Upon what datum is the NHD cast?
North American Datum of 1983. What is the file size of a subbasin? Compressed ARC coverages average 0.80 MB; uncompressed ARC coverages average 3.34 MB. Compressed SDTS files average 0.19 MB; uncompressed SDTS files average 1.86 MB. How current are the data? It will depend upon the production date of the initial linework and whether this linework was updated when the DLG files were created by USGS. Therefore the vintage of the linework can vary from the 1950s to the mid-1980s. Can users who have linked attributes to EPA Reach File Version 3 (RF3) move those attributes to NHD? In the process of building the NHD from RF3 and DLG3 (Digital Line Graph Version 3), the rules for delineating reaches were changed in some significant ways. For example, RF3 has reaches on the shorelines of wide rivers and lakes, while NHD has reaches on artificial paths that run through wide rivers and lakes. Another example is, RF3 did not have waterbody reaches and NHD does. Consequently the spatial representa
