This paper addresses the development of digital geologic map databases through the extraction of information from existing published geologic maps. As in any mining operation, the objective is an efficient and cost effective method for extracting and processing the ore while (in an environmentally safe way) leaving behind the tailings. The underlying concept of this paper is that information contained in existing small-scale geologic maps can be related to a high quality, larger-scale, topographic maps in the same manner that the information is commonly related to the real world (which has the ultimate scale of 1:1). The characteristics which are most useful in this process involve the relationship which exist between geologic boundaries (separating distinct and identifiable geologic units) and non-geologic features (cultural or topographic). Although the nature of sedimentary stratigraphy without complicating unconformities makes it the easiest environment for application of these principles, the interactions between geologic boundaries and variations in topography in almost any geologic environment will create patterns and spatial relationships to which the principles of this paper may be usefully applied.
Proverbs are generally the outcome of experience blended with wisdom. People who violate old proverbs may survive the experience, but they do so at their own risk. There is an old proverb among geologists and cartographers:
Don't take a map drawn at a smaller scale and enlarge it for work at a larger scale.
As stated by Robinson et al. (1984, p.427), ". . . the accuracy of the source data must always be a matter of primary concern for the map compiler." The fundamental concern addressed by this proverb is map accuracy; more specifically, the accuracy of feature locations on the map. Implicit in the proverb is the understanding explicitly stated by Thompson (1981, p.31) in Maps for America: "Generalization is used to some extent . . . at any map scale . . . The amount of detail omitted varies inversely with the map scale." In the drawing of geologic outcrop patterns, greater omission of detail corresponds to the introduction of greater location inaccuracies.
Addressing the current issue of map scale changes, Robinson gives his own version of our proverb:
"Progressive generalization with smaller scales is an inevitable aspect of the mapping process. For this reason compilation should always be from larger-scale sources rather than smaller. The temptation to enlarge a smaller-scale source map is bad enough. But it would be even worse to blow up a smaller-scale map of one feature . . . to be overlaid on a compilation worksheet containing other features . . . that were compiled from larger scales. (p.427-428 emphasis added)"
The first temptation is probably not so much a sin as Robinson suggests, unless the resulting enlargement is used in a way which implies location accuracy only possible from original compilation at the larger size (scale).
Robinson's warning against the second temptation suggests a more specific statement of our first proverb:
Don't enlarge features compiled and generalized for a map at a smaller scale in order to integrate those features, on a composite map, with features compiled and generalized for a map at a larger scale.
Violation of this proverb probably generates the worst possible results when it involves features of the same type. Newell's (1935) geologic map of Johnson County includes the outcrop pattern for the base of the Westerville Limestone (now considered a member of the Cherryvale Formation). The map is published at a scale of 1:126,720. O'Connor (1971) did not include this unit in his more recent map, compiled for publication at a scale of 1:48,000 from field work done on 1:24,000 scale topographic maps. Believing that information about the Westerville Limestone Member would improve O'Connor's map, a poorly trained cartographer might digitize the outcrop from Newell's map, enlarge the data to a scale of 1:48,000, and plot the result with data from O'Connor's map. The superimposed outcrop pattern for the base of the Westerville would cut back and forth between outcrops derived from O'Connor's map of geologic contacts above and below the Westerville Limestone, possibly crossing one or the other. Cartographic transgression could easily violate a fundamental geologic principle, inverting the order of formations within a normal stratigraphic sequence.
The location distortion of the transferred outcrop pattern for the Westerville Limestone Member in relation to surrounding units results from differences in the quality of the base maps and, more significantly, a higher degree of generalization inherent in Newell's smaller-scale map. Rather than improving the information content of the composite map, violation of the cartographic principles expressed in our first proverb (as revised) would degrade and call into question all the information presented on the composite map.
Geologic maps are models of information transferred from a 3-D, topographic relief map at the scale of the real world.
This is the first axiom of geologic mapping. With new field mapping, geologists follow Robinson's admonition against the use of smaller-scale sources in the compilation of maps. Working in the real world they use the largest available source, at the scale of 1:1, where a foot is a foot (and a rose is still a rose).
