Purpose

Vegetation acts as an integrator of many physical and biological properties of an area. IL-GAP characterizes the extent and distribution of existing vegetated cover in Illinois for the following objectives: determination of species and natural community representation within areas being managed for biodiversity conservation, to provide geospatial data that can be used to model wildlife habitat distributions, and development of a single temporal data set for monitoring trends.

Land Cover Mapping

A detailed Gap Analysis classification of the vegetated areas of the state has been completed using the same methodology as for the Land Cover Data for Illinois 1999-2000, also referred to as the Illinois Interagency Classification Project (IILCP). There is one notable exception of methodological differences in forested and urbanized areas of northeastern Illinois. Within each of the ten Landsat Thematic Mapper (TM) satellite Path/Row scene areas that comprise the state, the initial classification of forested lands accomplished in the IILCP were used as masks to segregate all forested areas for further analysis.

The same Landsat TM triplicate imagery used for the IILCP classification was used for the IL-GAP forested lands classification analysis. An additional channel vector, referred to as a Slope-Aspect-Index (SAI), was added to the TM multispectral channel data prior to the classification procedures. The SAI was produced using a statewide composite of U.S. Geological Survey 1-arc second Digital Elevation Model (DEM) data. The SAI combines the measurements of slope and aspect into a unique scalar value, and was incorporated with the multispectral imagery to further refine the classification of forested lands, especially within areas of the state that exhibit dissected surface topography.

The forested lands were divided into Floodplain and Upland types, with each having three sub-categories. Swamp was also delineated. The ability to detect differences between Floodplain and Upland forest was largely based on the DEM component. The sub-categorization of community level forest types was performed using soil surveys as ancillary data. These classification guidelines are taken from the Natural Areas Inventory of Illinois Technical Report (J. White and M. H. Madany 1978).

The IL-GAP urbanized land cover differs from the IILCP only in Landsat TM scene 2331 which contains Chicago. The presence of a highly urbanized city necessitated the classification of the urban landscape into finer levels. In TM Scene 2331 three density levels of urban were delineated (High, Medium, and Low) and retained in the IL-GAP land cover. In the IILCP the Medium and Low Density categories were combined. For a detailed description on how the urban areas were classified, see Challenges in Land Cover Classification in Areas of Rapid Urban Expansion.

The additional categories which have been identified with IL-GAP are denoted in red.

References

J. White and M. H. Madany, 1978. "Classification of natural communities in Illinois." Pages 310-405 (Appendix 30) in J. White, Illinois Natural Areas Inventory Technical Report. Volume 1: Survey Methods and Results. Illinois Natural Areas Inventory, Department of Landscape Architecture, University of Illinois at Urbana Champaign.