The map is based on the Hydrologic Unit Maps published by the U.S. Geological Survey Office of Water Data Coordination, together with the list descriptions and name of region, subregion, accounting units, and cataloging unit. The hydrologic units are encoded with an eight- digit number that indicates the hydrologic region (first two digits), hydrologic subregion (second two digits), accounting unit (third two digits), and cataloging unit (fourth two digits). This data was originally collected at a scale of 1:250K. Some areas, notably major cities in the west, were recompiled at a scale of 1:100K.
The original data set was compiled to provide the National Water Quality Assessment (NAWQA) study units with an intermediate scale river basin boundary for extracting other GIS data layers. For use with the GeoStac database, this data set was compiled in order to simplify pesticide risk assessment and provide a common data set for all stakeholders.
The original source dataset, available at http://water.usgs.gov/GIS/huc.html, provides additional details regarding the compilation of the hucs250 boundaries.
publication date
These data were digitized at a scale of 1:250,000 with some portions of coverage at 1:100,000- and 1:2 million scale. Limitations of the data strictly revolve around this scale input. Use of these boundaries with larger scale data (i.e. 1:24k hydrography) is not recommended as it would be beyond the resolution capabilities of the data set.
897-B Harrison Street, S.E.
U.S. Geological Survey Office of Water Data Coordination. The original source data, prior to compilation into this geodatabase, is located at http://water.usgs.gov/GIS/huc.html.
Polygon and chain-node topology present.
Resolution as reported
Unspecified
Converted source data downloaded from USGS Web Site, http://water.usgs.gov/GIS/metadata/usgswrd/XML/huc250k.xml to ArcGIS geodatabase feature class file format.
Projected data set to NAD 83 Albers Equal Area.
Imported and edited original source metadata, available at http://water.usgs.gov/GIS/metadata/usgswrd/XML/huc250k.xml.
Internal feature number.
ESRI
Feature geometry.
ESRI
Length of feature in internal units.
ESRI
Area of feature in internal units squared.
ESRI
For definition of drainage basins, there are two principal tabular data elements of interest. The first, "HUC", stands for the Hydrologic Unit Code and includes the 8-digit cataloging unit as assigned to the basin polygon by the U.S. Geological Survey. The second item, "SOURCE", is an attribute of each bounding line segment (arc) and incorporates a code to define the source and scale of the source linework. Because this data set includes linework from different scale sources, it is important to recognize the presence and use of this item in quality evaluation. The SOURCE item has four special codes to describe the source linework. Code 1 stands for all arcs that were from 1:100,000 scale GIRAS files and were internal to the quadrangle (no neatlines). Code 2 stands for all arcs from the 1:250,000-scale GIRAS files which were internal to the quadrangle. Code 3 is for lines from either GIRAS source scale but were part of the neatline which may be used to connect arcs that dont join cleanly between quads. Code 4 is for linework in the southern Central Valley and southern Coastal California where the GIRAS data were replaced with 1:2 million scale basin boundaries.
USGS Huc250K metadata document, available at http://water.usgs.gov/GIS/metadata/usgswrd/XML/huc250k.xml.
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None
Data can be downloaded from www.geostac.org with a registered user ID and password provided by the Spatial Sciences Laboratory.
Not Applicable
897 B Harrison Street, SE