Once they identify exposed rock units, geologists search for geologic contacts; where the top of one mapped interval of rock units is in contact with the base of the next mapped interval. Using the best available methods, the geographic location and elevation of these geologic contact observation points are carefully determined. The process used in compilation of field maps from these potentially sparse observations is described by Sawin (1996, p.3):
"Geologic maps are compilations of data and inference. Because most bedrock is covered by soil and vegetation, the information gleaned from outcrops are pieced together to build a map. Because outcrops may be a mile or more apart, geologists must use their training and experience to connect the data points by extrapolating and interpreting what happens between the scattered points of information. . . The geologist's job is to visualize the bedrock near the surface without the soil cover and to make a map that reflects this image."
Accurate positioning of outcrop patterns is probably the principle concern for the field geologist and the cartographic compiler of geologic maps. However, it is the forms of those outcrop lines in relation to the forms of the landscape (or the representation of those land forms on topographic maps) and their position in relation to cultural features (such as section corners and boundaries in the Public Land Survey System) which provides the most useful information regarding the location and spatial relationship of rock units. To a large extent these information-loaded aspects of form and spatial relationships are maintained in the preparation of geologic maps over a large range of scales. In mathematics and cartographic applications, geometric properties which are invariant with scale transformations define the topology of the mappings. For a geologic map to accomplish the geologist's objective of representing the spatial relationship between rock units, maintaining the topology of geologic features is more important than the absolute position of those features. In many cases the scientist is unaware of the distinction between position and form, and the primary significance of form, to the success of the map making effort.
The interaction between geologist and cartographer, trying to achieve location accuracy despite small map publication scales, produces cartographic generalization of map features dependent on the attributes as well as the relative positions of the features represented on the map. The result is preservation of the geologic information which the geologist seeks to convey through the map.
When a geologic map is used in the field, the information content of the smaller-scale map is related back to a 3-D, topographic relief map with the larger scale of the real world.
This is the first axiom of map use. Significant errors between the map coordinates of points along a line representing the outcrop pattern of a geologic contact and the corresponding geographic location of the outcrop occur as the result of generalization, and increase with decreases in scale. Despite this obvious fact, geologic maps published at small scales (ranging from 1:24,000 down to 1:320,000 for county maps and 1:500,000 for the state geologic map in Kansas) maintain a high degree of usefulness in a wide variety of applications. The fundamental quality of a truly good geologic map is the fact that users derive most of the available information from map characteristics other than the precise position of outcrop lines on the map.
Consulting geologists, highway engineers, civil engineers, zoning boards and a multitude of other users of the end product of the field geologist's efforts take advantage of the topological characteristics of geologic maps. They use their training and experience to relate the forms of outcrops on geologic maps to the corresponding topographic form of the real world where knowledge of near surface geology is crucial for success in performance of their jobs.
High quality topographic maps, such as the 7.5 minute quadrangles published by the USGS, provide excellent models of actual land forms in the real world. When smaller-scale geologic maps are used to locate geologic features in the field, the user commonly invokes the following principal:
The information content of all geologic maps can also be related to models of the large 1:1 scale topographic relief map.
This is an important corollary to the first axiom of map use. With emphasis on the information content of maps, it is a concept which must be adequately conveyed to geoscientists, technicians and program managers responsible for geologic database development. For many, its acceptance requires a major paradigm shift. Without its acceptance, the typical responses from all these groups, based on a misunderstanding of the nature of the information content of geologic maps, pose roadblocks to development of high quality digital geologic databases from existing published maps.
Computer mapping technicians generally see the task of data compilation from published maps as a challenge to find the best technology for capturing the precise location of each and every relevant line as shown on the map or maps in question. They focus on resolutions of scanning equipment, repeatability of point location measurement on specific digitizing tables, the medium on which the source map is printed, possible distortions in the medium, and the quality and clarity of lines on the map. Their efforts for accurate reproduction of the lines on the map will result in accurate reproduction of the cartographic generalizations (i.e., the position errors) built into the particular scale at which the map was drafted.
Project managers for geologic map database development intuitively recognize the merits of Robinson's admonition against blowing up a small-scale geologic map and printing it as an overlay on base map data derived from larger-scale sources. Large programs such as development of state or national databases are established in relation to a "standard" scale (e.g., 1:100,000 for the National Geologic Map Database). It is commonly presumed that no maps published at a scale smaller than the "standard" can be acceptable sources of information for the program. The scales at which existing geologic maps were actually published can become a constraint on the selection of the target scale for a database development project. The goal of a national database referenced to a 1:24,000 base is rejected a priori as impractical or infeasible. This rejection is based on the assumption that maps do not generally exist at this large scale and that resulting compilation efforts would be extremely expensive. The result is an unfortunate restriction in the use of information available from many published maps.
