Superfund GIS - 1:250,000 Geology of Tennessee

The data set was developed to determine relative geology at scales suitable
for use at a scale 1:250,000. The data set should not be used in situations
where positional accuracy is an issue.

This data set is a digital representation of the printed 1:250,000 geologic
maps from the Tennessee Department of Environment and Conservation, Division of
Geology. The coverage was designed primarily to provide a more detailed
geologic base than the 1:2,500,000 King and Beikman (1974). 1:24,000 scale
coverage of the state is available for about 40 percent of the state. Formation
names and geologic unit codes used in the coverage are from the Tennessee
Division of Geology published maps and may not conform to USGS nomenclature.
The Tennessee Division of Geology can be contacted at [phone]

Procedures_Used:

SCANNING THE LINEWORK
The Tennessee Division of Geology provided the negatives and/or scribecoats
for the west, west-central, east-central, and east map sheets for the
1:250,000 Geologic Map of Tennessee. The negatives for the west and
west-central sheets were scanned and saved as TIF image files at 400 dpi
black and white using a Contex FSS-3012t scanner. Positives were made from
the scribecoats for the east-central and east sheets. The positives were
scanned using the same system. The resulting images were transferred to a
Unix workstation for processing in Arc/Info 7.0.

GEO-REFERENCING
The original negatives and scribecoats did not have any positional tic marks
which could be used for geo-referencing. The 1:250,000 USGS Tennessee state
boundary had to be used as a reference. Each of the four scanned images were
geo-referenced into the Universal Transverse Mercator Zone 16 coordinate
system using the Arc/Info REGISTER command. Because of the lack of positional
registration on the source material, positional accuracy cannot be
guaranteed.

DELINEATION
Due to time constraints and personnel shortage, hand delineation of the
geologic features was not possible. An automated method of delineation from
the scanned data was tested. The images were converted into GRIDs using the
Arc/Info IMAGEGRID command. Several attempts at automated delineation were
made using the GRIDLINE command. After several variations on command
parameters, the resulting linework was acceptable and passed onto the next
step.

CLEANING
The linework resulting from the GRIDLINE command was a good conversion from
the raster data, but many problems had to be solved. Because of scanner
limitations, some of the closer linework merged together into single lines.
These had to be deleted and digitized by hand. There were many spots where
linework had "spider-webbed". This occurred where the linework was close but
not close enough to merge and cause the automated delineation to create
spurious lines between the actual lines. These extraneous lines were removed
as well. After linework errors were corrected, the resulting linework was
passed through the SPLINE command in ArcEdit. This smoothed out most of the
jagged linework caused by the GRIDLINE command. As a side effect, fault lines
were altered where intersected by other linework. Some of these effects were
corrected when they were visible on a paper plot while most were left as they
were for future correction. After errors were corrected, polygons were built
from the linework and then checked for closure. Errors found were corrected
and the coverage was rebuilt. This process was repeated for each of the four
sheets until all obvious errors were corrected.

ATTRIBUTION
Once the linework was clean, each line was then tagged as being a nonfault,
fault, or metamorphic isograd. During this tagging, more errors were found
and corrected. Polygons were attributed with the geologic symbol code found
on the printed maps. During this process, more errors were found with unclosed
polygons which were fixed and retagged. Each of the four sheets went through
this process. Each sheet was then plotted to scale using a custom Arc/Info
shadeset designed to mimic the printed shade patterns as closely as possible.
This process allowed easier verification of the geologic formation
attribution. Any errors in linework, line attribution, and polygon
attribution were corrected. Geologic symbol codes were based on the published
Tennessee Division of Geology maps and may not conform to USGS nomenclature.

MERGING
The two sheets for western Tennessee were appended together. The resulting
linework at the sheet boundaries had to be cleaned and the map boundaries
removed. The polygons were then checked for correct attribution along the map
boundary and any errors were corrected. The same process was repeated for the
eastern Tennessee sheets. The two resulting coverages where then merged into a
single coverage using the same process of removing map boundaries and
checking/correcting polygon and line features and attribution. Once all four
sheets were merged into a single coverage, all polygons with an area of less
than 5,000 square meters were removed. These polygons were impossible to
recognize when plotted at scale and, in most cases, were "spider-webbing"
errors not found during the initial delineation process.