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Fig. 1.2. The vertical direction is defined as the direction of the force of gravity

                      1.2 Measuring Distances and Angles
                      Horizontal distance is measured along a level surface. At every point along that
                  length, the line tangent to the level surface is horizontal. It can be measured by tape
                  or Electronic Distance Measurement (EDM). A true horizontal distance is actually
                  curved, like the surface of the earth.
                      A vertical distance is measured along the direction of gravity and is equivalent
                  to a difference in height between  two points.  When  the height  is measured with
                  reference to a given level surface, like mean sea level, it is called an elevation. An
                  instrument called level, which  is used to  observe the rod at different points, can
                  measure  elevation.  The  relative  vertical  position  of  several  points  separated  by
                  long distances can be determined by a continuous series of level rod observations.
                  This procedure is called leveling.
























                           Fig. 1.3. A true horizontal distance is actually curved, like the surface of the earth


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