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an angle-graphic survey of terrain, performed with the simplest instruments—a plane table with paper attached to it, a compass, and a sight (aiming) rule. In an exploratory survey a map—although not highly precise—is produced directly on the locale. Distances are measured by steps, using a pedometer, by a motor-vehicle speedometer, by a counter on a bicycle, or simply by eye using a table of object visibility. The directions to objects are traced at each surveying point, laying the plane table horizontally and orienting it by means of a compass; this is done by holding the sight rule to the image of the given point, setting the upper edge of the rule on the marked object, and tracing a line along the lower part of the rule adjacent to the point.
To make a survey net, a path is laid out along roads, lines of communication, and well-expressed contours. Survey paths for assessing the precision of the map are closed. The objects (the “situation”) are also surveyed by the method of cross bearings in open terrain, by the method of perpendiculars to survey lines near the path, and by sketching in a range (roads, borders, and similar lines that intersect the path). In making an exploratory survey, the direction from the first point of the path to the second point is traced, and bearings are taken for all necessary objects lying to the side. The distance to the second point is measured and is then laid out in scale on the plane table, and bearings are taken for the same objects from the second point, producing their representation at the intersections of the lines. Using these and other surveying methods, all essential objects within the field of visibility are recorded, and the map is drawn. If a survey of the terrain is also intended during an exploratory survey, heights, saddles, and divides are entered on the map in advance, and the angles of inclination of the terrain are determined by a gradient recorder, or its relative elevations are determined by an aneroid barometer. Then the terrain is drawn with contour lines concurrently with the survey. Exploratory surveys are performed along routes both when surveying a strip of terrain and when surveying large sectors. In the latter case the survey routes should comprise the entire sector.
Exploratory surveys may be performed using a topographical map. In this case the “skeleton” of the map is drawn on the paper (plane table) with the basic contours of the terrain in the scale adopted for the survey, and the methods of exploratory surveying are used to supplement this skeleton with the necessary terrain details.
Exploratory surveys are used to produce maps of small sectors or reconnaissance routes (in geological and other field research and in military affairs when scouting the terrain under combat conditions) on a scale of 1:25,000 and larger or for the purpose of making corrections and additions to a topographical map. With the development of aerial photography, the exploratory survey lost its importance as a fast method of producing maps of unexplored regions. In school, exploratory surveying is studied in order to give an idea of the basic methods of the topographical survey of a locality.
REFERENCESNikitin, N. D. Glazomernaia s”emka. Moscow, 1960.
Voennaia topografiia, 2nd ed. Moscow, 1969.
Shuvalov, Ia. A. Glazomernaia s’emka, 2nd ed. Moscow, 1963.
M. M. TIKHOMIROVA