The Ice, Cloud and land Elevation Satellite (ICESat) mission determines land surface vertical structure within laser footprints due to topographic relief and vegetation using received waveforms recorded by the Geoscience Laser Altimeter System (GLAS). In low-relief areas with tree cover the waveforms and derived elevation products provide useful biophysical parameters, including maximum canopy height, crown depth, the outer-canopy ruggedness, and a measure of canopy cover. For areas where within-footprint topographic relief is large compared to vegetation height, interpretation of the waveforms is complex. The contribution of canopy and ground to received waveforms is illustrated by comparing them with co-located waveforms computed using an instrument model applied to high resolution Digital Elevation Models (DEMs). The model includes representations of the transmit pulse's spatial and temporal distributions, and the receiver field-of-view sensitivity and temporal smoothing. This provides a means to validate GLAS waveforms, elevation products, and footprint geolocation.