Spatial data in GIS format on land agriclture production

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

This coverage contains estimates of land in agricultural production in
counties in the conterminous United States as reported in the 1987
Census of Agriculture (U.S. Department of Commerce, 1989a). Land in
agriculture data are reported as either a number (for example, number
of Farms), acres, or as a percentage of county area. Land in
agriculture 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 for 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 land 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
conterminou United States were digitized in 15 sheets and published in
the Digital Lin 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,
effect 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. 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 plo polygons with a large number of vertices -- made it
necessary to separate them into four polygons. The dividing lines are
located in narrow channel 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 coverag 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 error 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).