Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . etermination of an unknown point graphically onthe plane-table is performed by the method of resection,which consists in the occupation of the unknown point withthe plane-table and the sighting from it to the three knownpoints, on which well-defined signals must be erected and the 186 PLA NE- TA BLE TRIA NG ULA TION. positions of which are previously plotted upon the plane-table sheet. The determination of the unknown positionmay be accomplished by several
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . etermination of an unknown point graphically onthe plane-table is performed by the method of resection,which consists in the occupation of the unknown point withthe plane-table and the sighting from it to the three knownpoints, on which well-defined signals must be erected and the 186 PLA NE- TA BLE TRIA NG ULA TION. positions of which are previously plotted upon the plane-table sheet. The determination of the unknown positionmay be accomplished by several methods, the earlier of whichis known as Bessels, from its inventor, though the most sat-isfactory method, and that now almost universally employed,is known as the Hergesheimer or Coast Survey method. Inaddition there is an approximate but rapid and practicalmethod by means of tracing-paper, generally known as thegraphic method, and there are also the less well-known andrarely employed Lehmanns and Nettos methods. 75. Three-point Problem Graphically Solved—Threesimple, practical rules for determining the location of an un- Class 3. Class 3Fig. 59,—Three-point Locations. known point on the plane-table by means of the three-pointproblem are the following (Fig. 59): I. When the new point is on or near the circle passingthrough the other points, the location is uncertain. THREE-POINT PROBLEM. 18/ 2. When the new point is within the triangle formed bythe three points, the point sought is within the triangle oferror, 3. When the new point is without the circle, orient onthe most distant point, then the point sought is always on thesame side of the line from the most distant point as the pointof intersection of the other two lines. The last rule is that most usually called into requisition,and is perhaps the most important in aiding in the quick de-termination of the unknown position. 76. Tracing-paper Solution of the Three-point Prob-lem.—By the use of tracing-paper the three-
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