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Correction due to curvature and refraction AND combined correction in surveying

In geodetic surveying, error due to curvature and refraction is taken into action when the area is greater than 256 km²

 Correction due to curvature (Cc)
           Error due to curvature is taken into action because during leveling with theodolite  or Autolevel the horizontal line and level line do not coincide. Level line is curved line parallel to the earth surface and horizontal line is straight line.

             this correction is given by Cc =0.07849d² OR 0.0785d²
                                                                here, d is the linear distance (in km)
  Correction due to refraction (Cr)
           Error due to refraction is taken into action due to the changing of medium of light either from a denser medium to lightier medium or viceversa.
                   it is 1/7 of correction due to curvature 
                                             1/7x(0.785d²)
                                                  it is equals to 0.112d²
                                                         
  Combined correction due to curvature and refraction (C)
             it is the sum of both the corrections (correction due to curvature and correction due to refraction).
                                          C=Cc+Cr
                      it is 6/7 of correction due to curvature
                               C= 6/7x(0.785d²)
                                  = -0.6728d²
            NOTE- d is in kilometres and the answer obtained is in metres.

                            THANKS FOR YOUR KIND VISIT

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