- Data source
- Input directly
Maps were created from regional and transverse field work that was conducted over a period of three years. Regional field work included collecting gravity and elevation data at a distribution density of approximately one station per square mile. The density of stations was lower in less accessible areas and higher in more accessible areas. Stations were plotted on vellum sheets overlain on 7.5 minute topographic maps. Each station was assigned a 4 character code detailing its location within the entire surveyed area. The precise latitude and longitude of each station was measured with a 0.01-minute accuracy with a device constructed by John Kick. All gravitation data collected were reduced with a USGS gravity reduction program modified by John Kick. Latitude and longitude data were used to compute a simple Bouguer anomaly and complete Bouguer anomaly for each station location. Some data were converted to a simple Bouguer anomaly using the formula subtracting theoretical gravity (dependent on latitude) from the observed gravity, adding the free air correction (dependent on latitude and elevation above sea level) and subtracting the Bouguer correction (dependent on elevation and a 2.67 g/cc density). In high relief areas, data were terrain corrected using the Bible (1962) modification of the Hammer (1939) method. A complete Bouguer anomaly was obtained by subtracting a curvature correction (dependent on elevation) from the terrain correction value and adding the simple Bouguer value. These gravity anomalies were used to create gravity contour maps. To digitize the maps, the 16 vellum maps were scanned on an Ideal Contex FSC 5010/3010 DSP Wide Format Large Document Color Scanner and cropped to remove excess collars using Adobe Photoshop Version 14.2.1 x64. Maps were imported into ESRI ArcGIS 10.03 and georeferenced using the four corners of the 7.5-minute quadrangles as control points. Once the coordinates of the four points were entered a projective tranformation was applied and the georeferencing updated. This process created a georeferenced tif file with associated tfw and ovr files
