Plane Table

The plane table is a versatile instrument for surveying, particularly when covering a relatively small area, such as several fields or part of an estate. It consists of a flat wooden board mounted on a tripod stand, and a detached sighting rule or alidade. A compass is needed to orient the table, and this can be attached to one side of the board. In use a sheet of paper is pinned or clamped to the board, a suitable point on the paper chosen, taking account of the expected progression of the survey, and lines of sight drawn on the paper towards the other features to be located on the map. The table is then carried to the next station, the distance measured by a chain, and transferred to scale on to the relevant sight line on the paper. The other positions can then be located by triangulation or by further traverses. In this way a map can be drawn on the paper by a purely graphical method, without the need to measure angles in degrees.

The instrument became popular in the later 16th century with practising surveyors, although the geometers did not really approve of its use, since the graphical method bypassed the need for a theoretical knowledge of geometry and trigonometry. A variety of sophistications could be added, by including scales on the alidade or on the square surround used to clamp the paper on to the board.

Jim Bennett
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