Digital Mapping Techniques '99 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 99-386

The Development of a Digital Surface and Subsurface Geologic Map Database of Charleston County, South Carolina

By Peter G. Chirico

U.S. Geological Survey
National Center, MS 926A
12201 Sunrise Valley Drive
Reston, VA 20192
Telephone: (703) 648-6950
Fax: (703) 648-6953


Since 1974, the USGS has conducted geological investigations relating to the tectonic and earthquake history of the Charleston, SC region. Earlier studies, such as Rankin (1977) and Gohn (1983), were a combination of geophysical, tectonic, and seismic investigations. Near-surface, and surface geologic maps were published by Force (1978a, b) and McCartan and others (1980). Mapping of the region at a small scale was completed by McCartan, Lemon, and Weems (1984), and subsequent mapping efforts by Weems and Lemon provided a more detailed coverage of the Charleston area at 1:24,000 scale (Weems and Lemon, 1984a, b, 1987,1988, 1989, 1993, 1996, Weems, Lemon, and Chirico, 1997). Weems has continued mapping surficial and near-surface formations in and adjacent to Charleston County and it is this latest effort which has included the advent of digital geologic mapping in the region.


The goal of the project is to develop a comprehensive large-scale, digital geologic dataset of surface and near-surface geology and a smaller scale subsurface geologic coverage for the Charleston county region. Thus the mapping project was divided into two sections. The first was the conversion and continued development of a large-scale surface geologic database for the area. The second was a subsurface stratigraphic framework for the same region.


The large-scale surficial and near surface mapping project consisted of three types of map development. First, seven printed maps of Charleston County 7.5 minute quadrangles were converted into a geographic information system (GIS) database. Second, all new mapping in the region was compiled on mylar separates for the development of new digital databases. Third, several datasets previously compiled but unpublished, consisting of hand drafted linework on greenline quadrangles, needed to be converted to digital format. Although the same process was employed for all types of data conversion, printed versions and unpublished compilations, the GIS staff experienced different challenges during the conversion steps.

The GIS staff employed a method of scanning, vectorizing, editing, and attributing for digital database creation. Although the process was basically the same for previously published maps, unpublished compiled greenlines, and new mapping compilations, the differences in the original medium required differences in methodology. Previously published maps, which had map separates available, were easily scanned and compiled, whereas greenlines of unpublished data were more difficult to work with.

Scans that were made directly from hand drafted compilations of greenline quadrangles contained written text leader lines and other extraneous information that produced complex linework, much of which required deletion (Figure 1a). For new mapping, geologists drafted on mylar that was overlayed on plotted digital basemap files (Figure 1b). These digital basemap files were updated datasets of topographic features and are available in Arc/Info format through the South Carolina Department of Natural Resources, Colombia, South Carolina at In the case of published maps, photographic reproductions of linework were procured for scanning. These black-line positives of scribed linework were ordered from Eastern Region Publications Group of the USGS where some of the scribed map separates still reside. Scans derived from this medium were clean and contained only geologic contact information (Figure 1c).

Figure 1a
Figure 1b
Figure 1c
Figure 1. Examples of three different geologic data mediums that were digitized. A. Greenline with hand drafted data. B. New hand drafted mylar overlay. C. Black-line positive map separate.

The scanning process utilized an Anatech Eagle 4080 ET large-format scanner, capable of scanning documents up to 40 inches in width. Separates, blackline, and mylar overlays were scanned at different resolutions to provide the best possible file for the vectorization step. Blackline positives are scanned at higher resolutions, between 600 and 800 dots per inch (dpi). Hand drafted greenlines and mylar overlays generally were scanned at 400 dpi. The threshold setting on the scanner regulates the amount of light passing through the medium and therefore affects the clarity of the resulting scan. This threshold setting was altered for each type of media to produce the best possible raster image for vectorization. Blackline positives were scanned using a threshold of approximately 160-180, whereas the hand drafted media needed a higher threshold (190-210) to eliminate some background noise and produce cleaner lines. The raster scans were stored in .tiff or .rlc file format. These raster data formats are accepted by the GTX-OSR (GTX Corporation - Phoenix, Arizona) vectorizing software.

Vectorization of the scanned raster data was a straightforward process and resulting output files of vector linework were saved as Drawing Exchange Format (.dxf) files. Dxf files are easily imported into Arc/Info where the editing process occurs. The editing of vector linework to build topologically correct datasets of geologic polygons and lines was the most important and time comsuming step. Vector linework was projected into the same coordinate system as the digital basemap files (UTM 17, NAD 1927).

