Trail leading to Mt Lafayette, Franconia Ridge
Topographic maps are used by hikers and backpackers to plan routes, estimate travel times, find water, good campsites, and track their progress on hikes. They’re designed to depict the three dimensional shape of the world (mountains, valleys, plains, etc.) in two dimensions using flat squiggly lines that represent different elevations above or below sea level. When contour lines are close together on a map, they depict a steeper slope than if the lines are father apart.
Called contour lines, every point along a single continuous line has the same elevation, which is labelled somewhere along the line. The elevation difference between two contour lines is called the contour interval. You can often find the value of the contour interval in the map’s scale or figure it out by finding two contour lines with printed elevations, subtracting the smaller from the larger and dividing the total by the number of contour lines between the two elevations.
Let’s look at two different ways to use a topographic map, called “orienting a map”, and for estimating travel times.
Orienting a Map
With a little practice, you can learn to match landscape features you see in front of you to ones on maps that depict the area you’re hiking in. This is useful for orienting a map to determine your current location on it and make it match the land features you’re looking at. Using a compass or a GPS makes can make it easier to orient your map, but you can also learn how to do it simply by matching the terrain features you can see with the contour lines shown on a topographic map.
Map 1: Topographic Map of Franconia Ridge
For example, take the top photo shown above, which shows the Appalachian Trail running north to Mt Lafayette on Franconia Ridge in New Hampshire. That same trail is shown in the topographic map in Map 1, above, and marked in pink. Note how the contour line elevations to the left and right of the pink line increase until they meet at the middle at the trail. This is what the contours lines for a ridge line look like on a topographic map, two steep slopes that meet at the top.
Map 2: Three-dimensional shaded version of Franconia Ridge
Here’s a shaded rendering of the same area that helps emphasize the elevations and gullies represented by the contour lines and the ridge line in between them. With practice you can take the contours lines shown in Map 1 and visualize them in your head like they’re shown in Map 2. Most topographic maps are printed or displayed like Map 1, so you can’t rely on having ones like Map 2, which are easier to visualize.
If you could see the ridge line from where you’re standing, you could turn your map so that the ridge was pointed in the same direction (parallel) with your map if you were holding it flat in your hands on flat on the ground. This is called orienting your map. How do you know it’s the right ridge line if you can see many? You’ll need to identify other features in the landscape, such as mountain peaks or hills, and match them to the topographic map.
Once you’ve oriented a map, you can usually tell where you are on it, and plan your next destination by following trails to where you want to go.
Estimating Travel Times
There are lots of reasons to estimate travel times on hikes:
- You meet someone at a certain time and place down the trail, but need a good estimate of how long it will take you to cover the distance.
- You want to finish your hike before nightfall, because it’s easy to lose a trail in the dark and get lost.
- You want to minimize the amount of food weight you need to carry, so you need a good estimate of the number of meals you’ll require.
You can estimate the time it will take you to hike a known distance on a topographic map by adding up the trail distance you need to cover and the elevation gain along your route. On average, most hikers can walk at a 2 mph on level ground, but take an additional 30 minutes to climb 1000 feet of elevation gain. For example, it would take you 1.5 hours to hike a distance of 2 miles with 1000 feet of elevation gain.
You can determine the distance you need to hike by using the map scale printed (or displayed) on your map. To calculate elevation gain, you’d add up the number of uphill contour lines and multiply that number by the contour interval. Make sure you just count the contour lines along your route that are climbing up hill and ignore the ones that represent descents. Use estimating formula described above and you’ll come up with a surprisingly accurate estimate of the time it will take for you to cover the required distance and elevation gain, not including any long breaks you take along them way.