What do 'base heights' refer to in the context of a digital terrain model?

In the context of a digital terrain model (DTM), 'base heights' refer to the z-values that can be applied to an image during draping. Draping is the process of applying a two-dimensional image or texture to the surface of a three-dimensional digital terrain model. This involves assigning elevation values to the image based on the terrain model's z-values, which represent the height or depth at specific locations.

Base heights are crucial because they help create a realistic visualization of how the image, such as an aerial photograph or satellite imagery, fits over the terrain, accurately reflecting the changes in elevation across the landscape. By matching the image to the terrain using these z-values, the rendered output maintains visual coherence with the underlying topography.

Other options do not accurately describe base heights. The minimum elevation of flat terrain refers to a specific characteristic of landform, while the average height of structures might provide useful context for urban modeling but is unrelated to the concept of base heights in a digital terrain model. Negative elevation values may appear in certain geographic models, such as those depicting underwater terrain, but they do not define what base heights are within the framework of a DTM.