This is a mathematically generated grid in which each polygon represents one quarter of a standard USGS 7 1/2 minute quadrangle. The result is a 3 3/4 minute quadrangle layer which nests within the standard USGS quads and whose tile naming scheme includes reference to the parent quadrangles. Known as the "Quarter Quad" tile index.
To provide a standard framework for accessing and creating data sets tiled by "quarter-quad."
Processing occurred in 2001.