Aspects of the disclosure relate to generating a digital terrain model (“DTM”) from a digital surface model (“DSM”). For example, the DSM may include a plurality pixels associated with coordinate information and elevation information, including the elevation of features such as buildings and large vegetation. Initially, a set of vertices defining a polygon may be identified. A set of seed pixels, corresponding to points of the DSM of “bare” earth (without buildings or large vegetation) may also be identified. These may be the same or different from the set of vertices. An adaptive flood filler algorithm is used to evaluate the pixels of the polygon starting with the seed pixels. This process validates or invalidates the state of the pixels. The result is an approximation of a DTM for the polygon.