In the present invention, data of a digital altitude map of a cell in a cellular communication system is compressed in a first node by defining a polygon essentially representing the boundary of the cell. Each polygon corner is defined by a point having two lateral coordinates and an associated altitude parameter. In this manner a set of at least three points is defined. This set of points constitutes a relatively limited amount of data, which easily can be transferred over an interface to a second node. In the second node, the set of points is used as parameters for creating an altitude model. By using a pair of lateral coordinates as input arguments of the model, an estimation of an associated altitude can be derived.