The invention relates to a method, performed by at least one apparatus. The method comprises obtaining sample measurements at least in part comprising altitude information and observed in an area at least in part represented by an altitude map, the altitude map comprising sub-sections representing respective sub-areas of the area; The method further comprises determining altitude estimates from the altitude information. The method further comprises updating one or more altitude estimate distributions associated with respective sub-sections of the altitude map based on the determined altitude estimates. The method further comprises determining, for one or more sub-sections of the altitude map, whether a respective sub-section of the altitude map represents a single level sub-area or multilevel sub-area, based on respective altitude estimate distributions associated with respective sub-sections of the altitude map.