STRUCTURAL GEOLOGY GEOMETRIC OPERATIONS IN A GIS PROGRAM AND GOOGLE EARTH

GIS software provides 3D visualization and computation for structural geology problems. By using a digital elevation model (DEM), aerial photography, and georegistered geological maps, both professionals and students can rapidly assess geometric relationships. Digitizing a geologic contact on the geologic map, or a key bed on the imagery, allows computation of the dip and strike using the classic three point problem. The program can rapidly compute the orientation from each grouping of three sequential points along the line. It can also create a planar trend surface from all points on the contact, and the individual deviations from the planar surface, which can highlight changes from a simple plane, or errors in mapping or digitizing the contact. By digitizing two contacts, the program can compute unit thickness, and how it varies along strike. The software can also project planar contacts beyond the digitized contacts, include showing where units should emerge on the back side of topography.

Readily available digital data supports this analysis. The US Geological Survey (USGS) provides 1/3” (about 10 m) DEMs for all of the continental United States for easy download, and Web Map Service (WMS) orthoimagery at 1 m resolution that GIS software that download on demand to cover the current map view. In addition, USGS geologic maps are available in georegistered formats including Keyhole Markup Language (KML or zip-compressed KMZ) that can be displayed in both GIS software and simple viewers like Google Earth. Google Earth allows easy 3D viewing of maps, and intuitive digitizing of contacts. While Google Earth cannot perform the geometric calculations, its files can be imported into GIS software for analysis there, or the digitizing can be done in GIS software. The algorithms have been implemented in the freeware program MICRODEM (http://www.usna.edu/Users/oceano/pguth/website/microdem/microdem.htm).