Digital basemap files of hydrography and hypsography as well as roads and miscellaneous transportation were downloaded and used as boss layers or backcoverages (drawn on screen in background during the digital editing phase) for the digital compilation. The digital basemap files of hydrography were of particular importance as boss layers for compiling surficial geologic maps because hydrographic and topographic features such as marshes, swamps, mudflats, and islands needed to be copied directly into the new digital geologic coverage. This was done to ensure that geologic and hydrologic features would coincide exactly on future plotted copies and for any subsequent GIS analysis. Hypsography (topographic contour lines) was used in a similar manner. Instead of directly copying lines however, geologic contacts were edited to follow hypsographic lines where required by geologic conditions. For example, geologic contacts describing alluvium and artificial fill were edited to follow the appropriate lines of the topographic backcoverage.

Because the utility of having geologic data in a digital format is the ability to view, query, and analyze the data, a database was developed separately and included a variety of qualitative information that might be useful to data users. A simple database was constructed in Microsoft Access for each of the units depicted on the map. This database was saved as a .dbf file. The .dbf file format was converted into an Arc/Info database file or .info file through the dbaseinfo command. The .info file was then joined to the spatial database with the joinitem command. An explanation of those database items is contained in Appendix 1. An example of the database for surficial geologic map units follows:

AREA = 9504.188
PERIMETER = 376.169
ID = 9
UNIT = Wando Formation
FACIES = clayey sand and clay
DEPO_ENV = backbarrier (estuarine/lagoonal)
RELATIVE_AGE = late Pleistocene
MIN_AGE = 90000.00
MAX_AGE = 110000.00
ABSOLUTE_AGE = 90-110 ka
MODIFIER_1 = clayey
MODIFIER_2 = sandy
MODIFIER_3 = very fine-medium
MODIFIER_4 = bioturbated
FRESHCOL1 = pale-yellowish-orange
FRESHCOL2 = medium-light-gray
WEATH_COL1 = grayish-yellow
WEATH_COL2 = dark-yellowish-orange
COMPACTION = moderate

The final digital databases are stored in an Arc/Info format but are commonly displayed and distributed in an ArcView project. Figure 2 illustrates the combined set of 7.5 minute quadrangles for the northeastern part of Charleston County, South Carolina.

Figure 2

Figure 2. Surficial geologic maps in an ArcView project.


The subsurface geologic dataset is a collection of auger holes, water wells, and core holes drilled and logged throughout the South Carolina coastal plain. A preliminary dataset of only the Charleston County subsurface data layers was completed to demonstrate the organization of the data. This database was developed for six units from the Maastrichtian to the middle Eocene age.

Data for each corehole includes gamma and electric logs, and lithologic and fossil data. Geologic formations are recognized at different depths throughout the core by analyzing the available data. Depths to each geologic formation are entered into an Access database for each core analyzed. Structure contour maps of any surface can then be prepared through contouring the data for a particular formation. The database is stored in a relational format to prevent duplication of information and to simplify the process of contouring surfaces of selected geologic units. The main data table stores identifier information for each core hole, and examples are shown in Table 1.

Table 1. Example of the main data table for subsurface geologic information.
CHN_635 16DD-y3 Town of Sullivans Island Charleston 32.76444 79.83278 2540.00000
DOR-37 23CC-I1 USGS-Clubhouse Crossroads #1 Dorchester 32.88806 80.35917 2599.00000

Ancillary data tables are linked to this identifier table through a common field (USGS_NO) and contain numerical values that show the depth to the top and bottom of the unit and each unit's thickness (Table 2). Several tables are created for each hole, each for a different geologic formation encountered at a different depth in that hole.

Table 2. Example of ancillary data tables containing information for a particular geologic formation
BRK-089 32.00000 328.00000 -296.00000 490.00000 -458.00000 162.00000
BRK-272 18.00000 541.00000 -523.00000 678.00000 -660.00000 137.00000
CHN-011 10.00000 749.00000 -739.00000 836.00000 -826.00000 87.00000
CHN-012 10.00000 752.00000 -742.00000 845.00000 -835.00000 93.00000

Figure 3

Figure 3. Geophysical logs and cross sections are stored as .dxf files in ArcView database.

Contour maps were developed for the subsurface geologic units using Arc/Info ver. 7.1.1. Structure contour maps of the top and bottom of each unit as well as an isopach map showing unit thickness were developed. These contour maps were then stored as ArcView shapefiles. The collection of these subsurface layers was stored in ArcView GIS for the traditional two dimensional display structure contour maps. Additional work was then done to include several cross sections to illustrate the regions stratigraphy. Originally these cross sections were created in Micrographix Designer as .dxf files. The .DXF files were then opened in an ArcView View to be displayed (Figure 3).


