TURNING A GOOD OLD MAP INTO A BETTER NEW MAP – AN EXAMPLE FROM ELK RIDGE, UTAH

Recently, the U.S. Forest Service approached the Utah Geological Survey with the need for a 1:24,000-scale geographic information system (GIS) geologic map dataset of the Elk Ridge area of southeastern Utah. To save costs, we preferred to create the dataset from published maps. The U.S. Geological Survey (USGS) published preliminary maps of the area in 1958-59, and a compiled map at 1:62,500-scale in 1965. They devoted several person-years to this mapping, which we could not duplicate. In addition, they had access to uranium mines, prospects, and road cuts that are no longer open, providing significantly better exposures than exist today. On the other hand, map features do not fit modern base maps in many areas, several important contacts have been redefined since publication, and the authors paid scant attention to surficial deposits, making the maps unsuitable unless modified.

Recognizing that the USGS compiled map is generally quite good and that we could not duplicate their field effort, we determined to use it, but to improve it as much as we could. We vectorized the map and imported the data into VROne™ CAD software. We then systematically revised the placement of nearly every contact and fault using 1:24,000-scale preliminary geologic maps, digital orthophoto quadrangles, stereo aerial photographs, spot field mapping, and field-collected strategic GPS points as guides. We had to make some compromises, primarily because we could not visit many questionable features in the field. For example: we could improve the spatial accuracy of many contacts, but some had to be left as they were due to poor exposures; we were able to adjust most contacts to properly “V” in washes and over ridges, but could not adjust some contacts in areas without distinctive topography; we hoped to significantly improve mapping of surficial deposits, but, other than a few specific changes, we soon realized that maintaining consistency required more field mapping than we could do; and we found that some redefined contacts are not readily apparent and could not be remapped. The final result is a GIS dataset that is significantly improved and generally approaches 1:24,000-scale precision (but locally is only accurate to about 1:62,500-scale). This map was produced at a large cost savings compared to new mapping, but will still meet the needs of nearly all users.