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USGS 1:6000 (Quarter-Quarter 7 1/2 Minute) Quadrangle Index
This is a mathematically generated grid in which each polygon represents one quarter of the already quartered 225 second (or 3 3/4 minute) data resource, or a quarter-quarter of... -
USGS 1:12000 (Quarter 7 1/2 Minute) Quadrangle Index
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...
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