Plane-table surveying

A surveying method in which a sheet of paper is attached to a flat plate attached to a tripod, and a ruler with a collimation line called an alidade, which determines the direction of the observation line, is used to draw direction lines to each target on-site, thereby determining the location of the target point. It is mainly used to draw schematic plane figures and topographical maps on-site, and before the practical application of photogrammetry, most topographical maps were made by plane table surveying. Some alidades are equipped with telescopes so that distant targets can be clearly identified, and also use a magnetic needle to determine direction, a tape measure to measure distance, or a tacheometer (distance meter). Plane table surveying can be done with simple tools and is relatively easy to operate, so it is still used today for surveying residential land and other simple, small-scale surveying and drawing.

For plane table surveying, two or more known root points must be laid out on the map paper. A plane table is placed on these two points, and the plane table is first rotated horizontally so that the direction lines of the other root points on the map paper and on-site coincide. This operation is called orientation, and to prevent errors, three or more root points are used for orientation. Once orientation is complete, direction lines are drawn to each of the other targets, and the target's position is determined on the map paper as the intersection of the direction lines from two or more observation points. This is called forward intersection. Alternatively, a plane table is placed at a point whose position has not yet been determined, and the position of the observation point is determined by matching the direction angles to three points whose positions are known with the on-site position, which is called back intersection. These intersections are repeated to determine the positions of the points one by one, and the map is completed on the map paper by connecting these points.

[Yukio Ozaki November 18, 2016]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

三脚を取り付けた平板上に図紙を張り付け、アリダードalidadeという観測方向線の向きを求める視準線のついた定規で各目標への方向線を現地で引き、目標点の位置を求めていく測量法。主として平面図形や地形図を現地で図解的に描くのに用いられ、写真測量の実用化以前は地形図もほとんど平板測量によりつくられていた。アリダードのなかには遠方の目標が明瞭(めいりょう)に識別できるように望遠鏡のついたものもあり、また方位を決めるための磁針や、距離を測るための巻尺、あるいはタキオメーターtacheometer（距離計）も用いられる。平板測量は簡単な器具だけで行えるうえに、操作も比較的容易なので、宅地の測量その他の簡単で小規模な測量や図面作りには、現在も、用いられている。

　平板測量には2点以上の既知の図根点を図紙上に展開しておかねばならない。この2点に平板を据え、まず他の図根点の図紙上と現地での方向線とが一致するように平板を水平に回転する。この操作を標定といい、誤差を防ぐためには3点以上の図根点を使って標定する。標定が終わると他の各目標への方向線を引き、2点以上の観測点からの方向線の交点として目標の位置を図紙上に定める。これを前方交会法という。また位置が未決定の地点に平板を据え、位置が既知である3個の点への方向角を現地とあわせながら観測点の位置を決めるのを後方交会法という。これらの交会法を繰り返して順次点の位置を決定し、これらの点を結んで図紙上に地図を完成していく。

［尾崎幸男　2016年11月18日］

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