Extracting Consistent Watersheds from Digital River and Elevation Data

The Terrain Resources Inventory Mapping (TRIM) data standard in BC, Canada, includes speci cations for river data and elevation data that are typically met by interpretation of stereo orthophotos. It does not specify that breaklines for watersheds be interpreted from the photos, thus we must extract them from the data. We seek a system of watersheds that, while not exact due to errors in the data, is at least self-consistent { having no overlapping watersheds and no unclassi ed points. We build a TIN terrain model using a Delaunay triangulation that combines both river and elevation data. Based on the standard de nition of water ow along steepest-descent paths, we create an algorithm that identi es ridges and channels in the TIN, and extracts watersheds. Several unexpected geometric con gurations, which we have not seen in the literature, follow from the standard de nitions. These must be correctly computed to obtain a consistent system of watersheds.