plane table

A portable drawing board mounted horizontally on a tripod and used for surveying sites and earthworks by constructing a map directly onto paper or film attached to the board. With the board positioned directly above a survey station whose position is marked on the paper, an alidade is used to sight onto a target point. A line can then be drawn along the straight edge of the alidade and the distance from the survey station to the target point scaled off to fix its position. If measurement is not possible, then two survey stations a set distance apart can be used to triangulation the target point as the intersection of the two alidade-defined lines. Plane-table surveys have the distinct advantage that the surveyor can fill in detail by eye as the work progresses.