The plane table

The plane table is used for constructing a map directly in the field. The paper is fastened to a board, like a drafting board, supported on a tripod with a device for levelling and clamping so that it can be oriented in any desired azimuth. The usual size of the table is 24 by 30 inc. On the map alidade is used to define directions and measure distances. It has a telescope equipped with stadia hairs and is mounted on a horizontal axis resting in Y supports. These are connected to a metal column which rises from the centre of a flat piece of metal about 18 in. long of which both edges are straight and parallel to the plane of the telescope. On this base are two spirit-levels for levelling the table. The telescope has only a vertical motion and a vertical arc. The entire alidade is moved about on the top of the map after the table has been levelled.

The use of the plane table has generally been limited to topographical maps drawn on a small scale. A system of triangulation or a transit and tape traverse may first be carefully surveyed and plotted on a map. Levels also may be run to determine the elevation of triangulation or traverse points which will then be used as a control for plane table survey. In this case the plane table is used only to fill in details and accurate maps on a scale as small as 40 ft. to the inch can be rapidly made.

The most important advantage of the plane table over other topographic methods is that all the sketching is done in the field where topographer can see the form of the ground that be is mapping. He can sketch details at once in their proper position without burdening his memory and without making elaborate notes, for this reason details may be accurately sketched from much smaller number of located points than would be required, for instance, by the transit and the stadia method.

If not practicable to run out system of triangulation or a transit and tape traverse the plane table can be used for making such a survey graphically. In all cases, however, the accuracy of these points located by the plane table is limited by precision of the plotting rather than of the measurements. In the field the map must be protected from distortion by moisture so as to preserve accuracy of the plot. Celluloid sheets are obtainable which can be used even in rainy weather.

The plane table method usually requires more time for field work than the transit and stadia methods. It is also more dependent upon favourable weather. But taking into account both the field and the office work the plane table has proved more economical than the transit and stadia for the work in open country and the results are more reliable. In open country a contour interval of 5 ft. being required, a party of 3 men should survey about 8 acres per day.

Дата добавления: 2015-07-19