On a topographic map, the contour interval is the physical elevation difference between each contour line, usually expressed in feet or meters. The smaller the contour interval, the more accurately landscape features and any elevation changes will be represented. Understanding contour lines and the interval when performing map and compass navigation are key skills to have, and especially useful for offtrail travel, though be aware that micro features are not always well reflected – a slope that looks to gently rise upwards could actually hold other features (such as a small cliff) between two contour lines where the interval is not close enough to reflect more minute changes in the landscape.

As an example, the popular National Geographic Trails Illustrated maps often utilize a contour interval of 50 feet. This contour interval will do a good job detailing medium and large scale terrain features but terrain features that fit "between the lines" will not be reflected.

Determining the Contour Interval

To determine the contour interval of your map, locate a bold contour line with a listed elevation (index contour line) as well as the next index contour line up or downslope. Count the number of lines (intermediate contour lines) between the two index lines, but do not count the line you started on. Then divide the listed elevation change by the number of lines counted, or simply look at the map legend where you can find the contour interval listed.

For a full guide to backcountry navigation techniques see our article in the Premium Issue 56 PDF ($10 or included with a Premium TrailGroove Subscription). You can also find more navigation tips here on the TrailGroove Blog.

Editor’s Note: This Jargon installment originally appeared in Issue 20 of TrailGroove Magazine. You can read the original article here.