Although this Federal Geographic Data Committee-compliant metadata file is intended to document the data set in nonproprietary form, as well as in ARC/INFO format, this metadata file may include some ARC/INFO-specific terminology.
Hydrologic Unit Code (HUC) areas in this file do not necessarily match those published in Water-Supply Paper 2294 (Seaber et al). The file has finer resolution than the maps used to determine areas published in the Water-Supply Paper.
The ARC/INFO version of this file contains two additional annotation layers. One layer is called anno.maplabel1, and contains the first six digits for each accounting unit. The labels for Accounting Regions 1 though 9 have a "0" in front of the number. The second layer is called anno.catnum, and contains the last two digits for each cataloging unit. All numbers were checked manually against a 1:2,000,000 plot of the data.
Also in the ARC/INFO version, imbedded in the INFO file is the HUCS_2M.TRACE file, which points to the next downstream cataloging unit for each basin. If a basin flows into another cataloging unit, that unit's code is given for the HUC_TO item. Not all basins flow into other basins; in these cases, the following codes were used for HUC_TO:
HUC_TO Meaning 0 Ocean 1 Flows across a national border 2 Closed drainageA second imbedded file is HUCS_2M.AML_INDEX, which is a listing containing an index of all user-defined items in the .PAT and .AAT files.
Note: HUCS_2M.TRACE and HUCS_2M.AML_INDEX are accessed through INFO by selecting for the file. These files have not been updated for Version 2.0. INDEX_HUC.AML, which creates the index file, is also available through INFO.
The polygon attributes were checked by comparing a 1:2,000,000-scale plot to the source.
The attribute accuracy is estimated at 99%.
Next, the hydrologic unit boundaries were compared to the 1970 1:2,000,000-scale DLG stream file and the 1:500,000- scale Hydrologic Unit Maps. Plots containing both the streams and the hydrologic unit boundaries were made at a scale of 1:2,000,000. Hydrologic unit boundary intersections were checked, as well as problem areas such as points where streams crossed hydrologic unit boundaries on the plot but not on the hydrologic unit map. More than 400 errors were found. All errors that could be corrected were corrected. The remaining errors could not be corrected because the streams were in error rather than the hydrologic unit boundaries.
Finally, the hydrologic unit boundaries were plotted at 1:2,000,000 scale and compared to the NMD 1998 1:2,000,000-scale hydrologic data set. Any discrepancies were addressed by reference to either the 1:500,000-scale or the 1:100,000-scale maps. The exceptions to this were the hydrologic unit boundaries for Alaska, Hawaii, Puerto Rico, and the U.S. Virgin Islands. The Alaska boundaries were checked on a 1:2,500,000-scale plot but not modified, because the boundaries cross glaciers, and it is difficult to distinguish flow from the glaciers. The Hawaii hydrologic unit boundaries were not reviewed because each island represents a cataloging unit. The Puerto Rico and U.S. Virgin Islands boundaries were not plotted but were reviewed on-screen using a 1:1,000,000 hydrologic data set created for the 1991 National Water Summary.
The Horizontal positional accuracy of the data matches the positional accuracy of the original compilation.
a) 1:2,000,000-scale stable-base map plots of drainage (streams) and the latitude/longitude grid were obtained from the USGS National Mapping Division (NMD). These plots were in the Lambert Conformal Conic projection, and used 1:500,000-scale State base map projection parameters. Fifteen sections were plotted using 1970 National Atlas 1:2,000,000-scale sectional boundaries.
b) Individual State 1:500,000-scale hydrologic unit maps in the Lambert Conformal Conic projection were photo reduced to 1:2,000,000.
c) Hydrologic unit boundaries were compiled and the edges were matched on scale-stable 1:2,000,000 plots. The boundaries were combined with the streams plots for digitizing.
d) The hydrologic unit boundaries were digitized by Automated Datatron, Hyattesville, MD., which digitizes materials on contract for NMD.
e) Digital data from Automated Datatron was converted to an ARC/INFO coverage, then checked and corrected.
f) A detailed, annotated list was compiled, listing locations where hydrologic unit boundaries and streams were coincident, or where 1:2,000,000-scale DLG streams were in error. Wherever possible, conflicts and errors were resolved.
The Region, Subregion, Accounting unit, and cataloging unit names from Seaber were added to the data set and spot-checked. Cataloging unit names for Alaska were added from the Alaska Hydrologic Unit Map (HUCAKmap).
Values for BNDTYPE were determined from the bounding polygons using INDEX_HUC.AML. This same procedure also indexed the coverage on all user-defined items in the .PAT and .AAT files. A listing can be found in the file, HUCS_2M.AML_INDEX (imbedded in the INFO file).
The attributes Error_type, Reg_name, Map_label, Catalog_Unit_Nam, and Catalog_unit_num were added to the file.
Two annotation layers were also added to the coverage. One layer is called anno.maplabel1, which shows the first six digits for each accounting unit. The labels for accounting Regions 1 though 9 have the "0" in front of the number. The second annotation layer is anno.catnum, and this layer has the last 2 digit codes for each cataloging unit. All numbers were checked manually on a 1:2,000,000-scale plot of the data set. The cataloging unit names were spot-checked. The file was modified to agree with the official National Atlas coastline (NAStates)
The ARC/INFO coverage was exported to create the shapefiles and SDTS files.