With the acquisition of better desktop tools for presentation of complex subsurface datasets, the project scientists hope to better communicate this information to the public through visualization software. ArcView's 3D Analyst allows limited capabilities to show subsurface geologic units and their location with respect to the surface.

Future research and development in the use and applications of digital geologic maps will be an important step in the future of digital geologic mapping. Further studies with this dataset include designing maps to include pertinent quantitative data suitable for GIS analysis. Examples for surfical databases include detailed grain size information correlated across geologic units, and blow count or density data for geologic units for some general measure of strength. Geologic map databases must be prepared to address problems of cooperating agencies. In Charleston County, issues ranging from coastal erosion and storm surges, land use planning for growing coastal communities, groundwater resource issues, and further studies related to earthquake hazard and liquefaction potential all require geologic map data to produce viable derivative products.


Data Format of Database

1 AREA 4 12 F 3
5 PERIMETER 4 12 F 3
9 CHAR_GEOL# 4 5 B -
13 CHAR_GEOL-ID 4 5 B -
17 TYPE 5 5 C -
22 ID 8 11 F 0
30 UNIT 30 30 C -
60 FACIES 30 30 C -
90 DEPO_ENV 40 40 C -
130 RELATIVE_A 25 25 C -
155 MIN_AGE 8 20 F 5
163 MAX_AGE 8 20 F 5
171 ABSOLUTE_A 15 15 C -
186 LITHOLOGY 15 15 C -
201 MOD_1 20 20 C -
221 MOD_2 20 20 C -
241 MOD_3 20 20 C -
261 MOD_4 20 20 C -
281 FRESHCOL1 25 25 C -
306 FRESHCOL2 25 25 C -
331 WEA_COL1 25 25 C -
356 WEA_COL2 25 25 C -
381 COMPACTION 15 15 C -

Definitions of Fields

TYPE: These unique designations are the geologic map symbols used for the various geologic units. The upper-case letters refer to the general geologic ages of the units (Q=Quaternary, T=Tertiary). The lower case letters refer to the name, specific age, and (or) facies of the geologic units (for example, Qhs=Quaternary+Holocene+barrier sands; Qwf=Quaternary+Wando Formation+fossiliferous shelf sand).

ID: Consecutively numbered units.

UNIT: Names of geologic units. Names with upper-case letters are formally defined (for example, Wando Formation). Units with names using lower-case letters are informally defined (for example,Ten Mile Hill beds, artificial fill).

FACIES: General statement of the depositional environment and (or) lithology of a geologic unit. FACIES represents the appearance and characteristics of a sedimentary unit, typically reflecting the conditions of its origin.

DEPOSITIONAL ENVIRONMENT: Describes the environment in which the sediments originally accumulated (for example, estuary,barrier island, marine shelf).

RELATIVE AGE: Age of the geologic units as identified on the standard geologic time scale. Defined in terms relative to other geologic units rather than in terms of years.

MIN_AGE: Numeric field containing the approximate minimum ages of geologic units in thousands of years. This field incorporates the minimum age range from the Absolute Age field but in a numeric form.

MAX_AGE: Numeric field containing the approximate maximum ages of geologic units in thousands of years. This field incorporates the maximum age range from the Absolute Age field but in a numeric form.

ABSOLUTE AGE: Ranges or values for approximate ages of geologic units as determined from fossil ages, amino-acid racemization ages, and (or) radiocarbon ages (ka=thousands of years, Ma=millions of years).

LITHOLOGY: The physical characteristics of rocks or sediments as seen in outcrops, hand samples, or subsurface samples. Described on the basis of characteristics such as color, mineralogic composition, and grain size. Principal sediment types found in the geologic units. Sands consist primarily of quartz grains between 0.0625 mm and 2.0 mm. Silts consist primarily of quartz grains between 0.0039 mm and 0.0625 mm. Limestone/marl deposits consist of true limestone or limy clay (marl). Muck consists of variable percentages of sand, silt, and clay mixed with a high percentage of fine-grained organic material. Descriptions of sediments in the LITHOLOGY and MODIFIER sections are qualitative visual descriptions.

MODIFIER 1, 2, 3, 4: These descriptive terms modify the principal lithologic terms. They are listed in a hierarchical order according to predominant characteristics. Modifiers may refer to size of sand grains (for example, fine-medium), to admixtures of materials of contrasting grain size (for example, silty, sandy, clayey), to special components (for example, fossiliferous, phosphatic), to grain-size sorting (for example, well-sorted, clean), or to stratification type or other fabric characteristics (cross-bedded, bioturbated).

