USGS Map Indices Downloadable Data Collection
This data consists of data grids for the entire United States area, including
                1 X 2 Degree, 1 X 1 Degree, 30 X 60 Minute, 15 X 15 Minute, 7.5 X 7.5 Minute, and
                3.75 X 3.75 Minute. The grid was generated using ESRI ArcInfo GIS
                software.