Geology Reference
In-Depth Information
2.2.2
Geologic Mapping in 3-D
Widely available computer technology now makes it possible to construct and inter-
pret geological maps in 3-D. Three-dimensional geologic outcrop mapping usually
begins with a digital elevation model (DEM). A DEM is a grid of elevation values and
their corresponding
xy
coordinates (Bolstad 2002). The DEM is contoured (typically
by triangulation because it is computationally fast and fits the control points, see
Chap. 3) to produce a computer visualization of the topographic surface (Fig. 2.3). The
accuracy of the most widely available DEMs is not as great as the corresponding topo-
graphic map, which may be important in critical applications. For DEMs produced at
the scale of a 7.5-minute quadrangle and derived from a photogrammetric source,
90 percent have a vertical accuracy of 7-meter root mean square error or better and
10 percent are in the 8- to 15-meter range (source USGS EROS Data Center).
Geological contact lines can be constructed as three-dimensional traces on the surface
of the contoured DEM. This can be achieved by extracting lines from the 3-D topo-
graphic surface within 3-D software (Fig. 2.4), or by mapping 3-D lines in outcrop using
a Global Positioning System (GPS) receiver, which provides a digital file that can be
used with the computer map (Bolstad 2002). Extracting lines from the contoured DEM
usually is done over a superimposed image of the hard-copy geological map. Geographic
information system (GIS) software allows the image of a geological map to be draped
over the DEM surface, giving a realistic view of the map surface. Images used for this
purpose need to be ortho rectified, aligned and georeferenced to fit the DEM. Once the
Fig. 2.3.
Oblique view to the NE of the Blount Springs area, Alabama, 50 ft topographic contours derived
from a 30 m Digital Elevation Model (NAD-27). Horizontal coordinates are UTMs in meters, no vertical
exaggeration
