1. A method for predicting a radio field level and/or a duration of radio propagation of a mobile telephone network for coverage of a cell, the method comprising: accessing, by a calculator, at least one database comprising a grid of pixels, each pixel comprising a type of land cover and a height of the land cover; applying, by the calculator, a vectorization preprocessing to the grid of pixels to: linearize contours of obstacles in the grid of pixels by: considering only pixels having a type of land cover corresponding to an obstacle; and identifying the contours of the obstacles by: identifying pixels having a non-zero height of the land cover; and identifying straight-line segments in the grid of pixels representative of a position and an orientation of facades of the obstacles; produce a grid of feature types, the feature types comprising one or more of an inside of an obstacle, a facade of an obstacle, or a corner of an obstacle; produce a grid of feature angles, the feature angles comprising one or more of a facade angle or a corner angle, wherein the grid of feature types and the grid of feature angles allow prediction of a reflection and a diffraction and scattering of a ray launched across the grid of pixels; fictitiously launching, by the calculator, rays over the grid of pixels; applying, by the calculator, a vectorization processing to the grid of pixels to linearize the contours of the obstacles encountered while fictitiously launching the rays over the grid of pixels, the rays being either reflected or diffracted and scattered according to the obstacles encountered; and calculating, by the calculator, a global attenuation and/or the duration of radio propagation, wherein the vectorization processing of the contours of the obstacles applied to the grid of pixels linearizes the contours of the obstacles encountered by forming straight lines based on the grid of feature types and the grid of feature angles, the straight lines accurately describing a position and an orientation of the facades of the corresponding obstacle encountered.