Individual topographic maps are commonly referred to as quadrangles (or quads), with the name of the quadrangle indicating the area covered by the map. The largest area covered by most topographic maps used for scientific mapping purposes (i.e. geologic mapping, habitat studies, etc.) is two degrees of longitude by one degree of latitude.

A map of this size is referred to as a ‘two degree sheet’. One, two degree sheet can be divided into four smaller quadrangles, each covering one degree of longitude and 1/2 degree of latitude (‘one degree sheet’).

Each one degree sheet is subdivided into eight ‘fifteen minute quadrangles’, measuring fifteen minutes of latitude and longitude.

Finally, the smallest topographic quadrangle commonly published by the U.S. Geological Survey is a 7.5-minute quadrangles, which measure 7.5 minutes of latitude and longitude. There are four 7.5-minute quads per fifteen-minute quad, 32 per one-degree sheet, and 128 per two-degree sheet.

You can determine what type of quadrangle you are looking at by subtracting the longitude value printed in the upper (or lower) left corner of the map from the longitude printed in the upper (or lower) right corner of the map. This can also be done using latitude values, just remember that a two-degree sheet only covers one degree of latitude and a one-degree sheet only covers thirty minutes of latitude. This information is also commonly printed in the upper right-hand corner of a map, under the title of the map.