DIGITAL GEOLOGIC MAPPING IN THE YUCCA MOUNTAIN AREA, NYE COUNTY, NEVADA

A stand-alone, handheld Global Positioning System (GPS) receiver with a real-time display device, field-mapping software, and high-resolution digital ortho-rectified aerial photographs were integrated into a digital mapping system. This digital mapping system was field tested during detailed geologic mapping (1:6000 scale) in the vicinity of Yucca Mountain, Nevada, in an area of exposed volcanic terrane, where high-silica rhyolitic welded tuffs are displaced by numerous normal faults with a few to hundreds of meters of offset. Mapped contacts included boundaries between major pyroclastic flow deposits and subtle zonal variations within welded tuffs.

The display device integrated with the GPS receiver showed the geologist's location on the digital orthophotograph to a designed accuracy of about 10 meters. However, GPS location accuracy was usually much better than 10 meters. The accurate location of geologic features was recorded commonly using two techniques: (1) the geologist could digitally trace a contact or fault by walking them out as the display device recorded continuous location data provided by the GPS, or (2) the geologist could digitally draw the features on the digital orthophotograph. The first mapping technique was useful for recording locations of subtle geologic features that required close inspection, such as lithostratigraphic zonal variations, fractures, and breccia. The second technique was useful for recording large-scale features viewed at a distance across valleys and washes. Geologic features and their attributes were digitally recorded in the field, saving time by not digitizing field maps in the office and avoiding transcription errors from field maps to compilation maps. Digital geologic mapping saved time and improved accuracy for locating features in the field. Final map products were digitally produced and edited more efficiently than previous maps of this area.