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Illinois Gap Analysis Program Land Cover Classification


The National Gap Analysis Program, administered within the Biological Resources Division of the U.S. Geological Survey, seeks to manage biodiversity at the landscape and community levels. The main goal of Gap Analysis is to prevent additional species from being listed as threatened or endangered. The program uses a "bottom-up" approach that allows for creativity and collaboration at the state and local governmental levels where most land management decisions are made. The intent of Gap Analysis is to provide focus and direction for proactive, rather than reactive land management activities. It is anticipated that Gap Analysis is a step toward comprehensive land management planning that transcends political boundaries.

The Illinois Gap Analysis Project (IL-GAP, was initiated in 1996 at the Illinois Natural History Survey (INHS,, which is a division of the Illinois Department of Natural Resources. The INHS has been conducting research on Illinois' biological resources since 1858, which makes it the ideal institution for conducting the state's Gap Analysis. There are four major components of IL-GAP, which are as follows: (1) land cover mapping and classification, (2) vertebrate distribution mapping and modeling, (3) land stewardship mapping and categorization, and (4) data analysis. The focus of the Gap Analysis project included within this web site is the land cover mapping and classification component.


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: 1) determination of species and natural community representation within areas being managed for biodiversity conservation, 2) to provide geospatial data that can be used to model wildlife habitat distributions, and 3) 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 Illinois Interagency Landscape Classification Project (IILCP), with the notable exception of methodological differences in forested and urbanized areas. 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 (High, Medium, and Low) were delineated 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.

Value Land Cover Category
11 Corn
12 Soybeans
13 Winter Wheat
14 Other Small Grains and Hay
15 Winter Wheat/Soybeans
16 Other Agriculture
17 Rural Grassland
21 Upland


Dry Upland *


Dry-Mesic Upland *


Mesic Upland *

25 Partial Canopy/Savannah Upland
26 Coniferous
31 High Density
32 Low/Medium Density (excluding TM Scene 2331)


Medium Density (TM Scene 2331) *


Low Density (TM Scene 2331) *

35 Urban Open Space
41 Shallow Marsh/Wet Meadow
42 Deep Marsh
43 Seasonally/Temporarily Flooded
44 Floodplain Forest


Mesic Floodplain Forest *


Wet-Mesic Floodplain Forest *


Wet Floodplain Forest *

48 Swamp
49 Shallow Water
51 Surface Water
52 Barren and Exposed Land
53 Clouds
54 Cloud Shadows

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



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.