Illinois Natural History Survey - University of Illinois

A Land Cover Map of Illinois

 A land cover database for all of Illinois was recently completed by researchers at the Natural History Survey as part of the Critical Trends Assessment Project. The Land Cover Database of Illinois represents the most current and comprehensive inventory of the state's surface cover. It is available on CD-ROM to support the ecosystem watch programs of the Illinois Department of Natural Resources (DNR) and is also included on DNR's second release of digital spatial datasets on CD-ROM. This database provides a much needed baseline for evaluating environmental change, supports research, and supports efforts to educate the public.

Satellite data, image-processing software, existing spatial digital databases, and aerial photography were used to classify and map the surface cover of the state's 36 million acres. The Landsat Thematic Mapper satellite collects spectral information for each 28.5 x 28.5-meter piece of the earth, creating a pixel. The distinct spectral value for each of Illinois' nearly 300 million pixels was used to classify them into 1 of 19 land cover classes. The resulting database can be used with the wealth of existing digital information technology and geographic information systems (GIS) to produce maps, perform analysis, and assess change.

To improve discrimination and coding, two dates of satellite imagery were used. The vegetation changes that occur throughout the year are detected in the satellite data and the use of two dates exploits these phenological differences. To further improve classification, urban areas and small towns were analyzed separately from the non-urban landscapes. Crop compliance data from the Natural Resources Conservation Service were essential in discriminating the homogeneous but spectrally diverse agricultural landscape. Color infrared photography was manually interpreted to help classify the satellite data into distinct land cover categories, such as deciduous woodland, water, high density urban, and so on.

Figure 1. Middle Fork of the Vermilion River west of Danville. Predominat cover types
(dark to light) include woodland, grassland, water, and cropland.

The land cover database and map are spatially complex, reflecting the fragmented character of Illinois' landscape. Features such as railroad corridors, greenways, airport runways, commercial strips, and small wood lots are discernible (Figure 1). The size and complexity of the data often require further processing to facilitate analysis. For an inventory of resource-rich areas in Illinois, recently completed at the Survey, the data were aggregated. The percentage of woods and wetlands in each of 816 watersheds was determined from the land cover data. This information was used in conjunction with the acreage of natural areas and occurrence of biologically significant streams to identify the sites depicted in Figure 2. These sites cover less than 20% of the state but they contain one-third of the state's woodlands and nearly half of its wetlands.

To identify potential habitat for the reintroduction of elk in southern Illinois (see the September/October 1996 issue of Illinois Natural History Survey Reports) these data were recoded and spatially aggregated. Each cover type was coded to reflect its value as habitat and cover for elk. The data were then averaged to a spatial resolution of 1 hectare.

Figure 2. Resource-rich areas identified by INHS researchers.

While of great value, the land cover database is but a point on a continuum. With funding from the national Gap Analysis Program (GAP), work is under way to further discriminate natural vegetation. This program seeks to reclassify the land cover database at a resolution that identifies plant communities and alliances. GAP will also map the current distribution of all land vertebrates in Illinois and model habitat suitability.

As data from across the state are integrated, information will increasingly support both regional and local efforts. These efforts will allow biologists or land managers to utilize site-specific data to perform landscape analyses, evaluate ecosystems and watersheds, determine population distribution and dispersal, and quantify habitat fragmentation. Spatially integrated data are critical to the continued understanding and management of the state's natural resources. Continued refinement of the land cover and other databases expands our ability to better understand the nature of Illinois.

Mark Joselyn, Liane Suloway, and Tony McKinney, Center for Wildlife Ecology

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Subject: INHSPUB-00440
Last Modified 11/05/96

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