Land navigation basics: Contour lines |

Land navigation basics: Contour lines

Greg Foley
Mountain Rescue

Last fall we had a search mission for a 65-year-old who was looking at spending his second night out without shelter, food or water. He had planned an aggressive 10-mile loop hike starting at the Lake Evelyn trailhead which summited Bills and Byers Peaks, but had gotten off route after summiting Bills Peak. When he finally called 911 the next day he was dehydrated, hallucinating and disoriented. For more than 24 hours he had been "bushwhacking" in the Byers Peak Wilderness.

He had a guidebook map, but no compass. Part of the route was off trail, along a ridge. He really had no idea where he had been, or how far he had traveled, before he found his way to the summit of Byers Peak and hiked north down the old Byers Peak Trail. He was unable to find the trail that would bring him back to his vehicle. One of his comments was that he had hoped for better signage along the trail.

One of the "Ten Essentials" that every backcountry traveler should have with them is a map and compass. The ability to actually understand how use a map and compass for land navigation is critical. Why carry a tool if you don't know how it works?

In a previous article I discussed various types of maps available, large scale versus small scale maps and where to get free topographic maps. Bottom line is that in order for a map to be useful it has to be at a large enough scale to provide good detail and it has to be "topographic."

"The distinctive characteristic of a topographic map is that the shape of the Earth's surface is shown by contour lines. Contours are imaginary lines that join points of equal elevation on the surface of the land above or below mean sea level. Contours make it possible to measure the height of mountains, depths of the ocean bottom, and steepness of slopes."

Contour lines are those squiggly lines that are all over the map. By understanding what contour lines represent you can visualize land forms like peaks, valleys, ridges and cliffs.

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Since each line represents a certain elevation, they can never cross each other. Well, almost never – visualize an overhanging cliff. Depending on the scale of the map the elevation difference between adjacent lines, the "contour interval", will vary. A smaller contour interval means more elevation detail. Typical contour intervals, which are noted in the map legend, are 20, 40, 50 and 100 feet. Metric contour maps are also available.

Some of the contour lines are thicker or darker than others. These are "index contours" and are spaced at five contour line intervals. If the contour interval is 50 feet, each index contour represents 250 feet in elevation. Index contours are marked with the elevation they represent.

One way to visualize terrain with a topographic map is to understand that the closer the contour lines are to each other, the steeper the terrain. A 500 foot vertical cliff would have ten 50 foot contours stacked up on the map. On "Fraser Flats" along US 40 between Fraser and Tabernash, there is about one half mile between 40 foot contours. With some practice in the field you can get a good feel for how steep terrain is and how you might plan a route by using your map.

Two of the most important mapping considerations when planning a hike in the mountains are distance and elevation gain or loss. Most experienced hikers have a good idea of how fast they hike, and that speed is obviously different for uphill and downhill. With a topographic map it is very easy to determine the distance between two points and also determine the change in elevation. Based on your personal knowledge you can then estimate the time it will take to complete the route.

Average numbers in mountainous terrain might be two miles per hour or 1000 feet in elevation gain per hour. In a mountain rescue application, we could use these numbers to calculate, on a map, how far a missing subject may have traveled in a particular direction since he was last seen.

Contour lines are only a part of the data set available on topographic maps. Information about natural and man-made features, grid lines for determining location and declination information for determining compass bearings are also important for land navigation.

With a bit of study and practice you can become proficient with your map and compass. Then you will have a tool that will make you a more proficient mountaineer and maybe even save your butt some dark night.

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