Standard Geologic Grain Size Chart:
Gravel: larger than 2.0 millimeters
Very coarse: 1.0 to 2.0 mm
Coarse: 0.50 to 1.0 mm
Medium: 0.25 mm to 0.50 mm
Fine: 0.125 mm to 0.25 mm
Very fine: 0.0625 mm to 0.125 mm
Silt: 0.0039 mm to 0.0625 mm
Clay: smaller than 0.0039 mm

FRESH COLOR 1, 2: Dominant (1) and secondary (2) colors of sediments in fresh exposures and subsurface samples. Color identification was visually determined in the field from the Geological Society of Americas standard color chart.

WEATHERED COLOR 1, 2: Dominant (1) and secondary (2) colors of sediments in weathered exposures. Color identification was visually determined in the field from the Geological Society of Americas standard color chart.

COMPACTION: Qualitative field assessments of sediment compaction stated as "loose", "moderate", or "dense". Younger sediments (Pleistocene and Holocene) tend to be loose or moderately compacted. Older Eocene, Oligocene, and Miocene sediments tend to be dense relative to the younger deposits but still may be penetrated with a knife blade using moderate pressure. (Weems and others 1997).


Force, L.M., 1978a, Geological studies of the Charleston, South Carolina area - Elevation contours on the top of the Cooper formation: U.S. Geological Survey Miscellaneous Field Studies Map MF-1021-A, scale 1:250,000.

__________ 1978b, Geological studies of the Charleston, South Carolina area - thickness of overburden map: U.S. Geological Survey Miscellaneous Field Studies Map MF-1021-B, scale 1:250,000

Gohn, G.S., ed., 1983, Studies related to the Charleston, South Carolina earthquake of 1886 - Tectonics and seismicity: U.S. Geological Survey Professional Paper 1313, 375 p.

McCartan, Lucy, Weems, R.E., and Lemon, E.M. Jr., 1980, The Wando Formation (Upper Pleistocene) in the Charleston, South Carolina area, in Sohl, N. F., and Wright, W. B., Changes in stratigraphic nomenclature by the U.S. Geological Survey, 1979: U.S. Geological Survey Bulletin 1502-A, p. A110-A116.

McCartan, Lucy, Weems, R.E., and Lemon, E.M. Jr., 1984, Geology of the area between Charleston and Orangeburg, South Carolina: U.S. Geological Survey Miscellaneous Investigation Series, Map I-1472, scale 1:250,000.

Rankin, D.W. ed, 1977, Studies related to the Charleston, South Carolina, earthquake of 1886 - A preliminary report: U.S. Geological Survey Professional Paper 1028, 204 p.

Weems, R.E., and Lemon, E.M. Jr., 1984a, Geologic map of the Mount Holly Quadrangle, Dorchester and Charleston Counties, South Carolina. U.S. Geological Survey Geologic Quadrangle Map GQ-1579, scale 1:24,000.

Weems, R.E., and Lemon, E.M. Jr., 1984b, Geologic map of the Stallsville Quadrangle, Dorchester and Charleston Counties, South Carolina. U.S. Geological Survey Geologic Quadrangle Map GQ-1581, scale 1:24,000.

Weems, R.E., and Lemon, E.M. Jr., 1987, Geologic map of the Monks Corner Quadrangle, Berkeley County, South Carolina. U.S. Geological Survey Geologic Quadrangle Map GQ-1641, scale 1:24,000.

Weems, R.E., and Lemon, E.M. Jr., 1988, Geologic map of the Ladson Quadrangle, Berkeley, Charleston, and Dorchester Counties, South Carolina. U.S. Geological Survey Geologic Quadrangle Map GQ-1630, scale 1:24,000.

Weems, R.E., and Lemon, E.M. Jr., 1989, Geology of the Bethera, Cordesville, Huger, and Kitteridge Quadrangles, Berkeley County, South Carolina. U.S. Geological Survey Miscellaneous Investigation Map I-1854, scale 1:24,000.

Weems, R.E., and Lemon, E.M. Jr., 1993, Geology of the Cainhoy, Charleston, Fort Moultrie, and North Charleston Quadrangles, Charleston and Berkeley Counties, South Carolina. U.S. Geological Survey Miscellaneous Investigation Map I-1935, scale 1:24,000.

Weems, R.E., and Lemon, E.M. Jr., 1996, Geology of Clubhouse Crossroads and Osborn Quadrangles, Charleston and Dorchester Counties, South Carolina. U.S. Geological Survey Miscellaneous Investigation Map I-2491, scale 1:24,000.

Weems, R.E., Lemon, E.M. Jr., and Chirico, P.G., 1997, Digital Geology and Topography of the Charleston Quadrangle, Charleston and Berkeley Counties, South Carolina. U.S. Geological Survey Open File Series, OF 97-531, scale 1:24,000.

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