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Digital Mapping Techniques '98 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 98-487

Issues in the Application of Digital Geologic Data

By Jim Giglierano

Iowa Geological Survey Bureau
109 Trowbridge Hall
Iowa City, IA 52242-1319
Telephone: (319) 335-1594
Fax: (319) 335-2754

The Iowa Geological Survey Bureau (GSB) has recently completed a three year STATEMAP project which included mapping the surficial geology of five 1:24,000 quadrangles, and the bedrock and surficial geology of Linn County at 1:100,000 scale. STATEMAP is a component of the National Cooperative Geologic Mapping Program. The results of this project were many: the development of new computer-aided geologic mapping techniques; training of staff geologists in geologic mapping particularly of surficial materials; and the digital compilation and collection, storage, and analysis of new and existing subsurface data into GSB archival databases. We also focused on the development and use of geologic data and derivative products for local decision making and working closely with local government, training them to use and understand geologic data.


The Linn County project was a great learning tool for GSB and allowed us to develop numerous techniques for producing and using digital geologic data. These included use of digital soils, terrain, orthophotos and digital raster graphics as background material for on-screen digitizing by mapping geologists to compile their original line work. On-screen compilation required more time by the mapping geologist, especially when learning to use the system. However, the mapping geologist created better interpretations through integration of many data sources while also mapping at a larger scale than the target scale. Over the course of the three year project, desktop tools developed significantly and allowed us to move away from using high-end Arc/Info products on UNIX workstations by GIS professional staff, and move to ArcView 2 and ArcView 3 products on desktop PCs with the mapping geologist doing the digitizing on-screen with little or no help from GIS staff. This last year, nearly all of the digital work was done using ArcView 3 with Avenue scripts for editing and merging line and polygon data. Some gridding of bedrock surface contours and structural surfaces was still done using the high-end Arc/Info Topogrid tool because the ArcView Spatial Analyst module was not fully developed enough to perform this task. Geologic mapping done on the geologist's desktop computer with relatively inexpensive and easy to use software is a viable alternative to traditional methods requiring field mappers to draw their maps on paper or mylar first and then having GIS staff digitize and edit the paper map.

Finished geologic maps were produced with ArcView 3 software by GIS staff. The ArcView map layout functions are less comprehensive than the high-end workstation Arc/Info product, but were successfully used in the production environment to make interim products for error checking and finished products for distribution. The ability to create a reasonable printed product with inexpensive desktop hardware and software was seen as an advance in the methodology and a viable alternative to traditional printing methods. The software for printing maps should greatly improve with time just as the on-screen digitizing features already have. STATEMAP products for Linn County were not formally published and printed in large quantities because it was anticipated that there would not be a large demand for paper products requiring the cost efficiency of mass printing. Instead, digital project maps are printed on-demand at the cost of reproduction, which were calculated at $2 per linear foot, or generally $5 to $10 per map sheet. Printer files are stored on the network and sent to the printer when needed. It is estimated that a dozen or so have been sold this way. It is anticipated that front office secretarial staff can take the necessary steps to send the stored printer files to the printer when needed by a customer, and this will eventually include standard 24k topo quads stored as DRGs as well as more specialized maps, thus saving shelf space, printing and restocking costs and staff time. This process will become more viable with the development of more durable printer paper and inks that are not sensitive to moisture.

The Linn County project also allowed us to work closely with local users of geologic data including the county departments of planning and zoning, health, engineer, and the solid waste authority, local National Resource and Conservation Service office, plus local quarry operators and citizens. Linn County has a good mixture of urban and rural natural resource issues, including development pressures in the county and urbanization around Cedar Rapids, water quality and quantity, protection of aggregate resources, and environmentally sensitive areas. After the Linn County project began, a local users group was formed which also provided some financial support for additional geologic well data entry, not covered by STATEMAP resources. Members of the local users group were provided interim products for comments and feedback, and finished products at the end of yearly mapping phases. Interim digital products were provided on CD-ROM to the county planning and zoning and health departments who were developing their own GIS capabilities concurrently with our project. The planning and zoning department particularly showed serious commitment to developing their own GIS, by hiring a new planner with GIS experience, purchasing ArcView 3 and ArcCad, and an E-size Hewlett-Packard inkjet plotter.


During the course of the Linn County project, several versions of the geologic GIS coverage and geologic map were produced for various purposes. This led to some confusion as to which version was current or which version was used to produce a map. During the actual mapping, editing and revisions were made on a working copy of the GIS coverage. From this working coverage, several test plots were printed to check for errors and omissions. At the time of the STATEMAP contract deadline, no further changes were made to the working coverage; documentation or metadata was written, a good quality map was composed and printed with all the necessary collar and ancillary information, and everything sent to USGS. To help local cooperators use the geologic data, several derivative products were tried, using combinations of bedrock and surficial geologic coverages and attributes to produce various scenarios for aggregate resources, groundwater aquifers, potential hazards, and groundwater vulnerability. Some of these derivative maps were sent to the local cooperators for comment and evaluation. When new quad maps were completed during the project, the derivatives had to be updated, thus creating more versions of the same GIS coverages and maps. Final versions of the GIS coverages and the maps made directly from them will be produced and distributed to the local cooperators and the public. Questions arise as to how to name and cite the initial versions to distinguish them from the final versions, and whether the earlier version should be archived. Obviously, the interim products could have been withheld until completed versions were available at the end of the project. Because events within the county were moving faster than the mapping, we felt obligated to distribute incomplete geologic and derivative maps and GIS coverages to local users, who knew that they were not using the final products. Many of these questions have not been satisfactorily resolved. Plans for the next large-scale STATEMAP project involve maintaining a creation or modification date attribute connected to individual features of geologic and derivative coverages, and displaying a coverage history on printed maps to indicate which version was used.


