Determination of High and Low Points
The high and low points of Illinois counties were determined through the use of a Geographic Information System (GIS) and data kept online at the Illinois State Geological Survey. A GIS is a tool that can be used to store, manipulate, and analyze geographic data. The source data used in this processing are United States Geological Survey (USGS) 3-arc-second Digital Elevation Model (DEM) data. The DEM is a rectangular array of elevations at every third second of latitude and longitude (approximately 300 feet or 90 meters in Illinois).
Determining the amount of relief
The GIS was used to clip the DEM to each individual county. The highest elevation and the lowest elevation were determined for each county, and point locations for each were produced. Because of the coarseness of the source DEM, there were often several identical high or low elevation points. The actual high or low elevation was then determined by comparing the high and low location points to the USGS Digital Raster Graphic (DRG) file of the appropriate 7.5-minute quadrangle. The DRG is a scanned image of the traditional 7.5-minute topographic map. High and low point locations were adjusted to the more precise location, and elevation values were corrected.
Determining the average percent slope
The GIS was used to calculate the slope of each county based on the county DEM. The average percent slope is determined through the following equation: percent slope = (rise / run) × 100. Therefore, a 45 degree slope is a 100% slope (5/5) × 100 = 100). Percent slope can range from 0% to very high percentages. For example, a nearly vertical cliff face that is 100 feet tall over a distance of 1 linear foot has a 10,000% slope.
You can find the data for your own county at the Illinois Natural Resources Geospatial Data Clearinghouse.