Topographic maps and sketch mapping . om the point of set-up. The alidade is now pivoted about the point c for example,and sighted at the corresponding object, in the field, C,which c represents on the map. A ray drawn backwardfrom this point intersecting the previous ray will locate thepoint of set-up / on the first ray, / b. Fig. 60. The pointof set-up is now known and the board oriented, and themapper may proceed to locate details. The three-point method, mentioned above, gives evenmore freedom to the topographer than the resection method,as it is not necessary to select the next set-up bef
Topographic maps and sketch mapping . om the point of set-up. The alidade is now pivoted about the point c for example,and sighted at the corresponding object, in the field, C,which c represents on the map. A ray drawn backwardfrom this point intersecting the previous ray will locate thepoint of set-up / on the first ray, / b. Fig. 60. The pointof set-up is now known and the board oriented, and themapper may proceed to locate details. The three-point method, mentioned above, gives evenmore freedom to the topographer than the resection method,as it is not necessary to select the next set-up before leaving THREE POINT METHOD 109 a station. The mapper can pick up his board and move overthe area until he finds a desirable position for the board can be set up in this new position, oriented and thepoint of set-up plotted provided at least three objects alreadyplotted on the map are visible in the field. Several methods are used for solving the three-pointproblem. One of the simplest of these is known as the trac-. Fig. 61.—The Three Point Method. ing paper method, Fig. 61. A piece of tracing paper is placedon the board and held down with tacks. A point G is selectedin the center of the paper and rays are drawn from this pointin the direction of the three above-mentioned objects in thefield, C, D, and E. The paper is now untacked and is movedaround over the map until these three rays pass through thethree corresponding objects c, d, and e, as plotted on the 110 FLAT MAPPING map. It will be found that only one position of the tracingpaper will satisfy this condition and the center point of therays for this position of the paper gives the location of thepoint of set-up on the map. In Fig. 61 the map of Fig. 60is shown faintly by dotted lines while the three rays drawnon the tracing paper from the point of g are shown by solidlines. The position of the tracing paper illustrated showsthe three rays passing through the three points as required,thus locating the
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectcartogr, bookyear1920
