Estimates of herbicide use for the 20 most-used herbicides in the conterminous United States

The herbicide-use estimates in this coverage are intended for use as a means
for estimating regional herbicide use, and for producing maps showing relative
rates of herbicide use across broad regions of the United States.

This coverage contains estimates of herbicide use for the 20 most-used
herbicides in the conterminous United States as reported in Gianessi and Puffer
(1991). Herbicide-use estimates in this coverage are reported for each county
polygon as acres treated, pounds of active ingredient used, and pounds used per
square mile. The herbicide-use estimates provided by Gianessi and Puffer (1991)
list acres treated and pounds of active ingredient applied for a given crop in
each county for which use has been estimated. Cropping data are from the 1987
Census of Agriculture, and are subject to occasional suppressions of acreage
estimates at the county level due to problems of confidentiality and census
disclosure rules. The herbicide-use estimates included in this coverage are
totals of use on all crops treated in a given county.

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).

Herbicides Herbicide use Counties United States

Procedures_Used: HERBICIDE-USE DATA

An automated procedure was developed to process the raw herbicide-use data into
ARC/INFO coverage attributes. The procedure is summarized below:

(1) copy county2m coverage to coverage called herbicide%#%, and (2) run the AML
herbadd.aml for each herbicide to be added.

The herbadd.aml program runs a fortran program to total estimates of herbicide
use on all crops by county, then processes these data, finally adding them as
three columns of attribute data to the county coverage. Other programs were
developed to calculate summary statistics of the herbicide-attribute data and
to produce maps that show 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;
one of the sheets was chosen arbitrarily as the "correct" border. Edgematching
operations were used to adjust the linework as far as was necessary so that the
coverages would fit to a tolerance of 100 meters (328.1 feet). 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 counties. The accuracy of these adjustments is believed to be
comparable to that of 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 (65.6 feet), which resulted in few, if any, effects on small offshore
islands. The coverage also was checked to ensure that it represented current
counties or county equivalents.

The coverage in Version 1.0 ended at the coastline. No attempt was made to
depict offshore areas. This created 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 represented logically 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 tick marks were relocated to places that are easily visible on maps of the
United States, to help in registering maps that otherwise may not 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 this coverage is used to clip or intersect other coverages, a tolerance as
low as 2 meters (6.6 feet) 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.