jgg     2013 33 (3):  59-63    ISSN: 0412-1961  CN: 21-1139/TG ESTIMATION OF GPS ZENITH TROPOSPHERIC DELAY BASED ON  TERRAIN AND METEOROLOGICAL ELEMENTS 1)School of Civil Engineering, Central South University, Changsha 410075 2)Hydrogeology and Engineering Geology Prospecting Institute of Heilongjiang Province,Haerbin 150030 Received: 2012-03-18  Revised: null  Online: 2013-06-18 Supporting info