REMOVAL OF TREE OFFSETS FROM SRTM AND OTHER DIGITAL SURFACE MODELS
The recently completed 1 second Digital Elevation Models (DEMs) for Australia are based on the 1 second Shuttle Radar Topographic Mission (SRTM) elevation data. The SRTM data was corrected by removing voids, striping, tree offsets and random noise and finally by integrating mapped drainage lines. This paper describes the removal of the tree offsets, which was a crucial step in the production of a credible bare-earth elevation model and was one of the most technically challenging aspects of the project, and the possible application of the methods to other digital surface model (DSM) sources.
Methods for the removal of tree offsets rely on maps of tree presence/absence from sources such as remotely-sensed imagery, and the height offsets are computed from the DEM at the boundaries of tree patches. Tree offsets over most of Australia were successfully removed, but were underestimated in areas of extensive forest cover and poorly estimated where the mapping of tree patches did not match the patterns of offsets in the SRTM elevations.
The tree offset removal methods could be applied to the near-global SRTM DSM to produce a near-global bare-earth product, provided that a suitable map of tree presence or density can be compiled from satellite remote sensing and other sources. The process could be improved by using supplementary tree-height information from ICESat or other sources.
High resolution global DEMs other than SRTM are becoming available, notably ASTER GDEM and TANDEM-X. Both those products are subject to offsets due to vegetation in the same way as SRTM. The tree offset removal methods developed for SRTM could be adapted to the characteristics of these and other DSMs to provide a largely automated processing system to derive bareearth DEMs from new sources.