Ellipsoid expansion method in establishing independent coordinate system in high-altitude area

The causes of projection length deformation in engineering control networks and the methods to eliminate length deformation were analyzed. The principle of the ellipsoid expansion method and four methods for calculating the change in ellipsoid length radius were detailed. Taking the GNSS control network of a certain project in a high-altitude area as an example, four different methods for calculating the change in ellipsoid length radius, including the average curvature radius method, the Mao You curvature radius method, the plane analysis method, and the generalized differentiation method, were used to establish an independent coordinate system. The changes in ellipsoid length radius, latitude of control points, and ground height calculated by the four methods were compared and analyzed, and verified by the measured distance of the total station. It is believed that the Gaussian plane coordinate accuracy obtained by the four calculation methods is the same. Still, the plane analysis method and the generalized differentiation method are the most rigorous. The conclusion can provide a reference for establishing independent coordinate systems in high-altitude areas.