Planar coordinates are encoded using coordinate pair Abscissae (x-coordinates) are specified to the nearest 0.000000 Ordinates (y-coordinates) are specified to the nearest 0.000000 Planar coordinates are specified in meters The horizontal datum used is North American Datum of 1983. The ellipsoid used is Geodetic Reference System 80. The semi-major axis of the ellipsoid used is 6378137.000000. The flattening of the ellipsoid used is 1/298.257222. The WBD was produced using Enhanced Digital Raster Graphics (DRG-e's) digital images as source map. Data completeness for DRG files reflect content of the source graphic and may therefore be reflected in the completeness and accuracy of the WBD. The map was digitized from USGS 1:24,000-scale digital raster graphic base maps, with an inherited error of +/- 40 feet according to USGS National Map Accuracy Standards. Digitized line locations were held to the +/- 40 foot accuracy standards by ensuring that all lines were within a 12 meter buffer of where visual interpretion put the actual boundary. Any lines outside the buffer were corrected. It is estimated that the errors detected were less than 10%. It should be noted that while general rules of hydrology were used (i.e. water flows downhill), the location of boundaries is still somewhat subjective as the 1:24,000-scale DRG-e do not always provide enough information for identifying the location of the boundaries. In these instances group consensus was used. Please refer to the methods section for more details on watershed delineation rules. Unquantifiable errors may be associated with coordinate shift. The default was used for all other ArcEdit tolerances except fuzzy tolerance which was set to .05 meters, and dangle which was set to 0. As with the original DRGs, DRG-e is cast in the Universal Transverse Mercator projection and is referenced to the NAD27 datum. When the DRG-e's are used in the NAD83 datum the alternate world files only approximate the NAD83 by shifting the coordinates of the imagery. The total error introduced using this shift is less than 2 meters for all DRG-e imagery. All fields for all polygons in the .pat are attributed except for HU_8_DS and HU_10_DS. These will be attributed at a later date. NCONTRB_A is only attributed if the polygon is a 100% closed area. Otherwise the NCONTRB_A field will have a 0 value, even if it is partially or even mostly non-contributing. All fields for the lines in the .aat are attributed except hu_level. This field in the .aat will attributed at a later date. The lines adjacent to the universal polygon may be attributed with a 0. These cannot be attributed until the adjacent states complete their linework at which point the highest level of hydrologic unit can be determined. There are no unclosed polygons, intersections without nodes, or polygons without labels or with more than one label. Arc/Info's topological correction program CLEAN was used to correct intersections without nodes, identify unclosed polygons and remove duplicate lines with the same beginning and ending nodes. All sliver and wedge polygons were removed manually in ArcEdit. Source.