Reading Topographic Maps

February 24, 2012

When I began hiking, I was a college student working summers in AMC’s Reservations Department in Pinkham Notch. One of the job perks was a free copy of the White Mountain Guide, fat with maps.

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In those early years, my map reading was pretty rudimentary. I determined the route, read the associated trail descriptions in the guide, and then tucked the requisite map in my backpack in case I needed it. I felt prepared, but I wasn’t.

To better navigate the three-dimensional terrain under foot, I needed to be able to first read that landscape two-dimensionally on a map. Contour lines would be my guides.

Reading the lay of the land
Topographic maps, and the contour lines that pattern them, are detailed blueprints depicting elevation changes, valleys and streambeds, ridges and cliffs, and other key characteristics of a trail.

Deciphering that blueprint before and during a hike is essential. Fallen trees can change or obscure a trail. Signs at critical junctions can fall or become buried by snow. Visibility can diminish or trail blazes fade. Contour lines are reliable reference points to the surrounding landscape. Reading them correctly reduces your chances of getting lost and better prepares you for the terrain ahead.

Knowing where you stand
There are two types of contour lines. The thicker lines—or index lines—denote the elevation above sea level and are labeled. Each contour line is continuous and will never intersect with another (unless an overhanging cliff is present), meaning the elevation on that line is constant.

The thinner, unlabeled lines that fall between index lines are intermediate lines, indicating changes, or intervals, in elevation. All of AMC’s maps—and most good trail maps—will list the “contour interval” in the map legend, which is the amount of elevation change between each line. The maps included with the White Mountain Guide, for instance, list an interval of 100 feet. If you then look at an index line of 2,500 feet, the interval line to the inside (or above) that is an elevation of 2,600 feet while the interval to the outside (or below) is 2,400 feet. Generally, every fifth line on a map is an index line.

Be aware that a map’s topography or scale will dictate the contour interval, so don’t assume it will be constant from one map to the next. Also note if U.S. or metric system measurements are used.

Getting the grade right
Interpreting elevation change is the first step toward ensuring a particular route is one you can handle. Contour lines provide great visual cues indicating the slope of the land. On gentle grades, contour lines are spaced widely apart. Crowded lines denote a rapid gain in elevation (see the grade on Kearsarge North versus Rickers Knoll in Image No. 1, above).

Hilltops and peaks aren’t always labeled, but they are often easy to pick out. Look for a whorl of contour lines. The smallest circle in the middle is the summit or high point (see Kearsarge North in Image No. 1).

Keep your topo map handy once you are on the trail and reference it often. When the terrain changes—like the grade—note where you are on the map. You’ll be more prepared for what’s ahead and better able to retrace your steps should you take a wrong turn.

Learning your alphabet
Contour lines also form certain letter shapes that provide additional clues to the landscape. U-shaped lines with the bottom of the “U” pointing downhill represent ridges (see the contours around the Howker Ridge Trail in Image No. 2). Narrow valleys or streambeds, on the other hand, are V-shaped lines with the point of the “V” pointed upstream (uphill; see the contours of Cowboy Brook in Image No. 3).

“M” or “W” lines form upstream from a stream junction (see Image No. 4). Think of two upward pointing “V’s,” or streambeds, side by side. The contour shape is determined by how the two streams converge. Being able to locate a nearby streambed on a map, and knowing that water always flows downhill, can be a lifesaver if you find yourself off trail.

So, I now tuck my topo maps into a pocket and break them out regularly. I’ve learned they are one of the best tools for deciphering the backcountry.

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