One topic that was not fully explored during this project was the development of a good geologic data model for the geology of Linn County. There is much current interest within the geologic mapping community in developing a geologic map data model and standards for digital geologic maps and data. It is this author's opinion that the emphasis should be on development of simple data models that allow the exchange and combination of geologic GIS coverages between projects, states and regions of the country, including standardization of attribution such as lithology, stratigraphy, age and other physical descriptions of geologic/hydrogeologic properties. Development of standards or models for geologic maps including symbols, legends, text and other ancillary information, while important in the long run, should not overwhelm efforts to develop geologic data models. The geologic data model should be the framework around which standardized procedures for data capture, metadata creation, database storage schemas, planned analysis activities and map display would be constructed. For example, GSB currently needs a digital Quaternary geology data model that can describe the extent and physical characteristics of surficial materials in three dimensions and will easily transfer into a groundwater model to determine source water recharge zones for 1800 public water supplies in Iowa. Here the planned analysis function is the major, driving force behind development of the geologic data. A standard set of map symbols won't help do that.

In Linn County, GSB created a rather simple geologic data model and used it to produce maps showing aggregate and groundwater resources, hazards, and ground vulnerability, which were all derived from three geologic data layers: surficial geology, bedrock geology and depth to bedrock. This geologic data model consisted of physically merging all three vector GIS coverages and using queries of the combined attributes to define zones for the desired derivative model (for example, sandy surface units overlying karst forming bedrock units within 50 feet of surface equals highly vulnerable groundwater area). Not very elegant, but a simple geologic data model that provided useful results. However, this model would not be very effective in describing the three dimensional distribution of the surficial materials, and basically would not allow for any variation within the surface units at depth. For example, a simple geologic data model based on a 2-d surficial geologic map can only tell what the geology is at point x, y on the earth's surface. A better data model would need to be able to answer a query that asked what geology exists at point x, y and z. Relatively simple, horizontal geology such as that found in Linn County can be described with layers of gridded surfaces or vector overlays, but probably would not work well for complex faulted and folded terrains. Undoubtedly there exists in the petroleum industry geologic data systems that can handle complexity for a price. What is needed for small agencies is something usable on the desktop that combines simplicity, ease of use, compatibility with existing data, and relative longevity within advancing computer technology. Hopefully somewhere within the framework of the National Cooperative Geologic Mapping Program some of this research can be undertaken, either at the federal, state or university level.


Justification for the National Cooperative Geologic Mapping Program (NCGMP) has largely been that the massive development of new geologic maps is critical to the nation for making important natural resource decisions. While STATEMAP has provided the impetus and means for GSB to renew its geologic mapping activities in Iowa, producing new geologic maps for the sake of having new or better maps was never viewed by GSB as a total justification for pursuing this activity. Experience in Linn County has shown us that geologic maps in the traditional form of printed paper maps are by themselves of relatively little value to local officials faced with natural resource decisions. Digital geologic data in the form of GIS coverages are of marginally limited use as well, unless the local users really know how to use them, which involves some geologic training, or the digital coverages are transformed into some derivative products that are understandable to non-geologists. The act of creating and making available a new geologic map of an area does not by itself insure that good resource or environmental decisions will eventually be made. It is true that there are some persons within a local area who can make direct use of geologic maps, such as quarry operators and consultants, but mostly the local officials we have encountered are a new market for geologic information. They generally haven't had access to pertinent geologic information for their jurisdiction and don't have the training or resources to hire consultants for every issue that requires understanding geology. The NCGMP and STATEMAP do provide much needed resources for the creation of new geologic maps, but allow no resources to be used to produce useful derivative products or the development of educational or instructional materials or training for local officials in how to use these new geologic information resources. This is by no means a criticism of NCGMP or STATEMAP, merely a recognition that additional resources must be found to make the circuit complete and assure that local officials are aware of this information and can make use of it. It is up to the state surveys to find additional resources to train local officials to be geologically literate in order for the goals of the NCGMP to be fulfilled.

During the Linn County project, the solid waste authority started a new landfill siting process, and while GSB data was not complete for the whole county, it did become a crucial component used by the engineering consultant to run preliminary siting criteria for selection of 13 sites. In connection with a strategic planning process for the GSB, working with local decision makers and providing them with quality geologic information was identified as a high priority item. The Linn County project and the landfill siting process became a crucial test of this strategy, and an object lesson in the process of decision making. The landfill siting process is currently finishing its second round. Originally, depth to bedrock was a major limiting factor in the selection of the 13 preliminary sites. Public pressure has led to the addition of corn suitability rating as an exclusionary criterion and removal of depth to bedrock. Despite our best efforts to provide the best available information for choosing an environmentally appropriate site for the new landfill, the decision making process was able to avoid geological issues. Again, producing new geologic maps intended for use in natural resource decision making does not in itself guarantee that it will be used for that purpose or used well.


It's a new world in several ways for those trying to provide good quality geologic information, both in terms of new technology, and how geologic information can and is applied to society's environmental problems. Inexpensive desktop hardware and software is making it possible for small agencies to create and print their own digital geologic maps. Local government agencies are attempting to create their own GIS capabilities and want to integrate digital geologic data with their databases. Research is needed to develop consistent, reasonable geologic data models that can be plugged into groundwater and other environmental modeling software. While the ability to create digital geologic databases is progressing, local users are a new market for this information and frequently require training and educational opportunities to effectively use it. Finally, geologists need to realize that despite their best efforts at providing good information to decision makers, local resource decisions are commonly made on the basis of many criteria besides geology.

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Last updated 10.06.98