Defining earth radius

The earth radius is the radius of curvature of the earth at the location of the project. This parameter is optional since aerial photographs usually use a large scale (for example, 1:8,000) and the error due to the earth's radius is negligible. You only need earth radius correction for images with a scale over 1:20,000.

The earth radius equation is:

radius = a(1-e2)/(1-e2sin2phi)3/2