The construction of the accurate, economic topographic information for a sorts of work and a base facilities of country is one of the very important subjects. In this paper, Global network which include south KOREA and Regional network which include Chung-Nam/Chung-Buk provinces were designed, obtained WGS84 coordinates of control points in networks by GPS. The coefficients of transformation between WGS84 coordinates and Bessel coordinates in consideration of geoid undulations was derived by 7-parameter method. Also, the efficiency of 7-parameter method was proved through successful application of T.M. and U.T.M projection for the precise mapping, and the effectiveness of the aquisition of three dimensional topographic information by GPS was suggested.