The Plane Table, and its Use.
In making a plane table, discard all the complicated arrangements of sights, protractors, fixed compasses, spirit levels, and levelling screws, each of which has an individual error, which must be found and allowed for before correctness can be attained; while the approximation to perfection gained by their most careful use is almost certain to be vitiated by the contraction of the paper when removed from the table on which it has been stretched.
Take any flat board—Fig. 1 (an artist’s drawing-board) is as good as any—and stretch on it a sheet of drawing or stout cartridge paper: the best levelling apparatus is a wooden hemisphere (Fig. 2), screwed temporarily on to its back, and working in a circular aperture (Fig. 3) in the top of such a stand as is used for the photographic camera or theodolite; there is no necessity that the surface should be truly horizontal; indeed, it is much better that it should be capable of alterations of position, so that objects above or below its horizon may be sighted and mapped at pleasure. Set up a needle (a) in the centre of the drawing-paper, and lay against it a straight-edged ruler (b) with two other needles (c c), set perfectly upright in each end as near as possible to the fiducial edge, as sights; then, choosing some principal object—say a well-marked conical peak, the bluff edge of a precipice, or a deep and narrow cleft in distant mountains—and keeping your ruler pressed against the central needle, bring the others, which serve as sights, in careful alignment with the object, and draw a pencil line along the fiducial edge right across the paper; then, with your prismatic compass (resting on the table for greater steadiness, if there is no iron in it to affect the magnet), take very carefully the bearing of the object, and note it in degrees from north, or zero, say 40°, on the line you have drawn; then lightly sketch the object on the line, estimating its distance from the centre, according to the scale on which you are working, say one or more inches to a mile. Direct the sights to any number of well-marked and recognisable objects, draw lines, and sketch them lightly at their estimated distances. Then have a mark set up as far off as possible, say a mile, or any carefully-measured distance, direct the sights, and draw a line towards it; mark its distance accurately on the paper, and insert the pivot needle there; remove the table to the marked spot, and, with your prismatic compass, set the first line again to 40°; then bring your sights to bear upon the object, draw another line along your straight edge, and the point at which this cuts the first will be the true position of your object; correct your first sketch still lightly, but do not efface anything; sight all the other objects, and sketch them where the second set of lines cut the first. Then choose another station, forming, if possible, an equilateral triangle with the other two; mark its position on the map, remove the proof needle there, carry the table to the spot, set the first line again to 40°, and sight the same objects a third time all round, and the third set of lines crossing the other two will give their true distances from the centre with sufficient accuracy for all ordinary purposes.
A ball and socket joint for adjusting the table to the necessary alterations of level may be made by nailing on beneath it a hemisphere of wood (Fig. 2), working in a circular hole (Fig. 3) in the top of the stand.