A CONCEPTUAL FRAMEWORK FOR SYNTHESIS OF DIGITAL GEOLOGIC MAPS

We synthesize geologic maps to create synoptic views that educate, entertain, and elicit new hypotheses. We also synthesize to facilitate large-area queries such as: What is under my house, or any house in this state? Where, in a multi-state area, are there large bodies of gravel?

Synthesis of digital source maps proceeds by four steps: (1) Establish schematic consistency by translating sources to a common database schema. (2) Establish semantic consistency by classifying source-map units in a common taxonomy (a compilation stratigraphy); this is the meat of the synthesis effort. Similar classification of types of contacts and faults, fold axes, etc., is required but is relatively trivial. Once schematic and semantic consistency are established and source maps are appended, large-area queries can be posed and answered. The accuracy of the synthetic map is improved and visualizations are made more effective by two further steps: (3) Establish stratigraphic consistency by using Steno’s laws, Walther’s law of facies, Dahlstrom’s balanced-section principles, and other wisdom to refine source-map geology and minimize map-border faults. (4) Generalize by aggregating source-map units and simplifying feature geometry.

Unit taxonomies combine similar source-map units to ease comprehension and disclose regional patterns. Taxonomies may be ordered lists, matrices of time and geologic process, or—commonly—dendritic hierarchies organized mostly by age. Taxonomies may be designed to emphasize lithology (e.g., GeoMaterials in GeMS), significance for mineralization, vertical and lateral tectonics, or other themes. A single set of source-map unit polygons may support multiple taxonomies. Tests of the utility of a taxonomy include: Are source-map units easily classified? Are map-border faults minimized? Are unit distinctions within a single source map preserved? Does the taxonomy facilitate generalization? Does it portray a coherent and useful vision of regional geology?