If this is done and a standard classification system is adopted, this database would no doubt be one of the most valuable available for ground water vulnerability assessment of private lands at the county, watershed, and field levels. An accurate photo-image base, such as an orthophotograph, would need to be used instead of an aerial photograph in order to meet national standards and permit effective sharing of these data with others.
Assessment of ground water vulnerability to contamination is a complex task requiring information contained in a variety of geographic databases maintained by federal, state, and local agencies. Early efforts in compiling spatially-referenced data resulted in analog outputs (i.e., paper maps), but more recent efforts have led increasingly to data stored in digital formats. Currently, such databases are used routinely in a wide range of applications, including vulnerability assessments, to produce thematic maps for policy makers and resource management. The production of thematic maps, and similar decision aids, requires retrieval, transfer, manipulation, interpretation, and analysis of the digital information.
Since these data and information have been collected and maintained by a number of entities and for differing purposes, a myriad of problems have been encountered in their use. The National Research Council's Mapping Science Committee (MSC) recently reported that lack of coordination has resulted in duplicative efforts among the federal agencies, at significant cost to the public, and that existing spatial data may not always be compatible or reliable (NRC 1993). On this basis, the MSC report argued for the development of the National Spatial Data Infrastructure (NSDI), which is defined as the ''total ensemble of geographic information at our disposal" as well as all the other resources required to use such information. The MSC concluded that "unless a vision for the National Spatial Data Infrastructure exists and the spatial data bases, policies, and standards are in place to facilitate the access and use of the spatial data on a national scale, opportunities in areas from environment to development will be lost."
The MSC report also recommends that federal efforts expand beyond the compilation of various types of spatial databases and development of standards for data exchange to include "more specific measures and standards of content, quality, currency, and performance of various components" of the proposed NSDI. It is not enough to have easy access to existing spatial data; it is important to know how good is the information contained in these databases. The MSC also recommends that base data (also referred to as minimum data sets in some modeling literature) required for small-, medium-, and large-scale applications of spatial data be identified. Base data requirements for vulnerability assessments are clearly needed.