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Before beginning the IHABBS process, the root data -- in this case, NED elevations and NHD reaches and lakes -- must be formatted and cleaned. For the NED elevations this process entails reprojecting the data to WGS84 geographic projection (and mosaicing, if necessary) and pulling it into GISRS (NOHRSC GIS software which IHABBS is part of) through the use of an IDL function.

The streams and lakes coming from the NHD dataset were not quite as straightforward, however. The NHD data that was acquired through NSSL came with two files, one that contained just reaches and one that contained reaches as well as lakes and polygonal channels for larger rivers. Ideally the NHD reaches should be a perfect flow network from headwater to outlet. This reach file, however, contained both looped flows and misdirected stream segments. To use these reaches in IHABBS, it was necessary to manually go through the coverage and remove all looped flows and check that all reaches were flowing the correct direction. It is possible that some of this process could be automated. Lakes were then extracted from the polygonal coverage, again a fairly manual process. These coverages were then imported into GISRS as well.

Once all data was in the GISRS system, the IHABBS process could commence. IHABBS uses five steps to process flow directions. First all depressions up to a given threshold size are filled in the DEM. Next a flow direction grid is calculated from this filled dem. Third, the lakes and reservoirs coverage was used to route flow over these flat water bodies to their outlet points. Then, all pixels on the flow direction grid that fall along a river reach are flipped to match the flow of the reaches. Finally, all flow along flat areas is routed to an outlet point. This resulting flow direction grid has been corrected to use both streams and elevation data to create a coherent flow direction grid. After these steps, an additional step is made to create a flow accumulation grid.

The end result is a coherent flow direction grid that is forced to obey the basic hydrologic rule that a stream cannot cross a basin divide other than at its outlet point.

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