Process_Description:
This dataset was captured, edited, and verified by two Allegheny College students working under faculty and staff supervision. The work spanned from September 2009 to July 2010; technical and financial support was provided by Allegheny College. The base map for the thickness of unconsolidated deposits dataset is the published "Map Showing the Thickness of Unconsolidated Deposits, Locations of Selected Wells, and Seismic-Refraction Cross Sections" by Richards et al., 1987.
The following processing steps were performed using ESRI's ArcGIS Desktop 9.3.1 software. First, a digital copy (w062_pl2.sid) of the Richards et al., 1987 map was downloaded from the Pennsylvania Geologic Survey (PAGS) website. The image file did not include a world file, and therefore needed to be georeferenced in order for map overlay to occur. The image was examined and four control points (latitude and longitude lines near the outside edges of the map) were identified and coordinates recorded in degrees, minutes, and seconds. A shapefile was then created and points were plotted at the exact coordinate locations. This was accomplished by displaying digital copies of 1:24,000 USGS topographic maps, which were projected into UTM Zone 17N NAD 1983, and using the software to place points on the map at the recorded coordinates. Next, the image within an unknown spatial reference was overlaid onto the projected point shapefile and georeferenced to real-world coordinates using the point shapefile as guide for inputting the X and Y source coordinates. In addition, snapping was used to ensure data integrity and a 1st order polynomial (affine) transformation was applied. The resulting RMS error was 2.2 meters. Once georeferenced, a world file was created to store map coordinate information for the image file.
Next, a file geodatabase, feature dataset, and line feature class were created. A feature dataset was chosen in order that 1) the same coordinate system was applied to all feature classes and 2) topology rules might be integrated into the database. The coordinate system of the feature dataset was defined as UTM Zone 17N NAD 1983. The line feature class was added to the map and an edit session was started. The map scale was set to 1:5,000 and was maintained to ensure consistency throughout the digitizing process. Using the Sketch tools, lines were digitized by tracing over the study area boundary and contact lines as depicted on the map. First, the boundary of the study area was drawn. Next, the contacts representing lines of equal thickness of sediment thickness were drawn. Snapping was used to ensure that the contact lines touched the study area boundary and island polygons closed correctly. This process took nine months to complete, as the students only worked a few hours each week.
Once digitizing was complete, a line topology class was created to ensure data integrity. The following topology rules were applied: must not overlap, must not intersect, must not have dangles, must not self-overlap, and must not self-intersect. Topology was validated and errors (overshoots, undershoots, and incorrect intersections) were corrected using the topology editing tools provided by the software. Next, a polygon feature class was built using the features in the line feature class. A polygon topology class was created to ensure data integrity. The following topology rules were applied: must not overlap and must not have gaps. Topology was validated and errors (sliver polygons and holes) were corrected. In some instances, small polygons needed to be added, removed, or modified within the feature class.
To allow for descriptive information to be added to the geographic features, attribute fields were added and attribute domains were established. The following fields were added to the layer attribute table: minimum thickness, maximum thickness, and interpretation. Range domains were created for the minimum and maximum thickness fields and a coded value domain was created for the interpretation field. These domains helped to increase efficiency in attributing and to ensure that only acceptable values were entered. Next, the domains were applied to their respective fields. After an edit session was started, individual polygons were selected and attribute values were assigned according to the legend as depicted on the Richards et al., 1987 map. Once complete, all of the polygons were selected and the attribute values were validated. If erroneous values existed, they were corrected and edits were saved.
To ensure both digitizing and attribute accuracy, a poster-sized printout of the digitized map was visually compared to the Richards et al., 1987 map and digitizing and attributing errors were identified and corrected. This process involved modifying polygon boundaries, validating topology, and adjusting attribute values as necessary. Polygons representing offshore areas of Lake Erie were not assigned values, but were instead left as <Null>. Once the datasets were deemed error free, the polygon feature class was exported into a new file geodatabase and metadata documentation was constructed.
Users should note that a 25 foot contour line is missing from the area on Presque Isle; this error occurs in the original published map and has not been changed in the digitizing process.