From the site: "In May/June 2007, two hardcopy map sheets of the 1976 Structural Map of WV were scanned at 300 dpi 8-bit color by the WV GIS Technical Center. The scanned TIFs were converted to index color mode, and resampled to 150 dpi. The map sections were then mosaicked together and georeferenced to a 15-minute Longitude/Latitude grid."