Working principle of plane table survey

Plane tabling is a graphical surveying method in which fieldwork and plotting are done together on the spot, using a drawing sheet fixed on a board (plane table), an alidade for sighting and a tripod.

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).

Equipment: plane table with tripod, alidade (plain or telescopic), plumbing fork with plumb bob, spirit level, trough compass, drawing sheet, pins, ranging rods, tape or chain.

Working at a station

1. Fix the paper on the board and place the tripod; mount the board and make it roughly level by eye.
2. Level the board with the spirit level placed in two directions at right angles, adjusting the tripod legs.
3. Centre the table over the ground station with the plumbing fork and plumb bob, so the sheet point is vertically above the ground point.
4. Orient the board by trough compass (first station) or by back-sighting a plotted line (later stations).
5. Sighting and plotting: pivot the alidade at the plotted point, sight the object through the sight vane and draw a ray along its fiducial edge. Measure the distance and plot to scale (radiation) or fix the point by two rays (intersection).
6. Shift to the next station, re-centre, re-orient, and repeat. Close the traverse by checking the plotted position of the first station.

Basic geometrical idea

   Ground:  O ----- A          Sheet:   o ----- a
             \                           \
               B                            b
   Triangle OAB is similar to triangle oab: each ray
   on the sheet is parallel to its ground line and
   drawn to a constant scale.