Digital Mapping Techniques '01 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 01-223

Data Capture Techniques for the Digital Database of the Monterey Quadrangle, California

By David L. Wagner

California Division of Mines and Geology
80-1 K St. MS 12-31
Sacramento, CA 95814
Telephone: (916) 324-7380
Fax: (916) 322-4765
e-mail: dwagner@consrv.ca.gov

INTRODUCTION

This map and database are a step forward in the understanding of geology of coastal California. Monterey Bay is a marine sanctuary that has been intensively studied by the Moss Landing Marine Laboratories (MLML), the Monterey Bay Aquarium Research Institute (MBARI) and the U.S. Geological Survey (USGS). As a consequence, the geology and the physiography of Monterey Bay is sufficiently well known to prepare a seamless geologic map of the onshore and offshore. Digital geologic and physiographic data were compiled independently by CDMG, MLML, MBARI, and USGS and integrated into a single database. The Monterey Quadrangle is an example of the benefits of cooperation between research-oriented institutions that emphasize data-gathering activities and a state survey that emphasizes data distribution.

COMPILATION

The base for the Monterey quadrangle consists of a shaded-relief and a topographic map. The topographic base covers only the onshore part of the quadrangle. It is a digital line graph (DLG) of the Monterey and part of the Palo Alto 1:100,000 scale quadrangles provided by C. M. Wentworth (USGS). David Ramsey (USGS) assembled the altitude grid from 30 meter DEMs of the 7.5' quadrangles that make up the Monterey 1:100,000 quadrangle that he downloaded from the USGS database in Sioux Falls. George Saucedo (CDMG) generated the onshore part of the shaded-relief map from a 30-meter digital elevation model (DEM) using the technique described by Haugerud and Greenberg (1998). Norman Maher of the Monterey Bay Aquarium Research Institute (MBARI) prepared the shaded-relief image of the Monterey Bay submarine canyon and surrounding area from Simrad EM300 (30kHz) multibeam bathymetric data collected by MBARI and the USGS. He also provided the bathymetric contours. Steve Eittreim (USGS) provided imagery for the Monterey Bay shelf (water less than 150 deep). That imagery is available online at http://terraweb.wr.usgs.gov/TRS/projects/MontereySonar. Versions of the map with and without the shaded-relief base are included because the gray tones of the shaded-relief darken the lighter-colored tones of the map units. The shaded-relief base map particularly affects the yellow surficial units.

DATA CAPTURE TECHNIQUES

Table 1 shows a comparison of estimates of the time required for tasks in the preparation of a 1:100,000-scale quadrangle made by Wagner (1997) to the time each step actually required in preparation of the Monterey Quadrangle. The time required for digitizing individual 7.5' quadrangles was overestimated by a factor of 3 to 4. Thus the earlier estimate for a single 7.5' quadrangle of about $2280 was clearly too high and is now considered to be about $500 to $700. Part of the overestimation is due to the growing proficiency of the digitizers as they gain experience. Table 1 also demonstrates that while digitizing 7.5' quadrangles is a relatively inexpensive, straightforward process, tiling them together to make a seamless map is not. The main problem here is complexities of matching the geology across map borders (edgematching).

Table 1. Comparison of estimates by Wagner (1997) of time required for tasks in compilation of a 1:100,000-scale quadrangle to the time actually spent on the same tasks during compilation of the Monterey quadrangle. The first three steps are the digitization of the individual 1:24.000-scale quadrangles and last two are assembling the quadrangles to produce a seamless map. The edgematch and review steps are iterative, time-consuming tasks that are difficult to quantify. Close collaboration between the geologist/compiler and the digitizer is required for this part of the project.

