Spatial data in GIS format on agri. chem. use, LU, and cropping practices in US

The agricultural expenses data in this coverage are intend for use in
estimating regional agricultural expenses, and in producing visual displays and
mapping relative amounts of agricultural expenses across broad regions of the
United States.

This coverage contains estimates of agricultural expenses in counties in the
conterminous United States as reported in the 1987 Census of Agriculture (U.S.
Department of Commerce, 1989a). Agricultural expenses data are reported in
thousands of dollars. Agricultural expenses estimates were generated from
surveys of all farms where $1,000 or more of agricultural products were sold,
or normally would have been sold, during the census year.

Most of the attributes summarized represent 1987 data, but some information
from the 1982 Census of Agriculture also was included.

The polygons representing county boundaries in the conterminous United States,
as well as lakes, estuaries, and other nonland-area features were derived from
the Digital Line Graph (DLG) files representing the 1:2,000,000-scale map in
the National Atlas of the United States (1970).

Agricultural expenses Census of Agriculture Counties United States

Procedures_Used: CENSUS DATA

An automated procedure was developed for processing the raw census data into
ARC/INFO coverage attributes. The procedure is summarized below: 1) copy
county2m coverage to coverage representing type of census data (i.e. ag_expn or
ag_land), 2) run agadd.aml for each item added to the coverage, giving coverage
name and attribute field number as arguments.

The agadd.aml program runs a FORTRAN program to extract field data from the raw
census data files, and then processes that raw data finally adding it as a
column of attribute data to the county coverage. Other programs were developed
to calculate summary statistics of the census attribute data, and to make
graphics representing attribute values across the United States. COUNTY
BOUNDARIES

This series of maps was published as part of the National Atlas of the United
States (U.S. Geological Survey, 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. FIPS codes were
assigned to all polygons by 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. This 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 2.0 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 four
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 ticks 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 above. 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.
Revisions: COUNTY POLYGON DATA

Revision 1.0, 12/17/90. This revision represents numerous corrections and minor
modifications made to this set of coverages from its construction in 1985
through the revision date.

Revision 2.0, 3/18/91. Major reworking of the coverage, combining all
State coverages.

Reviews_Applied_to_Data: The Census of Agriculture data processing procedure
and attribute data have been peer reviewed in 1993 by Leonard Orzol and Barbara
Ruddy, both hydrologist with the USGS.

The county boundaries in this coverage have received no formal review. They
have, however, been used in numerous applications where serious errors would
have been obvious. Some State coverages were corrected following such use. The
offshore polygon extensions and the Canadian Great Lakes polygons have had no
review.

Related_Spatial_and_Tabular_Data_Sets: This coverage is part of series of
1:2,000,000-scale base maps covering the United States. Layers in this set
include:

COUNTY -- County boundaries. STATE -- State boundaries (formed from COUNTY).
WATERBOD -- Water Bodies. STREAM -- Streams. HUC -- Hydrologic cataloging units
(basins).