Research On Determining A Precise Quasi-Geoid For The Chengdu Region

With the recent advances in GPS technique, directly transforming GPS~derived ellipsoidal heights to orthometric heights or normal heights in centimeter level has been using. Its implementation can produce direct economic benefit by tremendously reducing laborious field tasks so to increase efficiency in practice. However, the most important step towards this goal is to firstly determinate an accurate geoid (quasi-geoid).Presenly, the precise geoid (quasi-geoid) determination is not only one main research content of geodesy and geophysics, but an important basic survey project. One mentionable goal of "National Spatial Information fiducial Frame Project" being about to start is to determine an accurate geoid of our country. Nowadays, determining a region geoid of cm- level is one research focus of geodesy.Aiming at the pertinent problems of the Chengdu existing elevation systems, technical problems, such as converting ellipsoidal heights to normal heights of the region, are discussed in the thesis. After systematically introducing the theory of GPS&leveling measuring height system and their similarities and discrepancies, the principles and methods of determining geoid are emphatically explained. Then using mathematic model, the scheme achieving height transformation with a cm-level precision by a few GPS/ levelling points is searched. Furthermore, the factors affecting height transformation and error sources byGPS single-point positioning are analyzed by certain tests. Finally, an approximate solution of the geoid computation with a cm-level precision in Chengdu is proposed based upon analyzing the approaches and corresponding errors of computing normal heights in cm- level for different areas. Taking the terrain characteristics of the Chengdu region in account, the approaches adapted to different surveying projects (linear interpolation, planar modeling and dual-curving facet modeling etc.) are used to realize height transform here. Proved by practice, the normal heights with a cm-level precision for the region can be achieved by a few but reasonably distributed leveling points (suggesting point density: one GPS/leveling point per 13-23 kilometer for a linear engineering; one GPS/leveling point per SkmxSkm for a planar model),and such precision can meet the demand of general surveying projects.