A tree structure represents a terrain area as nested polygons organized in a parent-child relationship, each polygon associated to a specific geographic location. The tree structure defines at least one parent node and a plurality of child nodes, some being leaf nodes containing a height value. A processor uses a distance measure to change the tree structure topology assessing whether all leaf node children of a first parent node lie outside a predetermined distance from an aircraft runway, and if so, converting the first parent node into a leaf node by storing in the first parent node a height value representing the greatest of the respective height values of the leaf node children and by removing the leaf node children; and iteratively repeating for each remaining parent node until it has been determined that every remaining parent node in the data structure cannot be pruned without violating accuracy requirements.