Even among geologists who are perfectly comfortable using a small-scale map in the field, there is strong resistance to the idea of taking information from the smaller-scale map and placing it on a larger-scale map. Failing to recognize the ease with which they use the information from existing maps while working (in the field) on the largest-scale map, geologists commonly, but incorrectly, believe that map information is mostly determined by the position of the lines rather than their topology.
These typical responses result in a failure to consider the potential for capturing important information from almost any geologic maps and then relating that information in useful ways to larger-scale maps of the local topography. Once the corollary in this section is accepted, the implications for database development are immediate. Highly effective procedures for capture of geologic information from published maps have been developed and tested at the Kansas Geological Survey. The procedures require interpretation of the topologic characteristics of geologic features represented on a map with subsequent transfer of information from the published maps to a common, 1:24,000 scale, USGS topographic base. The tasks involved in this process, are described by Ross (1996) and by Ross and Collins (1997).
As Ross explains, the idea of transferring the map information to larger-scale (1:24,000) topographic maps is not a matter of "trying" to make the data more accurate than the existing maps. Nor is it a violation of Robinson's principles of map compilation. Using the topologic, stratigraphic and geologic information presented in the smaller-scale map and the more accurate representation of real world topography on the 1:24,000 base map, it is possible to develop geologic data which more accurately represent what the geologists intended to map than could be done on the existing smaller-scale maps. Rather than flagrant "cartographic license," the process represents a redrafting of available information while eliminating much (but not all) of the "cartographic license" taken when results of field mapping were prepared for publication at small scale. Cartographers are not the only ones who take this license. Many geologists, in the process of completing outcrop patterns on their field maps, will fail to follow the structure implied by their field observations (often made at sparse critical points) when interpolating between actual mapped locations of a formation.
There is a new proverb among geocartographers:
Don't let an old proverb keep you from using the information content of a smaller-scale map when you want to make a map at a larger scale.
Scanned images of old maps can still be important historic resources in geologic literature. They provide an efficient, cost-effective means of preserving past geologic research and make possible electronic re-publication of the old maps, now generally out of print. Scanning is the only method recommended for this purpose because it replicates the original document in all its detail with far greater fidelity and much lower cost than any manual digitizing technique.
Digitizing or scanning geologic formation boundaries and outcrop patterns directly from existing smaller-scale maps or from bases prepared at a small scale such as 1:100,000 for the purpose of database development would simply perpetuate the errors introduced by cartographic generalization. Using these images, or elements vectorized from the images, in conjunction with databases derived from more accurate, larger-scale maps would be a return to map terrorism, violating the wisdom of old proverbs and basic principles of cartography. This a constant concern in other geographic information systems applications as well as in automated cartography.
Databases derived from published maps through transfer of map information to a common, larger-scale base provide an appropriate alternative to this violence.
Newell, N.D., 1935, The geology of Johnson and Miami counties, Kansas: Kansas Geological Survey, Bulletin no. 21.
O'Connor, H.G., 1971, Geology and ground-water resources of Johnson County, northeastern Kansas: Kansas Geological Survey, Bulletin no. 203.
Robinson, A.H., Sale, R.D., Morrison, J.L. and Muehrcke, P.C., 1984, Elements of Cartography, 5th Edition, John Wiley & Sons, New York, N.Y.
Ross, J.A., 1996, Compilation of Digital Geologic Map Data at the Kansas Geological Survey: a report to the Working Group on Data Capture, Digital Geologic Map Standards Committee, American Association of State Geologists and United States Geological Survey; Kansas Geological Survey Open File Report 96-45, revised October 31, 1996.
Ross, J.A. and D.R. Collins, 1997, Information Capture from Previously Published Maps: to be presented at Digital Mapping Techniques '97, a conference of the U.S. Geological Survey and American Association of State Geologists, Lawrence, KS, June 2-5; Kansas Geological Survey, Open File Report 97-27.
Sawin, R.S., 1996, Geologic Mapping in Kansas, Kansas Geological Survey, Public Information Circular 4.
Thompson, M.M., 1981, Maps for America, 2nd edition, U.S. Government Printing Office, Washington, D.C.