PLANE-TABLE. An instrument used to fill in the details of topographical work (see Suit 11.v1No), and, on account of the ease and rapidity with which it can be operated. popular where only a moderate a pprOaell '10 accuracy is re The planc-table consists essentially of a mounted draw•ing-board bearing an alidade or ruler surmounted by a telescope whose line of sight is parallel in azimuth to the ruler. In orienting the table a declinatcir, or compass box. and a plumbing arm serve to place a given point on the table over that on the ground which it represents. and to valise a line of the drawing on a paper mounted on the board to lie in o• parallel to the vert lea] plan' tlr•ouglh its counter part on the ground. After the paper has been attached to the table it is then leveled and ori ented as already deseribed. The observer then sights :-.neeessively at other different objects in the area under survey, revolving the tele scope and ruler in azimuth and drawing corre sponding lines along the edge of the ruler at each position. All of these pass through
the given point and give the anerular direction of the different objects in the field. The plane table is then taken to a second station located at some place previously sighted and whose distanee from the first can be measured by stadia, chain. or tape. and is there set up and oriented as before. The distance from the first station to the second is set otl' at the desired scale on the line connecting the two points, and the telescope is directed at the same objects as were sighted at from the first station. as well as others not visible from the first point, and new lines are ruled. The instrument is then taken to other points which are given by the intersection of the various lines ruled on the paper. and the positions of the various ohjeets in the area under survey are determined. In this way it is possible to fill in the detail work of a topographical sur vey with great rapidity and sufficient exactness for most map work. See SURVEYING.