DIGITAL PHOTOGRAMMETRY FOR STATEMAP GEOLOGIC MAPPING

At the New Mexico Bureau of Geology and Mineral Resources, digital photogrammetry techniques are being developed for collecting three-dimensional, direct-to-digital geologic data as an integral part of the STATEMAP program. We have digital stereo imagery coverage for ~96% of the state, and we use the ERDAS Stereo Analyst extension for the ESRI ArcMap program to draw contacts, faults, and other geologic data directly on the imagery displayed in stereo using an active shutter system. The program can automatically calculate surface elevations from the parallax shift between coinciding locations on overlapping images, such that lines and points digitized this way are attributed with elevation data in addition to horizontal location data. As a result, well-exposed geologies can be accurately mapped digitally, then immediately visualized in appropriate 3D-viewing software. The case study presented here is from recent mapping of the Grants SE 7.5’ quadrangle. This quadrangle lies immediately south of the town of Grants on the Colorado Plateau in the vicinity of the Grants-Mt Taylor uranium mining district. The geology here is dominantly of weakly deformed interbedded shales and sandstones that are inset upon and buried by Quaternary alluvium, eolian material, landslides, and basalt flows. Features visible on the imagery available for this area include topographic steps as low as 1 m tall; basaltic surface textures such as collapse pits, flow tubes, and tumuli; surface and rock coloration, which can be enhanced by the Stereo Analyst software; and gentle folds with interlimb angles as broad as 178°. These and similar features were used to draw contacts and structures directly onto the stereo imagery, and geologic data was collected and revised throughout as digital data. The resulting geologic map and cross-section were superimposed in ArcScene to visualize the geology in a surface-3D format. Drawbacks to this digital stereo technique for mapping include 1) small discrepancies between the aerial imagery base and the topographic map base, which locally results in visible misalignments between stereo-drawn geologic lines and contours; 2) difficulty of use for less computer-savvy mappers; and 3) increased data management complexity.