In this paper we present a new method for White Spaces estimation on a geographical region. The method is based on a low-cost signal sensors network, distributed in the areas of interest for the deployment of cognitive radio technologies. The sensors are connected to software that locates the data on the cartography and performs an estimation of the electromagnetic field, obtaining a radio environment map from the field measurements. The interpolation method used is the result of several interpolation methods in combination with models of propagation of radio signals and statistical, cartographic and urban distribution considerations. Also, we present the methodology for deciding whether a channel can be considered white space or a busy channel. This is done considering the characteristics of the technology used by the primary operator and the CR devices. The calculation of the resolution and other parameters of the obtained radio environment map are also shown and explained