Topographic Maps Definition

Use topo maps when you need to know the elevation

Topographic map
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Topographic maps are highly detailed maps that show both the natural terrain and man-made roads and buildings. They differ from most types of maps because they show elevation, but they have all the other elements that you find on maps including a legend, scale and north-pointing arrow. Topographic maps are frequently paired with handheld GPS devices, sports and fitness GPS devices and smartphone applications. Topographic maps in their paper form have been in use for many years and are a mainstay of outdoorsy people, urban planners and those who must understand landscape details for business purposes.

Topographic Maps Show Elevation With Contour Lines

When you look at a map, you are looking straight down at a representation of the Earth, so it is difficult to identify changes in elevation. Topographic maps use contour lines to indicate elevation. Each contour line on a map connects points that have an equal elevation. In theory, if you follow a single contour line, you walk at the same elevation all the way around until you return at your starting point. Contour lines follow certain specific requirements, including:

  • Each spot along a contour line has the same elevation.
  • A contour line forms a closed shape. 
  • Contour lines can't cross one another.

A tiny number appears on some contour lines indicating the elevation above sea level. Most U.S. topographic maps show the elevation in feet, but some show it in meters. However, not all contour lines are labeled with a number. In this case, you need to know the contour interval to figure out the elevation of some of the lines.

Explanation of Contour Intervals 

When you look at a section of contour lines on a map, you'll see they appear to be spaced at uneven intervals, but there is a logical explanation. They are spaced at intervals that change as the elevation changes. You need to know the contour intervals to interpret changes in elevation at a glance on a map.  To figure out the contour interval:

  1. Locate two contour lines on the map that are labeled with their heights and that have one or more unlabeled contours between them.  
  2. Subtract the smaller elevation number printed on one contour line from the larger number on the other unlabeled contour.
  3. Divide the result by the number of unlabeled lines between them to arrive at the contour interval.

For example, if you have two contour lines labeled 30 and 40 feet with a single unlabeled contour line between them, the contour interval is 5 feet. The elevation at any point on the unlabeled contour is 35 feet. The contour interval value remains constant for all the contours on the map.

You are unlikely to see a single contour line except in flat areas. The more abrupt the elevation changes are, the more contour lines are needed to illustrate the changes. 

 Where to Get Topographic Maps

The U.S. Geological Survey offers free downloads of current and historical topographic maps of the U.S. in PDF format at its website. Garmin offers several topographic map sets for sale at its website, and the Camping and Hiking section at Amazon has a selection of topographic maps available. Topographic maps are increasingly stored, transmitted and used in digital format.

Scale of Topographic Maps

Topographic maps come in different scales, and the differences are important. For example, the common 24K topo map is at the scale of 1:24,000 (1 inch = 2,000 feet) and shows great detail. The 24K map is also known as a 7.5-minute map because it covers 7.5 minutes of latitude and longitude. Another common format, the 100K topo map, is at the scale of 1:100,000 (1 centimeter = 1 kilometer) and shows less detail but covers a wider area than the 24K map.

What Is a Relief Map?

A relief map is a type of typographic map doesn't use contour lines. Instead, it is drawn and colored to show changes in elevation. This gives the map a realistic look, and you can easily distinguish between mountains and valleys just by looking. A globe with raised mountain ranges is also a type of relief map.