1. A computer-implemented method, comprising: identifying, by one or more computing devices, a first overpass in an overpasses model, the overpasses model describing respective locations and widths of one or more overpasses, wherein each of the one or more overpasses occludes a portion of terrain located below such overpass; identifying, by the one or more computing devices, at least a first span across the first overpass, the first span corresponding to a line across the width of the first overpass; identifying, by the one or more computing devices, a pair of endpoints of the first span; determining, by the one or more computing devices, a pair of terrain elevation values respectively for the pair of endpoints of the first span, the pair of terrain elevation values included in an elevation model; interpolating, by the one or more computing devices, the pair of terrain elevation values for one or more points across the first span to obtain an interpolated terrain elevation value for each of the one or more points across the first span; and updating, by the one or more computing devices, the elevation model to associate the interpolated terrain elevation values with the portion of terrain occluded by the first overpass.