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

Production of the 1:250,000-Scale Digital Geologic Map of Alabama

By Berry H. Tew, Douglas R. Taylor, and W. Edward Osborne

Geological Survey of Alabama
P.O. Box O
Tuscaloosa, AL 35486-9780
Telephone: (205) 349-2852
Fax: (205) 349-2861


In 1988, the Geological Survey of Alabama (GSA) published the first statewide geologic map of Alabama based on new mapping and compilation since 1926, when the now classic and collectable lithograph, Geologic Map of Alabama, was prepared in cooperation with the United States Geological Survey (USGS) and released as part of GSA Special Report 14, Geology of Alabama (Adams et al., 1926). The publication of GSA Special Map (SM) 220, Geologic Map of Alabama (Szabo et al., 1988) was the culmination of decades of work by numerous geologists, including not only those affiliated with GSA, but a number of others involved in mapping the geology of Alabama for various purposes. SM 220 was published as four map sheets, representing the four quadrants of Alabama, at the scale of 1:250,000, plus a fifth sheet of explanatory information, and comprises the largest scale and most accurate border-to-border depiction of Alabama's geology as presently understood. The map is now in wide distribution and has been generally acknowledged as a significant contribution to the science.

The decade that has passed since the publication of SM 220 has seen an explosive expansion in the routine use of Geographic Information Systems (GIS) and digital geospatial data for various applications, including: geologic, hydrologic, and environmental research; mineral exploration; land use planning; industrial development; and others. Soon after publication of the map, GSA began to receive inquiries about plans for release of a digital version of SM 220. We realized that demand for digital products would continue to increase as more and more agencies and organizations incorporated digital geospatial data into their routine operations. Further, GSA recognized an obligation to produce a digital version of SM 220 that carried the agency"s imprimatur as the "official" digital rendition of Alabama's geology.

Owing to circumstances, production of the digital version of SM 220 languished until 1996, when GSA developed a project with USGS-Eastern Mineral Resources Survey Team (USGS-EMRST) to evaluate the quality of data contained in the Mineral Resource Data System (MRDS) for Alabama, to correct obvious errors, and to classify major groups of deposits and mineral occurrences by deposit type. To assist in this assessment program, it was determined that a digital lithofacies map of Alabama was a necessary component and that this map would be developed on the basis of the geologic units shown on SM 220. Thus, the MRDS project provided a vehicle for production of the Digital Geologic Map of Alabama as a cooperative effort between GSA and USGS.


The production of the digital geologic map of Alabama is being undertaken as a joint project of the GIS Group (GSA-GIS) and the Economic Geology Division (GSA-EGD) of GSA, with support from USGS-EMRST. GSA-GIS has assumed responsibility for data capture and GIS development, whereas GSA-EGD is providing final review and quality control. The primary goal of the project has been to produce an accurate, fully georeferenced and attributed rendition of SM 220. Initial data capture, georeferencing, and attribution have been completed and the digital data is now being reviewed for completeness and accuracy.

Much discussion went into deciding the most accurate and efficient method for capturing vector data from SM 220. The final published version of the map consists only of offset-printed, full-color, paper copies and, thus, was deemed less than desirable for data acquisition due primarily to the inherent problem of scale-instability. In Alabama"s subtropical climate, which is characterized by relatively rapid fluctuations in humidity and temperature, paper is subject to dramatic and significant variations due to swelling and shrinking over the course of hours and days due to changing environmental conditions. Changes of 0.125" to 0.25" in the dimensions of a standard USGS 7.5-minute quadrangle sheet over the course of a day are not uncommon and, for this reason, we try to avoid using paper maps for data capture whenever possible. Therefore, we decided against using "off-the-shelf" copies of SM 220 for data capture.

Fortunately, after production of SM 220, GSA had archived the original scale-stable film scribe coats onto which the geologic contacts had been hand drawn and scribed, thus providing negative images of the geologic line work for the four map sheets. It was decided to: (1) use these scribe coats to produce scale-stable, contact-print film positives of the geologic line work; (2) add tic marks from the original map base to the film positives for subsequent georeferencing purposes; (3) have the film positives scanned; (4) convert the resulting raster images to vector format; and (5) process the vectorized data into useable, georeferenced, topologically structured GIS data sets. USGS-EMRST agreed to the scan the map quadrants, make a automated, unsupervised "first-pass" vectorization of the raster images, and provide GSA with the resulting data sets in Arc/Info (ESRI, Inc., Redlands, CA) format.

Upon receiving the vectorized data sets from USGS, GSA-GIS staff set about the task of georeferencing the four map quadrants and correcting the errors (undershoots, overshoots, missing lines, misconnected lines, unclosed polygons, etc.) contained in the data. Georeferencing was accomplished by the addition of real world coordinate tics at the intersection of the lines representing the original base map tics and transforming the data to the real world coordinate system. Error correction was a somewhat more significant chore, but proceeded in a relatively timely fashion using a combination of on-screen and manual digitization techniques to edit the line errors in each map quadrant.

