jgg     2007 27 (4):  1-7    ISSN: 0412-1961  CN: 21-1139/TG GPS GRAVITYPOTENTIAL LEVELING Wei Ziqing Xian Research Institute of Surveying and Mapping, Xi’an 710054 Received: 1900-01-01  Revised: 1900-01-01  Online: null Supporting info null