This paper shows a methodology that allows to obtain an up to date model of a primary distribution network. In the first part, the procedure used for the resistance and reactance calculation of each branch from the Geographic Information System data is described. Also, the approximations used in calculating each parameter are mentioned. Then, the methodology for obtaining the information from the GIS to obtain the network configuration is described and finally, the software structure developed for the automatic calculations of the parameters and the configuration is shown. The computational tool was tested in a real distribution system, in a medium size town (Buenaventura) that has about 6000 buses and 1300 customers, which belongs to a regional utility (EPSA:Empresa de Energía del Pacifico ESP).