After several iterations of editing and check plotting to assure accuracy, polygon topology was established for the corrected geologic contact data sets, feature label points were generated, and feature attribute tables were created. SM 220 depicts 163 unique geologic units in Alabama. Each of these units was assigned a numerical code (1-163), a field for the unit codes was added to the polygon attribute tables, and each polygon was attributed with the code corresponding to the unit that it represents. Again, an iterative process of check plots and edits was employed to find and correct any erroneous unit code assignments. All subsequent attribution was handled automatically on the basis of the unit codes, as described below.

At this point, the four quadrants were joined into a seamless layer for the entire state, polygon topology was re-established for the joined data set, and errors introduced in the joining process were identified and corrected. To facilitate detailed review of geologic unit codes for polygons prior to final attribution, a look-up table was constructed to assign colors to polygons on the basis of the unit codes. A standard Arc/Info color palette (shadeset) was used and colors were chosen from this palette to match as closely as possible those depicted for units on SM 220. Color plots at the 1:250,000 scale were then generated and used to check for incorrectly coded polygons.

The completed draft statewide geologic unit polygon data set contained over 6000 polygons that needed to be attributed with data regarding the geologic province in which they occur, a hierarchical arrangement of geologic nomenclature (system, series, group, formation, member, etc.), an alphabetic map symbol, and predominant rock types. For example, the Cusseta Sand Member of the Ripley Formation of the Selma Group of the Upper Series of the Cretaceous System occurs in the Coastal Plain province, is designated by the alphabetic symbol Krc, and consists primarily of sand and clay. Further, as stated above, there are 163 unique possibilities for these descriptive values. To accommodate these data, eight fields were added to the polygon attribute table for the geologic unit coverage and an Arc Macro Language (AML) script was written to automatically populate these fields on the basis of the geologic unit code entered earlier. The AML, written by the second author of this paper, was designed to take advantage of the cursor processing functionality of Arc/Info and essentially "steps through" the attribute record for each polygon, evaluates the geologic code in the record, and searches through a loop directive in the script until a match for the code is found. The AML then populates the fields in the attribute table with the appropriate data and proceeds to the next polygon until all polygons are attributed. In addition to the obvious savings in time spent on data entry, use of the AML also substantially reduced the possibility of inaccuracies being introduced into the data set due to inconsistencies (e.g., Pottsville Formation vs. Pottsville Fm.), misspellings, omissions, and other common data entry errors.

After completion of the attribution process, GSA-GIS plotted the digital map data in color on a county by county basis at the scale of 1:100,000 and closely checked these plots against SM 220. Identified errors and discrepancies were corrected. This process was then repeated at the 1:250,000 scale. At this point, GSA-GIS deemed the Digital Geologic Map of Alabama ready for final review by GSA-EGD.

GSA-EGD is the coordinating and principal reviewing entity for geologic mapping in Alabama and also oversees the State's stratigraphic nomenclature and, therefore, has the responsibility to approve the Digital Geologic Map of Alabama before its release to the public. Final review of the data is now underway and is being facilitated with a combination of color plots on paper and black and white geologic contact plots on film for comparison and overlay with SM 220. As with all steps above, errors identified with each check will be corrected and the process will be repeated until all reviewers are satisfied that the data set is ready for release.


GSA plans to release the Digital Geologic Map of Alabama as the initial offering in our CD-ROM series before the end of 1998. Publication this year of the first large-scale digital geologic map of Alabama as the first CD-ROM product of the agency is very appropriate in that GSA is celebrating its sesquicentennial in 1998. Since its inception in 1848, it has been the agency's mission to provide quality, accurate, timely, useful information regarding the mineral, energy, water, and biologic resources of Alabama to aid in economic and social development. The production of the Digital Geologic Map of Alabama is a continuation of this proud tradition and the harbinger of the direction that GSA will follow as we move into the 21st century. Digital data formats for both geospatial and tabular data, as well as text-based reports, will become the norm and electronic delivery of information via various media, including the World Wide Web, will evolve into our primary mode of data distribution. By utilizing these rapidly expanding technologies, GSA hopes to better serve the needs of the State of Alabama and its citizens.


Adams, G.I., Butts, Charles, Stephenson, L.W., and Cooke, C.W., 1926, Geology of Alabama: Geological Survey of Alabama, Special Report 14, 312 p. (Scale of included geologic map, 1:500,000).

Szabo, M.W., Osborne, W.E., Copeland, C.W., and Neathery, T.L., 1988, Geologic Map of Alabama: Geological Survey of Alabama, SM 220, 1:250,000 scale.

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