Knight's Mechanical Encyclopedia, P., Plane-ta′ble.

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19th-Century American

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Plane-ta′ble.

An ancient instrument for taking angles in mapping. The round table a is adapted to receive sheets of paper, and has a marginal, graduated circle. Over the center of the table an alidade [1727] is pivoted, one side of the alidade intersecting the center and reaching to the marginal graduations. The telescope being directed in turn to the different objects to be observed, a line is drawn on the table which represents the direction to the said objects from this station. Another station is then selected, and the distance between the two carefully measured. The telescope is then pointed to the same objects in turn from the new station, and the lines of direction marked on another sheet of paper. The two triangulations being placed on a plotting-table at the proper distance between stations, on the required scale, the lines of direction from each station are prolonged, and their intersections give the angles, as in figure b, which is supposed to be a fort whose angles are all distinguishable objects, and whose sides are straight lines between the said angles.

Wurdemann's plane-table is a geodetical instrument consisting of two main parts: (1.) The motion works on a tripod, including leveling-screws and a horizontal movement, with clamp and tangent, above which is fastened down the drawing-board, which receives the paper. (2.) On this is placed the alidade, consisting of a broad metallic ruler with level, an upright column, near the middle, carrying above a telescope on a horizontal axis, so as to have a vertical movement of about 30° each way from the horizontal position. The telescope itself is accurately adjusted parallel to the ruler's edge. A vertical divided are of 30° each way, with vernier reading to minutes, is commonly added for determining altitudes. The plane-table is used altogether for details of surveys, such as roads, shore lines, borders of forest, houses, etc. It differs from all other instruments in the results of the work, that it produces immediately a detail map in the field, whilst going over the ground, measuring distances, and triangulating within the limits of the sheet on the board according to the scale of reduction required. A telemeter rod or chain is used in connection with it.

1877.

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Knight's Mechanical Encyclopedia
Knight, Ed.

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Plane-ta′ble.

Knight's Mechanical Encyclopedia Knight, Ed.

This text is part of:

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Plane-ta′ble.

Knight's Mechanical Encyclopedia
Knight, Ed.