Several factors affect edgematching, including: 1) inconsistencies between mappers; 2) inconsistencies in geologic nomenclature; 3) complex geology where there are rapid facies changes and time transgressive units; and 4) scale differences. Inconsistencies between mappers are the most vexing problems facing any compiler of geologic maps. These inconsistencies may arise from scientific perspectives of the mapper, evolution of the understanding of the geology of an area, or the reason why the mapping was done. A map by an academic researcher may have a completely different emphasis than a map prepared by an engineering geologist concerned with the siting of a critical facility. Geologic nomenclature evolves over time and unless a compiler is familiar with how the nomenclature has evolved it may be very difficult to produce a coherent map. As the understanding of geologic relationships evolve, maps portraying the same rock units may be quite different. In California for example, most of the northern and central coastal ranges are underlain by the Franciscan Complex, a tectonically deformed, lithologically diverse collection of rocks now considered to be the type example of a subduction complex. Originally named the Franciscan Formation, mappers labored unsuccessfully for decades to describe it in terms of classical stratigraphic nomenclature. Eventually the Code of Stratigraphic Nomenclature was revised to account for such situations. As a result, newer maps depict the geology much differently than the older ones. Economic interests drove most of the early mapping done in California. Thus until fairly recently, most mapping was done in Tertiary marine terranes in search of hydrocarbons, or in igneous/metamorphic terranes where most mineralized rock occurs. These maps tend to lump most Quaternary units into alluvium and mostly ignore landslides or other features considered superficial. In contrast, mapping today tends to be driven by geologic hazards considerations, engineering applications, land-use, and environmental issues so the superficial features and materials have prominence. Time and effort expended in investigating and resolving inconsistencies before the digital compilation begins can lessen the seemingly endless revisions after the initial compilation is completed.

Prior to beginning the digital compilation, we compiled an analog version of the Monterey Quadrangle that proved invaluable. My experience as a compiler of regional geologic maps (scales of 1:100,000 and smaller) has convinced me that establishing the stratigraphic framework and the preparation of the map explanation requires more time and effort than the preparation of the map itself. Preparation of an analog map is expensive and time consuming but it more than pays off in the long run. It is analogous to a set of plans to guide the construction of a building. The compiler needs to visualize how the pieces are to fit together before the project starts and not have to make midcourse corrections. This however adds at least a year to the project and requires one to two person years of geologist time.

BENEFIT OF DIGITIZING AT THE LARGEST SCALE POSSIBLE

An example of the utility of this approach was recently demonstrated when CDMG received a request from a sister agency, the Division of Oil, Gas and Geothermal Resources (DOGGR). DOGGR has a program to mitigate the effects of improperly abandoned oil wells that are leaking. They were evaluating old wells in the Sargent Oil field central California Coast Ranges. The Sargent Oil field is on the Monterey 1:100,000 quadrangle and was compiled from several sources at 1:24,000. DOGGR asked CDMG to provide the most up to date geologic map of the area. In a matter of hours we were able to clip out a part of the map at 1:24,000 scale, add explanatory and border text using Adobe Illustrator and deliver a PDF image of a finished-looking, colored geologic map (Figure 1).

[Click on image to open a larger version in a new window]

Figure 1. An example of a map quickly produced for a customer (DOGGR). Derived from several 1:24,000-scale maps, this product was clipped from the database and a map explanation and text were added within hours of the request.

CONCLUSIONS

REFERENCES

Haugerud, Ralph, and Greenberg, H.M., 1998, Recipes for Digital Cartography: Cooking with DEMs, in David R. Soller, editor, Digital Mapping Techniques '98 -- Workshop Proceedings: U.S. Geological Survey Open-File Report 98-487, p. 119-126, https://pubs.usgs.gov/openfile/ of98-487/haug2.html.

Wagner, D.L., 1997, Digital mapping techniques employed by the California Division of Mines and Geology, in David R. Soller, editor, Proceedings of a workshop on digital mapping techniques: Methods for geologic map data capture, management, and publication: U.S. Geological Survey Open-File Report 97-269, p. 35-38, https://pubs.usgs.gov/openfile/of97-269/wagner.html.