1. Method for predicting the radio field level and/or the duration of radio propagation of a mobile telephone network for the coverage of a cell using the technique of ray-launching and utilizing geographical databases wherein the database directly utilized is a grid of pixels, each pixel carrying at least two items of information, an item of information specific to the type of land cover and an item of information specific to the height of the land cover, a vectorization processing is applied to the grid of pixels to linearize the contours of the obstacles (I 1 , I 2 , I 3 ) encountered while rays (Ri) are fictitiously launched, with the rays according to the obstacles encountered being either reflected (Rr) or diffracted and scattered (Rd), while the global attenuation and/or the duration of radio propagation are calculated, the vectorization processing of the contours of obstacles (I 1 , I 2 , I 3 ) being applied to the grid of pixels to linearize the contours of obstacles (I 1 , I 2 , I 3 ) encountered by forming straight lines whose positions and various angles are known accurately, with these straight lines accurately describing the position and the orientation of the façades of the obstacle (I 1 , I 2 , I 3 ) encountered.