An instrument by which the draught or plan of an estate, etc. may be taken on the spot, while the survey or measuring is going on. It consists of a perfectly flat rectangular board, sufficiently large for the purpose, the centre of which moves freely on a ball and socket attached to the top of three legs, on which the instrument stands; by this means, when the legs are fixed in the ground, the table may be inclined or moved round in any proposed direction. For the purpose of fixing a sheet of paper on the table, there is a frame of wood, which fits exactly round its edges; one side of this frame is graduated into equal parts, and the other side into degrees from the centre of the table; by which means this instrument is made to answer the purpose of a theodolite. To the side of the table is screwed a magnetic needle and compass, to take directions and bearings; and, lastly, there is a brass two foot scale, furnished with two open sights, or else a small telescope, serving as an index. The use of the plane-table is as follows: - Having moistened a sheet of writing or drawing paper, spread it flat on the table, and secure it in this position by pressing down the frame on its edges.
When this paper is dry it will be perfectly smooth, and ready to have drawn on it the plan of the proposed scene. We then begin by setting up the table at any part of the ground that is judged most proper; and having done this, a point is made in some convenient part of the paper to represent the spot where the instrument stands; we are then to fix in that point of the paper, on a leg of the compasses, or a fine steel pin, and apply it to the fiducial edge of the index, moving it round the table close to the pin till some desired point or remarkable object, such as the corner of a field, a tree, a picket, etc. be seen through the sights; from the station point an obscure fine is then to be drawn along the fiducial edge of the index. We then turn the index to another object, and draw a line on the paper towards it. The same process is repeated till as many objects are set as may be deemed necessary for the purpose. We then measure from our station to each of these objects, taking the necessary offsets to corners and bendings in the edges, etc, laying down the measured distances, taken from a proper scale, upon the respective lines on the paper. The table is then to be removed to any one of the objects to which the measuring was made, as a second station.
Here it must be fixed in its original position, turning it about the centre for that purpose, both till the magnetic needle points to the same degree of the compass as at first, and also by laying the fiducial edge of the index along the line between the two stations, and turning the table till the former station can be seen through the sights on the index: it is to be fixed in that position. From this new station repeat the former operations, setting several objects by the edge of the index, and measuring and laying off the distances. In this manner we proceed from one station to another, measuring such lines only as are indispensable, and determining as many as possible by intersecting lines of direction, drawn from different stations. If, before the survey be completed, the paper be full of lines, measurements, etc, recourse must be had to another sheet of paper. Draw a line in any manner through the farthest point of the last station line to which the work can be conveniently laid down; then remove the sheet from the table, and fix a perfectly clean sheet in its place, drawing upon it, in a part the most convenient for the rest of the work, a line to represent that drawn at the end of the work on the former sheet.
Cut or fold the old sheet by this line, and apply the edge so that it may exactly coincide with the corresponding line on the new sheet. While they lie together in this position, produce the last station line of the old sheet upon the new one, and place upon it the remainder of the measurement of that line, beginning where the work ended on the old. In this manner the process may be continued from one sheet to another, till the proposed survey is complete. When the survey is finished, the sheets are all to be fastened together, taking care that the lines in one sheet accurately meet the corresponding lines in another throughout.