Impervious surface area for Pennsylvania was estimated from Thematic Mapper data using algorithms developed by Dr. Toby Carlson. The Value attribute indicates percentage of the 25 meter grid cell that is impervious and range from 0 to 100 and use integer rather than decimal values for reduced storage volume. Date of the imagery ranged from 1999 to 2002, availability depended on extent of cloud cover at time of acquisition. All images were collected for the late Spring or Summer months (May-August). The Pennsylvania Department of Transportation supported the construction of the impervious surface data, with technical assistance from Eric Warner and Deborah Slawson.