The most significant change has been within agricultural lands, in that all of the major field crops have been differentiated. The USDA-NASS collected field-by-field ground reference data for 425 sites, each approximately 1 mi2, across Illinois during the 1999 and 2000 crop years. These reference data were then digitized onto same-year Landsat 5 Thematic Mapper (TM) or Landsat 7 Enhanced Thematic Mapper (ETM+) satellite imagery. Using these reference data as training sites, USDA-NASS remote sensing scientists then employed a directed supervised classification approach to identify the same field crops across the entire state.
|1991-1995 Thematic Mapper Coverage Map|
|1999-2000 Thematic Mapper Coverage Map|
In the 1991-1995 land cover inventory, two dates of Landsat 4 and 5 Thematic Mapper (TM) imagery for each of the ten Path/Row scene areas were acquired during the spring and fall seasons and used as the basis for the classification analysis. Six of the ten Path/Row scene areas contained TM imagery from different years (1991-1995 TM Coverage Map).
In contrast, the 1999-2000 land cover inventory used three dates of same-year Landsat 5 TM and/or Landsat 7 ETM+ imagery for each Path/Row scene area acquired during the spring-summer-fall periods (1999-2000 TM Coverage Map). p>
The time span of the primary Landsat TM imagery was four years for the 1991-1995 land cover inventory; two years of TM/ETM+ imagery were used for the 1999-2000 inventory. p>
The National Wetlands Inventory (NWI) was largely used as the surrogate for wetland cover in the 1991-1995 inventory. Because portions of the source information are now twenty years old, the NWI was not used in the 1999-2000 land cover inventory. Instead, the delineation was accomplished through a combination of unsupervised classification of same year, spring-summer-fall Landsat TM/ETM+ imagery and photo-interpretation of 1998-1999 U.S. Geological Survey (USGS) National Aerial Photography Program (NAPP 3) photographic prints. p>
Forested lands were given additional attention in that these areas were characterized as part of the Illinois portion of the USGS National Gap Analysis Project. The initial classification of forested lands by TM Path/Row scene areas were used as masks to segregate these forest areas for further analysis by remote sensing scientists at the Illinois Natural History Survey.
A comprehensive accuracy assessment procedure was conducted for the 1999-2000 land cover inventory. Field-specific ground reference data were provided by the USDA-NASS for agricultural lands, the IDNR CTAP program provided ground reference data for forested tracts, and a number of additional areas were checked using photo-inspection and interpretation of NAPP 3 aerial photography. Consult the Accuracy Assessment Section for details.
No accuracy assessment was conducted for the 1991-1995 land cover inventory.
A study was performed to assess the potential of conducting land cover change analysis with the 1999-2000 and 1991-19995 data products. The report and accompanying appendices are available: