drawing a radial line. At this line the object station's distance is marked by scale. The object's position on map could also be plotted through sighting the object from two various plane table stations and locating the object through intersection of radial lines.
Since survey is carried out through sighting, the table should be set up to give drawing board at convenient height about 1m above ground. The table should also be levelled and stable. The legs of the table are spread well apart to give stability and adjusted to provide the table in horizontal plane through means of the levelling screws along with reference to a level tube. A table is centered over the station accurately along with the help of a plumbing fork or U frame. The upper leg of the fork coinciding within the point on paper although a plumb bob is hung from the other leg directly over the peg.
The other necessary step in setting up the table is its orientation. It ensures in which the table is kept parallel to its original direction as it is moved from station to station. This step is essential to make the lines on the map parallel to the lines on the ground represented through them. This is achieved either through the use of a compass or through the process of back sighting. By using the compass, a line is drawn on map at first station within the direction of magnetic meridian. While the table is needed to be set up at a new station, the compass is placed within the direction of already drawn magnetic meridian and table rotated to bring the needle ends on zero reading of scale. The table clamped inside this position ensures accurate table orientation if no local attraction is present at station.
A more reliable and preferred method of orientation is through back sighting. While the table is needed to be shifted from instrument station O1 to O2, the O2 is sighted through placing a ranging rod at O2 and line of sight along O1 O2. A line O1 O2 is then drawn on map. While the instrument is shifted to O2, the alidade is remaining along O1 O2 and ranging rod placed at O1 is sighted from O2. The board is turn until the line of sight bisects the ranging rod at O1.