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A  horizontal  angle  is  measured  in  a  plane  that  is  horizontal  at  the  point  of
                  measurement. When horizontal angle is measured between points, which do not lie
                  directly  in  the  plane,  it  is  measured  between  the  perpendiculars  extended  to  the
                  plane from those points.
                      A  vertical  angle  is  measured  in  a  plane  that  is  vertical  at  the  point  of
                  observation or measurement. Horizontal and vertical angles are measured with an
                  instrument called a transit or theodolite.

                      1.3 Types of Surveying
                      There are two types of surveying: these are
                      1. Plane surveying
                      As mentioned earlier that most surveying  measurements are carried out as  if
                  the surface of the earth were perfectly flat. The method of surveying based on this
                  assumption is called plane surveying. In plane surveying, it is neglect the curvature
                  of the earth, and it is used the principles of plane geometry and plane trigonometry
                  to compute the result of our surveys.
                      The use of plane surveying methods simplifies the work of surveyor. Within a
                  distance of 20 km, the effect of earth’s curvature on our measurement is so small
                  that  we  can  hardly  measure  it.  In  other  words,  a  horizontal  distance  measured
                  between  two  points  along  a  truly  level  line  is,  for  practical  purposes,  the  same
                  distances measured along the straight chord connecting the two points.
                      In  plane  surveying  horizontal  lines  are  assumed  to  be  straight  line  and  all
                  vertical lines are parallel.


























                       Fig. 1.4. In plane surveying, the curvature of the earth is neglected, and vertical distances
                                           are measured with reference of a flat plane


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