THE NATIONAL GEOLOGIC MAP DATABASE (NGMDB): PROTOTYPE STATE DATABASES WITH ALASKA AS AN EXAMPLE

We report on a USGS effort, in collaboration with several State Geological Surveys, to produce an integrated digital national database of state-scale (1:500,000) geologic maps. This is part of a project to create and populate national databases of geological, geochemical, geophysical, and mineral deposit data. Contracts with state agencies have resulted in the updating or creation of some new state geologic maps. Our geologic map compilations require the development of a uniform data structure and attribution scheme, which contains information related to geologic units and contacts. As the NGMDB develops, we expect that the datasets we develop will be integrated into it. We are preparing a new geologic map of Alaska -- a composite map, showing bedrock and surficial deposits. The existing state map was produced non-digitally at a scale of 1:2,500,000 and is unsuitable for many uses due to base map issues and the generalized nature of the map. Our new map is based on published, unpublished, and new mapping at 1:250,000 scale combined with additions from 1:63,360 scale mapping. Stored as 1:250,000-scale quadrangle coverages, the state map is assembled on the fly. Digital methods facilitate the geologic compilation of Alaskan geology and creation of a rich database of geologic attributes. We use Arc/Info specifically for its ability to store data as “network” coverages. An important aspect of the coverage structure is the ability to store attributes, including type and direction of linear features, such as the relative upper plates of thrust faults, the downthrown side of a normal fault, and the direction of glacier movement relative to a terminal moraine position. All of this information can be utilized in a relational manner to allow production and analysis of specialized maps. Quadrangle coverages are created from multiple sources and contain coding to indicate the original sources, brief unit descriptions, and the geologic unit assigned by the original author(s). An additional attribute is added that correlates geologic units to adjacent quadrangles and serves as a regional unit assignment. This attribute is then linked through lookup tables and related files to labels, colors, lithology, form (flow, bed, etc.), and age. Other attributes including radiometric age and pluton characteristics are planned additions.