Like most state geological surveys, the Utah Geological Survey (UGS) is faced with the growing challenge of producing digital geologic maps. In 1991, the UGS made its first move into digital map production through the acquisition of an analytical photogrammetric stereoplotter. Traditional map production methods require hand-copying geologic features four or five times between field mapping and printing or digitizing. An analytical stereoplotter eliminates up to three of these steps. The UGS has digitally compiled 27 7.5' geologic quadrangle maps directly from aerial photographs. The UGS is now entering the next phase of digital map production, converting maps into Geographic Information System (GIS) databases and using software to create high-resolution line work and color separates for hard-copy publication. The UGS hopes to do all stages of map production digitally within the next few years.
A typical method of producing geologic maps from the 1960s to the mid-1980s was to visually transfer field mapping directly from aerial photographs to mylar base maps. This was done by placing the base map and aerial photographs side-by-side and comparing contours with stream channels, ridges, and other topographic features to visually estimate the locations of contacts, faults, etc. As a result, the precision was commonly poor and inconsistent. The advent of ground-rectified 7.5' orthophotoquadrangle maps in the 1980s was an improvement. Many more features could be seen on both aerial photographs and the orthophotoquads, aiding in properly locating geologic lines and symbols. In some cases, the formation contacts themselves can be seen. Transferring field mapping to orthophotoquads, and then tracing onto the topographic base map, is still a common method of creating geologic maps. However, geologic features are typically hand-drawn or copied five times between field mapping and final digital map production: (1) original field mapping on aerial photographs, (2) visually transferring to the orthophotoquad map, (3) tracing onto a mylar base map, (4) cartographic scribing for map publication, and (5) hand digitizing from the mylar or scribe plate. Each time, some detail is invariably lost and errors are introduced; even the most careful geologist or cartographer has a tendency to round corners, veer off lines, miss subtle line flexures, or introduce new lines, bends, or features.
Two ways to reduce hand-copying are to electronically scan the scribe or mylar plate, or digitally compile the field mapping directly from aerial photographs. Scanning, in which the scribed or mylar map is digitally scanned using a large-format scanner and then vectorized, eliminates the digitizing step. Results are mixed, depending primarily upon the amount of "noise" introduced during the scanning process. Cleanup of a scanned map commonly requires several hours to days of work.
Digital compilation of field mapping directly from aerial photographs using analytical photogrammetry offers four primary advantages over hand-copying: (1) map data are copied only twice instead of five times, saving time and reducing the loss of detail and introduction of errors; (2) the field geologists compile their own mapping directly from aerial photographs, allowing them to edit and correct their work in 3D stereo view, and reducing the possibility of cartographers misinterpreting their work; (3) analytical stereoplotters correct for inherent problems in aerial photographs, such as earth curvature, variations in aircraft altitude and attitude, and ground elevation changes. The resulting stereo model has great precision in the x, y, and z planes, allowing the geologist to actually improve placement of contacts, increase detail, and solve three-point problems; and (4) the digital map information is three-dimensional and amenable to either digital or hard-copy map production.
Primary disadvantages of digital compilation are: (1) contacts on steep slopes and cliffs can be difficult to see on aerial photographs because of shadows or "compression" of contacts into a small area; (2) locally, digital mapping may be more accurate than the topographic base maps, resulting in geologic contacts that do not fit the map contours properly; and (3) revisions late in the process may cause repetition of already-completed steps (though still less effort than making revisions after additional work is completed by hand).
After considering various options, the UGS purchased an Alpha 2000 analytical stereoplotter made by International Imaging Systems, Inc. Unfortunately, International Imaging Systems, Inc. was subsequently purchased by another company that discontinued stereoplotter production and support. However, other companies make similar analytical plotters for under $100,000. The stereoplotter is driven by a 25 MHz 386 personal computer. The proprietary software includes functions for rectifying aerial photographs and configuring digital data.
The stereoplotter software delivers a stream of three-dimensional UTM or state-plane coordinate data to a 166 MHz Pentium personal computer with CADMAP photogrammetry software, a product of Carl Zeiss, Inc. CADMAP is specifically designed for map preparation and contains many geologic map symbols and features. For example, it has a function to calculate strike and dip by digitizing three points on an inclined stratigraphic bed or fault directly from aerial photographs. Its type font selection is limited and unable to create special characters such as the Triassic and Pennsylvanian symbols. We use CADMAP to create all line work, geologic map symbols, and preliminary labels. Currently, the UGS is switching from a UNIX operating system to a Microsoft Windows NT based CADMAP system. CADMAP is excellent for geologic-map preparation, but it is not GIS software. For that purpose, the UGS uses Arc/Info software for Windows NT, which includes features for preparing maps for hard-copy publication.