I have LIDAR coverage for a route which crosses multiple UTM zones. I want to calculate the length of this route as accurately as possible correcting for projection error, elevation, and curvature of the earth. I have figured out the first two parts but the curvature issue has stumped me. By modeling the Earth in AutoCAD using UTM WGS 84 parameters I've come up with a length multiplier of 1.000457002 to account for curvature but I'm not sure if that is correct, or if that can be multiplied with a 3D length to get the proper answer.
All thoughts are appreciated.