COMPUTER MANIPULATION OF SIMPLE 3D STRUCTURES: SUPPLEMENT, COMPLEMENT, OR REPLACE THE FIELD EXPERIENCE?

Outcrop patterns of faults, dikes, and sedimentary beds reflect the interplay of 3D orientations and the topographic land surface. A key task for geologists, and hence geology students, requires extrapolating from the depiction on a geologic map to a visual model of the topography and geology. A related task starts with a measured orientation, and requires estimation of where the structure projects on the map. Two simple geometric problems from the basis for much of this visualization: the three point problem, and the projection of a measured dip and strike across topography. A geographical information system (GIS), starting with a digital elevation model (DEM), can easily manipulate both types of data with a variety of base maps: scanned topographic or geologic maps, air photos, or satellite imagery. Given suitable large scale digital base maps, students can digitize contacts and compute attitudes on the computer, observe 3D representations of the terrain and rotate them to any desired orientation. The MICRODEM GIS implements these geometric operations, in both a simple teaching version that emphasizes only the geometric relationships, and as part of a full-featured GIS program. MICRODEM can be used in preparation for going to the field to help students learn to visualize, for 3D stereo virtual field trips on the Geowall, and taken into the field on a tablet PC for improved mapping efficiency. GPS tied to the PC removes the tension students feel in locating their position on the map, and lets students can concentrate on matching the map features to the terrain in front of them and thinking about the geometric structures of the geologic features. In addition to improving geometric perceptions, using the GIS introduces students to valuable technology.