Process_Description:
The following processing steps were completed by four Allegheny College students as part of the French Creek Watershed Research Program (FCWRP) during the summer of 2005. Support for this work was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-0126808. The base map for the glacial deposits data set is the published "Map of Glacial Deposits of Northwestern Pennsylvania" by Shepps et al., 1959.
First, a hard-copy version of the Shepps et al., 1959 map was divided into northern and southern sections in order that it could be digitized on a 36" x 48", high accuracy Summagrid V digitizing tablet. Next, the northern section of the map was placed on the digitizing tablet and four registration points were identified using latitude and longitude lines near the edges of the map. ARC/INFO Workstation 9.1 software was then used to digitize the four registration points, the boundaries of the study area, and all contacts between lithologic units. In addition, unique arc-ids were assigned to both the boundaries of the study area and contact lines. Once completed, this process was repeated for the southern section of the map.
After both sections of the map were digitized, the coverages were cleaned and arc-node topology was generated. Next, the coverages were edited in order to correct node errors (overshoots and undershoots) and unassigned or incorrectly assigned arc-ids. To ensure digitizing accuracy, 42" x 36" printouts of the digitized coverages were visually compared to the Shepps et al., 1959 map and digitizing errors were identified and corrected. Once both coverages were deemed error free, they were transformed from digitizer (inches) units into Universal Transverse Mercator (meter) units. After transformation, the error in horizontal accuracy far exceeded National Map Accuracy Standards for source maps at a scale of 1:125,000. It was determined that the registration error (> 500 meters) was due to inaccuracies in the published latitude and longitude lines near the edge of the map. Instead of re-digitizing the entire map, a solution was developed.
The following solution was developed and completed by the Allegheny College GIS Laboratory Manager. Using latitude and longitude intersections, based on U.S. Geologic Survey topographic quadrangles within the original Shepps, et al., 1959 map, 16 new registration points were identified on both the northern and southern sections of the map. These new registration points and the boundaries of the study areas were digitized into two, new coverages and transformed into UTM coordinates. After transformation, the horizontal errors for the northern and southern sections of the map were 0.018 inches (56.127 meters) and 0.019 inches (59.401 meters) respectively. These errors in horizontal accuracy are less than those required by the National Map Accuracy Standards for maps at a scale of 1:125,000.
The Spatial Adjustment tool within ArcGIS 9.1 was then used to align the coverages containing the lithologic contact lines (RMS >500m) with the coverages containing the 16 registration points and boundaries of the study areas (RMS <63m). First, both coverages were converted into geodatabase feature classes using ArcCatalog, so that they could be edited using ArcMap. Next, the arcs within the feature class containing lithologic contact lines were aligned to the arcs in the feature class containing 16 registration points and study area boundaries using the rubber-sheet method of adjustment. After adjusting, the arcs within the northern and southern portions of the map were merged together to create a new feature class containing all lithologic contacts within the study area. After merging, the boundary line between the northern and southern sections of the map was removed and the new feature class was converted back into a coverage using ArcCatalog.
After adjusting the data, the coverage was edited and cleaned using ARC/INFO Workstation 9.1 software in order to remove topological errors. Next, polygon topology was generated and label points were created for each polygon within the coverage. A unique label-id scheme was then developed in order assign each lithologic unit within the map a unique-id. Using the label-id scheme, the label points within the coverage were then attributed accordingly. After attributing, a large printout of the attributed coverage was visually compared to the original Shepps et al., 1959 map. Label errors such as polygons with missing or multiple label points and attribute errors such as incorrect label-ids were identified and corrected. Once all label errors were corrected, additional items such as glacial deposit code, epoch, epoch code, stage, stage code, unit, unit code, symbol, symbol code, description, material, and topography were added to the polygon attribute table. These attribute items were designed to mimic that of the legend on Shepps, et al., 1959 map. Values for the new attribute items were then attributed accordingly and polygon topology was re-built. Finally, the attributes were visually verified to ensure accuracy and completeness and the coverage was exported as an ArcInfo export file (glacdepo_nwpa.e00).
On May 24, 2010 the coverage was migrated to geodatabase format in preparation of sharing with the Pennsylvania Spatial Data Access (PASDA) Clearinghouse. Using ArcGIS Desktop 9.3.1 the coverage polygon feature class containing lithologic contacts was exported to a file geodatabase polygon feature class and the AREA, PERIMETER, GLACDEPO_NWPA, and GLACDEPO_NWPA_ID fields were eliminated. The metadata was then updated to reflect this change.