HEADWATERS PROVINCE, IDAHO AND MONTANA: DIGITAL GEOLOGIC MAPS FOR LAND MANAGEMENT

Geologic maps have long been used to understand Earth history, mineral and energy resource potential, water resources, and hazards related to earth processes. The US Forest Service also uses geologic maps to delineate disturbance regimes--domains that have similar response characteristics to disturbances such as fires, road building, and timber harvesting. Recent findings involving tree growth response to fertilization and natural tree-mortality dynamics on different lithologies have demonstrated a significant influence of rock type on forest health.

Through the Headwaters Project, USGS is compiling a regionally consistent and integrated spatial database ("digital geologic map") for northern Idaho and western Montana, comprised of 43 individual 1:100,000- to 1:250,000-scale map tiles to meet the planning and science needs of the USFS and USGS. The effort brings maps published both traditionally and digitally by USGS, Idaho Geological Survey, Montana Bureau of Mines and Geology, and Washington Division of Geology and Earth Resources into a common database format.

A geologic map is an interpretive cartographic product that uses complex symbologies to record observations and concepts. However, standards and conventions used to prepare geologic maps vary from agency to agency, author to author, and with time. Descriptions of map units may be qualitative and inconsistent; the amount of detail varies depending on the interests of the author or the intended use of the map. Bringing these diverse styles and information contents into a common format has proven to be a significant challenge.

The common database format and rich attribution of polygons in the spatial database promotes regional analysis and integration with other spatial data including climate, forest patterns, wildlife distributions and habitat, fire patterns, logging, other disturbance regimes, or planning requirements. Derivative maps based on lithology, chemistry, age, or any combination of other attributes may quickly be made, enhancing the usefulness of the database for questions unanticipated at the outset of the project.