Plane table principles; orientation by back sighting

Principles of plane table surveying

Principle: the table is set up so that the rays drawn on the sheet from a station point towards the objects are parallel to the corresponding lines on the ground. The plotted figure is then similar to the ground figure. This needs three things at every station:

1. Levelling - the board is kept horizontal (spirit level on the board).
2. Centring - the point on the sheet is kept vertically above the ground station (plumbing fork and plumb bob).
3. Orientation - the board is turned so that all lines on the sheet stay parallel to their ground lines (by magnetic needle or by back-sighting a previously plotted line).

Orientation by back-sighting

This is the more reliable method, because it does not depend on the magnetic needle, which is affected by local attraction.

Condition: the table is at station B; the line ba (from the previous station A) is already plotted.

1. Level the table and centre b over station B.
2. Place the alidade along the line ba.
3. Turn the board until the ranging rod at A is bisected by the line of sight.
4. Clamp the board. The table is now oriented, since ba is parallel to BA.
5. Check by sighting another known point; the ray should pass through its plotted position.

   SHEET (table at B)             GROUND
   a o--------o b                 A o ........ o B
     \_ alidade on ba             (rod at A seen
       along the line)             along the line)

The board is turned about its vertical axis, with the alidade kept on the line ba, until the rod at A is bisected.