## Usgs Topographic Maps

It might be interesting to start this discussion of USGS maps with a refresher on how cartographers divide up the earth. The distance around our planet is divided into 360 units called degrees. A measurement east or west is called longitude-, a measurement north or south is latitude. Longitude is measured 180 degrees, both east and west, starting at the Greenwich meridian in England. Latitude is measured 90 degrees, both north and south, from the equator.

By determining the intersection of longitude and latitude lines, any point on the earth's surface can be pinpointed. New York City, for instance, is situated at 74 degrees west longitude and 41 degrees north latitude. Each degree is divided into 60 units called minutes, and each minute is further subdivided into 60 seconds. On a map, a latitude of 47 degrees, 52 minutes, and 30 seconds north would probably be written like this: 47°52'30"N.

One type of USGS map used by mountaineers covers an area of 15 minutes (that is, !/4 degree) of longitude by 15 minutes of latitude. These maps are part of what is called the 15-minute series. Another type covers an area of 7.5 minutes (that is, degree) of latitude by 7.5 minutes of longitude. These maps are part of the 7.5-minute series.

The scale of a map is a ratio between measure ments on the map and measurements in the real world. A common way to state the scale is to compare a map measurement with a ground measurement (as in 1 inch equals 1 mile), or to give a specific mathematical ratio (as in 1:24,000, where any one unit of measure on the map equals 24,000 units of the same measure on the earth). The scale is usually shown graphically at the bottom of a map.

In the USGS 7.5-minute series, the scale is 1:24,000, or roughly 2l/2 inches to the mile, and each map covers an area of approximately 6 by 9 miles. It is the standard topographic map for all of the United States except Alaska.

In the 15-minute series, the scale is 1:62,500, or about 1 inch to the mile, and each map covers an area of four times that of the 7.5-minute series (about 12 by 18 miles). The 15-minute maps are no longer in production and are being replaced by the 7.5-minute series for all states except Alaska. (Most 15-minute maps will be permanently out of print by the mid-1990s.) For Alaska only, the 15-minute maps are still the standard, and the scale is somewhat different: 1:63,360, or exactly 1 inch to the mile. The east-west extent of each Alaska map is actually greater than 15 minutes because the lines of longitude are converging toward the North Pole.

The 15-minute and 7.5-minute maps are the ones that mountaineers use for cross-country route-finding and navigation. The USGS also produces maps that cover larger areas (with such scales as 1:100,000 and 1:250,000), and these are suitable for some trip planning and trail navigation. In some areas, private companies produce maps based on USGS topographic maps, but they are updated with more recent trail and road details and sometimes combine sections of USGS maps. These maps are often useful supplements to standard topographic maps.

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