A method is disclosed comprising: obtaining a plurality of pieces of estimation information, the obtaining comprising receiving or determining the plurality of pieces of estimation information, wherein each of the plurality of pieces of estimation information represents one or more respective absolute values of an altitude, wherein each of the one or more absolute values of an altitude is associated with a respective position information, each of the plurality of pieces of estimation information determined based, at least in part, on a respective relative altitude information and, at least in part, on a respective absolute altitude information associated with a data element of a database comprising map data representing an altitude map; and aggregating the plurality of pieces of estimation information in a spatial grid, wherein the spatial grid is divided into a plurality of grid points. It is further disclosed an according apparatus, computer program and system.