Background
Introduction
In late 1999, the U. S. Department of Agriculture National Agricultural Statistics Service (NASS), the Illinois Department of Agriculture (IDA), and the Illinois Department of Natural Resources (IDNR) began a cooperative, interagency initiative to produce statewide land cover information on a recurring basis. The Illinois Interagency Landscape Classification Project (IILCP) completed Cycle 1 of this initiative in the summer of 2002, resulting in the Land Cover of Illinois 1999-2000 inventory and associated database.
In 1995, the original Land Cover of Illinois
(1991-1995) was completed, and the Illinois Department of Natural Resources produced, Illinois Land Cover: An Atlas, to provide statewide and county-based land cover statistics. While the printed publication is now out-of-print, there is an on-line version that can still be viewed at the following address: http://www.dnr.state.il.us/orep/ctap/map/land.htm. Presented in PDF format, this on-line Atlas provides county-by-county statistics for each land cover category, along with some explanatory information.Based upon the experiences of the Atlas, an enhanced on-line version has been developed for the Land Cover of Illinois 1999-2000 inventory. This Statistical Summary section is an integral part of the Land Cover of Illinois website and provides a wide variety of statewide and county-by-county summaries, graphics, and explanatory information compiled from the Land Cover of Illinois 1999-2000 inventory. Additional features are planned for 2003, including land cover statistics summarized by watershed, and a comparison of the Land Cover of Illinois 1991-1995 and 1999-2000 inventories.
Statistical Calculations
The information contained in the Land Cover of Illinois Statistical Summary section has been derived from initial tabulations using the Analysis –>Tabulate Areas function in ArcView v.3.2a. After completion of the initial tabulations, consisting of simple areas in square meters, the results were exported to a dBase file (.dbf file) and subsequently converted to an Microsoft Excel file (.xls file) for the purpose of derivation of the final statistics.
The files used for the initial tabulations of areas include the Illinois county and state boundary
coverages produced by the Illinois State Geological Survey, available at the Illinois Natural Resources Geospatial Data Clearinghouse, and the lcoi_99-00 ESRI GRID coverage. The use of any other boundary files or earlier versions (pre 1998) of boundary files available at the Clearinghouse for area calculations will yield different results. Both the county and state boundary coverages were reprojected to match the projection of the ESRI GRID coverage using ArcView v.3.2a.When a vector-based (polygon) boundaries file is used in conjunction with a raster-based land cover data file to derive areas, a parsing procedure must be employed to compute the initial area tabulations in ArcView. The example below shows a greatly enlarged segment of the lcoi_99-00 ESRI GRID coverage, and the smallest square-shaped pixels (picture element) shown represent a single 30 x 30 meter (98.4 x 98.4 feet, or 0.22 acres) ground resolution cell. A portion of the county boundaries coverage is overlain as a single black line. Note how this line does not bisect each land cover pixel evenly. When the pixel areas for each land cover category are tabulated, the individual 30 x 30 meter cells situated along the vector boundary are assigned to a particular county on a whole pixel basis. That is, the entire pixel is assigned (parsed) to a particular county and no partial cell areas are calculated.
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Because the pixel-based land cover data is not aligned with the vector-based county boundaries, the parsing procedure results in tabulated county areas that differ from the "true" county areas as computed using only the county boundaries data file. Analysis shows these absolute differences to be quite small, and the counties with discrepancies of 20 acres or greater are listed below.
| County | Absolute Difference (in acres) |
| Will | 47 |
| Ford | 43 |
| Cook | 42 |
| Champaign | 41 |
| McLean | 36 |
| Woodford | 30 |
| Grundy | 25 |
| LaSalle | 22 |
| Vermilion | 22 |
| Kane | 20 |
The descriptions of the various individual statistics and how they are computed is provided on the Calculations web page of this Statistical Summary section.