The data was then modified by USDA Forest Service Personnel for use in the Southern Forest Resource Assessment and exported to a shapefile (please see USDA Forest Service Modifications in process step description p.6).
Assessment-standard projection: Projection: Albers Equal Area Units: Meters Spheroid: GRS80 Datum: NAD83 First Standard Parallel: 27 degrees 30 minutes Second Standard Parallel: 37 degrees 30 minutes Central Meridian: -90 degrees 00 minutes Latitude of Origin: 25 degrees 00 minutes False Easting: 0.0 False Northing: 0.0
USGS_Procedures_Used: This series of maps was published as part of the National Atlas of the United States (USGS, 1970). The maps for the conterminous United States were digitized in 15 sheets and published in the Digital Line Graph (DLG) format as described by Domeratz and others (1983).
Each sheet was prepared by reading the DLG files of the political and water bodies layers, converting them to ARC/INFO, extracting the county boundaries and the coastline, respectively, and joining the two layers together. FIPS codes were assigned to all polygons using available sources, and were checked manually.
Boundaries with adjacent sheets of the 15-sheet set were edgematched manually, arbitrarily choosing one of the sheets as the "correct" border. Edgematching operations adjusted the linework as far as was necessary so that the coverages would fit to a tolerance of 100 meters. The coverage (referred to herein as Version 1.0) was stored as 49 separate coverages (48 states and the District of Columbia) because the ARC/INFO software in use at the time could not process the entire coverage. Individual states could be joined by specifying a tolerance of 100 meters.
From time to time, adjustments were made to the state coverages to reflect changes in U.S. counties. It is believed the accuracy of these adjustments is comparable to the original linework.
For Version 2.0, all state coverages were rejoined and manually edited to produce a perfect edgematch between all states. For states on the original map sheet boundaries, this adjustment averaged less than 20 meters and in no case was more than 100 meters. The whole coverage was CLEANed to a tolerance of 20 meters, which resulted in few, if any, effects on small offshore islands. The coverage also was checked to ensure that it represented current U.S. counties or county equivalents.
The coverage in Version 1.0 stopped at the coastline. There was no attempt to depict offshore areas. The created some problems when the coverage was used to assign county codes to sampling stations located near the coast. To help in this matter, Version 2.o includes offshore extensions of the county polygons. The (water) boundaries of many of these polygons are arbitrary.
The Canadian Great Lakes features are another new addition to Version 2.0. They were added to improve the utility of the coverage for visual displays. Although the Canadian Great Lakes are logically represented by a single polygon, practical considerations -- the inability of some software to plot polygons with a large number of vertices -- made it necessary to separate them into 4 polygons. The dividing lines are located in narrow channels to minimize interference with plotting patterns. Canadian islands within the Great Lakes also were included.
All tics were relocated to places that are easily visible on maps of the United States, to help in registering maps that may not otherwise have adequate registration information.
To expedite accessing parts of the coverage, certain items have been indexed with the procedure, index_county.aml. See Section 3. A spatial index also was created.
When using this coverage to clip or intersect other coverages, a tolerance as low as 2 meters can be used.
The processing used to derive this coverage moved boundaries from their positions on the original maps. In cases of conflicting lines, preference was given to forming the correct topology. Strictly speaking, this coverage is not identical to the source materials. These changes were unavoidable in producing a continuous coverage of the conterminous United States.
For revision 2.1, it was necessary to alter about 100 arcs. This was accomplished with ARCEDIT using the 1:100,000 TIGER county coverage as a background. These arcs were matched to the background so that they matched at least as close as other arcs that were not edited.
Related Spatial and Tabular Data Sets: This coverage is part of series of 1:2,000,000-scale basemaps covering the United States. Layers presently available in this set include:
county2m -- County and state boundaries.
waterbod2m -- Water Bodies.
huc2m -- Hydrologic cataloging units (basins).
For Version 2.3.1, USGS corrected the following 2 errors found in version 2.3. First, a duplicate label in the water polygon for Hampton, VA, FIPS 51650 was removed. Second, the PLYTYPE attribute was corrected to PLYTYPE=9 for the external polygon. Because this had been set to 0 in some prior processing, the BNDTYPE values for outside arcs were not correct. Ran index_county.aml to correct the BNDTYPE values.
Other References Cited: Domaratz, M. A., Hallam, C. A., Schmidt, W. E., and Calkins, H. W., 1983. USGS Digital Cartographic Data Standards: Digital Line Graphs from 1:2,000,000-scale Maps. U.S. Geological Survey Circular 895-D, 38p.
U. S. Geological Survey, 1970. National atlas of the United States of America: Washington D.C., U. S. Geological Survey, 417p.
USDA Forest Service Modifications:
Sept. & Oct.-2000: The coverages were downloaded by USDA Forest Service personnel on 22-Sep-2000 in compressed Arc Info Export format along with their associated metadata. The coverages were then reprojected to match Assessment standard Albers using the Arc Info PROJECT command, subsetted to the thirteen Southern states of the Assessment using Arc View's Theme Query tool and state FIPS code, then exported as shapefiles.
14-NOV-2000: In early November 2000 two problems were discovered with the two county coverages:
1. slivers were found associated with Virginia county boundaries, these were manually edited and removed 2. multiple records per county or municipality interfered with join operations and subsequent analyses, these were similarly identified and removed.
Both operations were performed on the Albers-projected shapefiles and renamed as scty100 and scty2m. These have replaced the USGS county coverages on the Assessment web site. Shortly after these errors were corrected, two additional coverages were created from scty100 and scty2m to address problems specific to mapping and analysis of FIA data. Particularly relevant in Virginia, FIA aggregate forest information for most of the independent municipalities into one of the adjoining counties. To match this tabular data, a coverage was created which reflected FIA's incorporation into counties of the various municipalities. Some of the largest municipalities are still tracked separately by FIA, these polygons were retained. Additionally, counties in central and western Texas and Oklahoma where FIA does not collect data were dropped from the coverages. FIA also does not conduct surveys in several counties in coastal Louisiana and Florida but these were retained in the coverage. The coverages were saved as fcty100 and fcty2m.
The USDA Forest Service, Southern Forest Resource Assessment or SFRA, provides data to users of digital geographic data. The USDA Forest Service is in no way condoning or endorsing the application of these data for any given purpose. It is the sole responsibility of the user to determine whether or not the data is suitable for the intended purpose. It is also the obligation of the user to apply the data in an appropriate and conscientious manner. The USDA Forest Service provides no warranty, nor accepts any liability occurring from any incomplete, incorrect, or misleading data, or from any incorrect, incomplete, or misleading use of the data. Much of the USDA Forest Service data is based on and maintained with Arc Info and Arc View Software developed by the Environmental Systems Research Institute (ESRI). In addition, much of the information presented uses conventions and terms popularized by Arc Info and Arc View and their user community. USDA Forest Service in no way represents the interests of ESRI nor acts as an agent for them.