Problem of Piecing Together Between China Mainland-Geoid and Altimetry-Derived Geoid in China Sea Area

There exists the discontinuity in coast area between continental geoid and marine geoid determined from terrestrial grarity data and altimeter data,respectively,which is mainly due to systematic errors in the two kinds of geoids.In this paper,the possible main causes of the errors are analyzed in brief.Considering the reality that there is lack of gravity data in China coast area,an extending method is advanced for piecing the two types of geoids determined by different principles and data sets.The main idea of this method is:①the terrestrial gravity anomaly data and marine altimetry-derived ones are combined as a whole which is a unified 5'×5' gridded data set,and the continental gravity geoid is extended to the sea area using Stokes formula and the data set;② the marine geoid directly determined only from altimeter data,i.e.altimetry-derived geoid,is fitted with the above continental gravity geoid,and the altimetryderived geoid will be corrected using the fitting function.The piecing together is accomplished in this step,and the continental gravity geoid is fixed in the process of piecing together;③the continent-marine gravity geoid after piecing together is then fitted with(quasi)-geoid determined by National GPS leveling network,and the corrected gravity geoid is obtained.Moreover,the systematic errors in the two types of geoid are estimated using the discrepancies between the continent-marine gravity geoid and the altimetry-only-derived geoid at corresponding points.