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Topo Maps

October 3rd, 2009

A good topographical map is essential to have on any hike, but only if you know how to read it properly.  Lets look at what all the lines and scales are about.

Contour lines

The heart of a topo map is its overlay of contour lines, each line indicating a constant elevation as it follows the shape of the landscape. Contour lines that are close together represent steep slopes.

topographic map

Grids

Topographic maps almost always carry a grid of lines which divides them up. This is either based on degrees of latitude and longitude or a special grid developed by the mapping authority such as UTM -Universal Transverse Mercator coordinate system.

Simply defined, scale is the relationship between distance on the map and distance on the ground. A map scale usually is given as a fraction or a ratio—1/10,000 or 1:10,000.  These “representative fraction” scales mean that 1 unit of measurement on the map—1 inch or 1 centimeter—represents 10,000 of the same units on the ground.  The second number (ground distance) is different for each scale; the larger the second number is, the smaller the scale of the map. In the small-scale map (1:316,800), there is less room; therefore, everything must be drawn smaller, and some small streams, roads, and landmarks must be left out altogether. On the other hand, the larger scale map (1:63,360) permits more detail but covers much less ground.

The U.S. Geological Survey (USGS) publishes maps at various scales. The scale used for most U.S. topographic mapping is 1:24,000. USGS maps at this scale cover an area measuring 7.5 minutes of latitude and 7.5 minutes of longitude and are commonly called 7.5-minute quadrangle maps. Map coverage for most of the United States has been completed at this scale.  Maps at 1:24,000 scale are fairly large and provide detailed information about the features of an area, including the locations of important trails and most campgrounds. Footbridges, drawbridges, fence lines, and private roads are also shown at this scale.   I’ve included a USGS scale grid on the website to give you an idea of what scales are used where.   US sectional maps are 1 : 2,000,000 scale. where 1 inch represents 32 miles.

USGS Maps

Series Scale 1 inch represents
approximately
1 centimeter
represents
Standard quadrangle size
(latitude by longitude)
Quadrangle area
(square miles)
Puerto Rico 7.5 minute 1:20,000 1,667 feet 200 meters 7.5 by 7.5 minute 71
7.5 minute 1:24,000 2,000 feet (exact) 240 meters 7.5 by 7.5 minute 49 to 70
7.5 minute 1:25,000 2,083 feet 250 meters 7.5 by 7.5 minute 49 to 70
7.5 by 15 minute 1:25,000 2,083 feet 250 meters 7.5 by 15 minute 98 to 140
USGS-DMA 15 minute 1:50,000 4,166 feet 500 meters 15 by 15 minute 197 to 282
Alaska Maps 1:63,360 1 mile (exact) 633.6 meters 15 by 20 to 36 minute 207 to 281
County Maps 1:50,000 4,166 feet 500 meters County area Varies
County Maps 1:100,000 1.6 miles 1 kilometer County area Varies
30 by 60 minute 1:100,000 1.6 miles 1 kilometer 30 by 60 minute 1,568 to 2,240
1 degree by 2 degrees or 3 degrees 1:250,000 4 miles 2.5 kilometers 1° by 2° or 3° 4,580 to 8,669
State Maps 1:500,000 8 miles 5 kilometers State area Varies
State Maps 1:1,000,000 16 miles 10 kilometers State area Varies
U.S. Sectional Maps 1:2,000,000 32 miles 20 kilometers State groups Varies
Antarctica Maps 1:250,000 4 miles 2.5 kilometers 1° by 3° to 15° 4,089 to 8,336
Antarctica Maps 1:500,000 8 miles 5 kilometers 2° by 7.5° 28,174 to 30,462T
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