Robust Conicoid Fitting in Converting GPS Height

Height difference between GPS height and normal height is called height anomaly. If height anomaly for a point can be estimated, GPS height above the surface of the WGS-84 ellipsoid can be converted into normal height, which is used in engineering applications. A new robust procedure by using the conicoid fitting method (CFM) with the Huber M-estimation was proposed, to convert GPS height to normal height using some survey marks of a GPS control net where both normal height and GPS height are known. The robust CFM was presented on the example of a B-order GPS control net. The accuracy was analyzed and the results are promising. The impact of survey marks distribution and density on fitting accuracy was also discussed. In flat areas, converting GPS height to normal height by using CFM arrives at sub-centimeter level accuracy under some conditions.