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Show 3843 Gerdine- D 1864 A Most of this map is dependent for orientation for direction on the magnetic needle; the elevations are dependent on spirit level elevations; run with a wire level ans spirit level. And the horizontal measurements are made with an instrument that we call a telescopic alidade, used on a plane table. The whole map depends on a system of graphic triangulation. You have got to have first a base, then from that original base you expand your triangulation graphically on your plane table sheet. Q What is the base you start from? A They measured out their own base. Frequently we have geodetic or geographic bases we tie to. In this case we didn't have any. I think we tied to one or two, but they were not controlling. Remember, all this survey is dependent for direction on the magnetic needle. Q As to elevation, how are the various elevations which are shown on that determined? A The basic elevation or the controlling elevation is run by spirit level lines; the elevations that control these contours are really referred to that; they are carried off from that basic line by means of vertical angles, stadia distances and vertical angles which determine the elevations. |