There is little controversy over how to conduct a visibility analysis over bare terrain. However, in many areas this results in a totally unrealistic estimate of a project’s visibility, and the question arises how to account for the screening effect of land cover when information about its height is not available. This paper investigates the difference between a digital model of the terrain (DTM) and canopy surface (DSM) from a 3-mile wide corridor in the state of New Hampshire, USA to evaluate the appropriateness of assigning a fixed height to land cover for the purposes of visibility analysis. The results find a 40-ft (12 m) height reasonable for forest cover, but suggest that using fixed heights for other land covers may be inappropriate.