One Degree Block

A one degree block measures 1 degree in latitude by 1 degree in longitude, and each block has a unique number corresponding the lower right hand corner of the block. For example, degree block 32097 has a latitude of 32 degrees north and 097 degrees west at the southeast (lower right) corner.

Each degree block is divided in 64 sections, and each section measures 7.5 minutes by 7.5 minutes – thus the 7.5 minute QUAD. Sections or cells are numbered 01 through 64 as shown below:

Sometimes a seven character alphanumeric code is used to identify the sections of a one degree block. In this case, rows are lettered A through H beginning at the lower right hand corner, and numbered east to west from 1 to 8. The bottom right corner is A1, the upper left is H8.

Literally hundreds of sites offer free downloads of USGS QUADs as well as other types of freely distributed data and filenames are often in the xxyyyzz format.

xx = latitude
yyy = longitude
zz = section number or alphanumeric code of the one degree block.

If a digital QUAD (typically downloaded as a .tif image) is collared, the original USGS collar with notes, distances, etc. is shown. If the QUAD is collarless, this information has been removed and the .tif image fits in the 7.5 minute by 7.5 minute square.