FINDSD: AN APPLICATION FOR DETERMINATION OF BEDDING-PLANE ATTITUDES FROM HIGH-RESOLUTION DIGITAL ELEVATION MODELS

We present the application findSD, which allows for determination of planar orientations using readily available high-resolution bare-earth digital elevation models (DEMs), such as the 1-m gridded data available through the USGS 3D Elevation Program. When such DEMs are displayed with a nadir hillshade and high vertical exaggeration (or equivalently a slope map displayed over the full range of slopes), the bedding is generally so clearly delineated that the images effectively become a new type of imagery. findSD presents coupled plan-view map, draped 3D surface, and stereonet views. It facilitates determination of planar orientations by digitizing “formlines” in 3D. These formlines represent the geologist’s interpretation of the local bedding (or perhaps other planar feature) on a visualized 3D surface with a draped hillshade or other imported imagery. A best-fitting plane is computed from the XYZ points using an orthogonal regression to solve for the eigenvectors of the covariance matrix. The minimum eigenvector represents the orientation of the pole to the plane, and the strike and dip are then computed. The centroid of the points is used as the location of the remotely extracted bedding-plane attitude. The coplanarity of the points can be evaluated, and the plane visualized with the rendered DEM surface along with planes from field measurements or other remotely-derived bedding planes. findSD can also be used to extract strike and dip measurements from existing geologic maps. This is done by digitizing in map view the strike symbol with two end points, and then manually entering the dip magnitude. The bearing of the XY locations of the two points determines the strike, and the mean the XY location. Z (elevation) is extracted at this location from a DEM. Other field attitudes can be imported in the program, and all measurements can be exported as text data or in standard formats used in GIS applications. Such an environment improves geologic mapping in inaccessible areas, in the integration and verification of field measurements, and visualization of geologic structures in an easy-to-use fashion. We demonstrate the functionality with examples from the Valley and Ridge province of the Southern Central Appalachians where significantly higher density of measurements can be obtained relative to conventional field work.