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While initially based on 1:100,000-scale data, the NHD is designed to incorporate and <br />encourage the development of higher resolution data required by many users. " <br />Figure 2.1 shows the status of NHD for Colorado (http://nhdgeo.usgs.~ov/viewer.htm). In Figure 2.1 <br />(taken directly from the USGS web site and best viewed in color), the numbered basins correspond to 6- <br />digit hydrologic unit codes and are referred to as subregions in NHD. The black outlines correspond to 8- <br />digit hydrologic unit codes and are referred to as subbasins. A medium resolution (100k scale) coverage <br />exists for the whole state. High-resolution (24k scale) subbasins are nearly complete for the entire state. <br />A few remaining subbasins are still being produced on the state boundary with Wyoming and Kansas. <br />These subbasins will likely become available in the near future. The data coordinates are in decimal <br />degrees using the North American Datum of 1983. The NHD web site makes new data available as they <br />are processed and therefore the NHD products are often updated. <br />M H U Status <br />Med Resolution Available <br />High Resolution Planned <br />High Resalutian in Work <br />High Resolution Available <br />Local Resolution Available <br />2.3 NHD History <br />Initially, the NHD data were only available in ArcInfo coverage format (NHDinARC) and shapefiles <br />(NHDinSHP). In April 2004, the NHD became available in geodatabase format (.mdb files). The new <br />implementation is called NHDinGEO. The new format takes advantage of new capabilities in ArcGIS <br />with the NHD geodatabase model. This model is consistent with ESRI's migration away from coverages <br />and shapefiles. Support for the NHDinARC data model continues while the NHD programs and user <br />community transition to the new design. NHDinARC is now generated from NHDGEOinARC, which is <br />an NHD geodatabase model that supports the NHDinARC model and is maintained in parallel to the <br />NHDinGEO. <br />Although the basic data content of NHDinGEO remains the same as NHDinARC, the implementation of <br />the new data model involves several changes. These changes are the result of a redesign effort to both <br />simplify the data and to take advantage of the new capabilities in the geodatabase environment. The most <br />Page 3 of 17 ~fifversFde TeChnnlo~yv ffTC. <br />4s':rkv Resuurce.s Ergmee: mg end ~C9 nsuNrng <br />Figure 2.1. NHD Status for Colorado <br />