A plane table is a small, horizontally mounted mapping board fixed on a tripod, usually 50–60 cm square, with a paper sheet attached for direct plotting. Before starting, the geologist selects two survey points in the field, set at a known distance apart (up to 200 m), and marks them with pegs or flagging tape.
The mapping begins by setting up the table at the first survey point and orienting it with a compass. A mark is made on the paper to represent this point, and an alidade (sighting instrument) is used to sight the second survey point. A line is drawn along the alidade, and since the distance between the two points is known, the second point can be accurately plotted according to scale. Bearings to other features of interest, such as geological, topographic, cultural points, or additional survey markers, are then drawn as radiating lines from the first point. An assistant with a survey pole helps in identifying and labelling these features.
Next, the table is moved to the second survey point and back-sighted to the first point. A second set of bearings is taken to the same features, and where the lines intersect, the exact positions of the features are determined. This process, called triangulation, ensures accuracy and allows the survey to be extended indefinitely by selecting new baseline points.
More advanced methods improve accuracy. A telescopic alidade allows longer and more precise sightings. Tacheometry, using stadia hairs and a survey staff, enables direct measurement of distances and vertical angles, making it possible to plot points without triangulation and even produce contour maps. Modern surveying instruments, such as infrared or laser-based distance measurement devices, can also be incorporated into plane table mapping for greater efficiency.
Author: Roger Marjoribanks
