This data set is a complete digital hydrologic unit boundary layer to the Subwatershed (12-digit) 6th level for the State of Alabama. This data set consists of geo-referenced digital data and associated attributes created in accordance with the "FGDC Proposal, Version 1.0 - Federal Standards For Delineation of Hydrologic Unit Boundaries 3/01/02" (<http://www.ftw.nrcs.usda.gov/huc_data.html>). Polygons are attributed with hydrologic unit codes for 4th level sub-basins, 5th level watersheds, 6th level subwatersheds, name, size, downstream hydrologic unit, type of watershed, non-contributing areas and flow modification. Arcs are attributed with the highest hydrologic unit code for each watershed, linesource and a metadata reference file.
The 8, 10, and 12 Hydrologic Unit Boundaries for Alabama provide a uniquely identified and uniform method of subdividing large drainage areas. The smaller sized 6th level sub-watersheds (up to 40,000 acres) are useful for numerous application programs supported by a variety of local, State, and Federal Agencies. This data set is intended to be used as a tool for water-resource management and planning activities, particularly for site-specific and localized studies requiring a level of detail provided by large-scale map information. The dataset will be appended to a larger seamless nationally consistant geospatial database as other states complete their portion of the watershed boundary dataset. For use with the GEOSTAC Database, this data set has been compiled in order to simplify pesticide risk assessments and to provide a common data set for all stake holders.
publication date
The distributor shall not be held liable for improper or incorrect use of this data, based on the description of appropriate/inappropriate uses described in this metadata document. It is strongly recommended that this data is directly acquired from the distributor and not indirectly through other sources which may have changed the data in some way. These data should not be used at scales greater than 1:24,000 for the purpose of identifying hydrographic watershed boundary feature locations in Alabama. NRCS should be acknowledged as the data source in products derived from these data. The Alabama Watershed Boundary Dataset is public information and may be interpreted by all organizations, agencies, units of government, or others based on needs; however, they are responsible for the appropriate application of the data. Federal, State, or local regulatory bodies are not to reassign to the Natural Resources Conservation Service any authority for the decisions they make. The Natural Resources Conservation Service will not perform any evaluations of these maps or purposes related solely to State or local regulatory programs. Photographic or digital enlargement of these maps to scales greater than that at which they were originally delineated can result in misrepresentation of the data. If enlarged, the maps will not include the fine detail that would be appropriate for mapping at the small scale. Digital data files are periodically updated. Files are dated, and users are responsible for obtaining the latest version of the data from the source distributor.
897 B Harrison St SE
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 polygons were removed either using eliminate, merge or manually in ArcEdit.
All fields for all polygons in the .pat are attributed. Some polygons in the Ncontrb_a field might have a 0 value. All fields for the lines in the .aat are attributed. The lines adjacent to the universal polygon are attributed with a 0 for the hu_level. These cannot be attributed until the adjacent states complete their linework at which point the highest level of hydrologic unit can be determined.
The WBD was produced using Enhanced Digital Raster Graphics (DRGEs) digital images as source map. Data completeness for DRGE 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 interpretation 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 DRGEs do not always provide enough information for identifying the location of the boundaries. In these instances group consensus was used. As with the original DRGs, DRGEs are cast in the Universal Transverse Mercator projection and are referenced to the NAD27 datum. When the DRGEs 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 imagery.
eight digit hydrologic unit map delineations
Level 6 (12-digit) watersheds were delineated within a nested hierarchical structure using the existing level 5 (11-digit) watershed delineations as a starting point. The delineation process began with review of the 1974 USGS Hydrologic Unit Map (1:500000) and the Soil Conservation Service (SCS) Hydrologic Unit and Sub-Watershed Map (1:500000). Initial delineation of the level 6 watersheds was done on the SCS Hydrologic Unit and Sub-Watershed Map. During this process the level 5 (existing 11 digit) watersheds were revised to conform to the guidelines in the Federal Standards For Delineations of Hydrologic Unit Boundaries. Draft level 6 watersheds were then transferred to paper USGS 7.5 minute topographic quadrangles using the USGS 30 x 60 minute topographic quadrangles as a reference guide. Each of the 920 7.5 minute quadrangles were edge matched to check line work between the topos.
Delineated hydrologic unit boundaries were hand-digitized from paper 7.5 minutes quadrangles on a CalComp 34362 digitizing table using Didger v.2 digitizing software from Golden Software. Each individual map sheet was registered to a mathematically generated grid of corner tics of 7.5 minute quadrangle maps for the State of Alabama. Hydrologic unit boundary lines were then digitally captured. This process created a seamless coverage of hydrologic unit boundaries for the state.
The digital seamless coverage of hydrologic unit boundaries was overlaid on top of 7.5 minute enhanced digital raster graphics (DRGEs). Hydrologic unit boundaries were then edited to conform to the topography represented by the DRGEs. Environmental Systems Research Institute, Inc. (ESRI), ArcGIS v.8.2 software was used in editing the hydrologic unit boundaries. The typical size of a level 6 watershed is 10,000 to 40,000 acres. Polygon topology was established for the level 6 watersheds using ESRIs Arc/Info v.8.2.
ref: AL02, combined Alabama and Georgia datasets at National Cartography and Geospatial Center. In merging, the state boundaries were removed and the boundaries and polygon labels reconciled. In a few cases new arcs were created.
Data set converted to geodatabase feature class file format.
Data set projected to Albers Equal Area and NAD 83 datum.
Internal feature number.
ESRI
Feature geometry.
ESRI
Length of feature in internal units.
ESRI
Area of feature in internal units squared.
ESRI
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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 St SE