A navigation system has a database which stores index data for, respectively, a tile of a tiling. The index data includes, for each one of plural cells of a grid which is superimposed on the tile, respectively an identifier for each triangular face of a triangulated irregular network which at least partially overlaps with the respective cell. The plural cells of the grid are defined so as to cover the respective tile. A processing device of the navigation system is configured to use the index data to determine a triangular face on which a map feature having given lateral coordinates is located, in order to determine a height coordinate of the map feature.