Abstract: Aiming at the pertinent problems of the Hulin existing elevation systems, technical problems, such as converting ellipsoidal heights to normal heights of the region, are discussed in the present work. After systematically introducing the theories on GPS and on leveling measuring height system, as well as on their similarities and discrepancies, the principles and methods of determining quasi-geoid are explained. Then using a mathematic model, the scheme achieving height transformation with a cm-level precision by a few GPS points and leveling points is searched. Finally, an approximate solution to the quasi-geoid computation with a cm-level precision in Hulin area is proposed, based on analyzing the approaches and corresponding errors of computed normal heights in cm-level in different areas. Taking the terrain characteristics of the Hulin area in account, the approaches adapting to different surveying projects are used to realize height transform here. Proved by practice, the normal heights with a cm-level precision of this region, can be achieved by a few reasonably distributed leveling points, which can meet the demand of general surveying projects.

Key words: GPS, quasi-geoid, normal height, precise model, isoline of height anomaly, deflection of the vertical