LULC data are usually represented as either mapped areas with associated attributes, or statistical indices for given areas such as a county or state. In both instances, data are collected using a standardized classification scheme. Most land use or land cover mapping projects manually interpret areas from remotely sensed imagery and then digitize class boundaries.
The classification scheme applied to LULC data is perhaps the most critical factor in determining the value of this information for ground water vulnerability assessment. For example, classes that are useful for forestry applications may not be appropriate for hydrologic investigations and vice versa. The spatial resolution of LULC data is also critical to use of these data. Large polygons generally will be more heterogeneous than smaller ones, and it is therefore more difficult to estimate meaningful average values for large polygons.
Land use and land cover are important data elements for many agencies of the federal government (USGS 1992). These data are used to evaluate current status, changing conditions, and resources over large areas in support of agency initiatives and planning operations. Many agencies have been digitizing existing and collecting new LULC data for their specific purposes, resulting in a mixture of data formats, LULC classification systems, and scales and accuracies.
The USGS is producing LULC maps and associated files for the entire United States. Most of the available files have been digitized using the Geographic Information Retrieval and Analysis System (GIRAS) format, and are available as either vector or raster files. The data are distributed in 1 degree of latitude by 2 degrees of longitude corresponding to the standard 1:250,000 topographic mapping series. The minimum resolution for GIRAS data is 4 hectares in urban and built-up lands and 16 hectares in other areas. The GIRAS digital data provide LULC information using the Level II Anderson classification (Anderson et al. 1976). Digital LULC data are distributed by the USGS/NMD on nine-track tape and will be distributed soon on CD-ROM disks.
Many other federal agencies, such as the Forest Service, Bureau of Land Management, Tennessee Valley Authority, and Fish and Wildlife Service, have collected their own LULC data or have modified existing GIRAS files. Many state and local government agencies also have extensive LULC data collection programs, primarily at the 1:24,000 scale.
LULC data are by nature transitional—each file provides only a snapshot of the environment at one point in time. Vulnerability assessment