jgg     2015 35 (1):  58-61    ISSN: 0412-1961  CN: 21-1139/TG Study on Topography Correction based on Spherical Approximation 1 Institute of Geospatial Information，Information Engineering University，66 Mid-Longhai Road, Zhengzhou 450052, China 2 Xi’an Research Institute of Surveying and Mapping，1 Mid-Yanta Road, Xi’an 710054, China 3 Xi’an Division of Surveying and Mapping，36 Xiying Road, Xi’an 710054, China Received: 2013-11-18  Revised: null  Online: 2015-02-06 Supporting info