1. A method comprising: receiving a three dimensional (3D) surface representing a geographic area, the surface having elevation values associated with points of the surface and the geographic area comprises a structure having a geographic footprint smaller than the geographic area; projecting, by a processor, the elevation values into a two dimensional (2D) plane; generating an intensity image based on the elevation values of the 2D plane; segmenting the intensity image; detecting a shape representing the geographic footprint of the structure in the geographic area from the segmented intensity image; and constructing a 3D model of the structure by assigning heights to the shape based on the elevation values projected into points of the 2D plane corresponding to the shape.