The altimeter helps in deciding whether to continue a climb or to turn back, by letting you calculate your rate of ascent. Let's say you have been keeping an hourly check on time and elevation during a climb. It has taken the party 4 hours to climb 3,000 feet, an average of 750 feet per hour. But you know that the actual rate of ascent has been declining with each hour. In fact, the party gained only 500 feet in the past hour, compared with 1,000 feet the first hour. You know that the summit is at an elevation of 8,400 feet, and an altimeter reading shows you're now at 6,400. So you can predict that it will take roughly 4 more hours to reach the summit. Take that information, courtesy of the altimeter, combine it with a look at the weather, the time of day, and the condition of the climbers, and you have the data on which to base a sound decision.
An altimeter also can help determine exactly
where you are. If you are climbing a ridge shown on the map, but don't know exactly where you are along the ridge, check the altimeter for the elevation. Where the ridge reaches that contour line on the map is your likely location.
Another way to ask the altimeter where you are is to start with a compass bearing to a summit or some other known feature. Find that peak on the map, and plot the bearing line from the mountain back toward the climbing party. You now know you must be somewhere along that line. But where? Take an altimeter reading and find out the elevation. Where the compass bearing line crosses a contour line at that elevation is your likely location. This could lead to an ambiguous answer, of course, because the line might cross that contour at several points. That's when you turn to further observations, common sense, and intuition.
Navigation gets easier with the aid of an altimeter. If you top a convenient couloir at 9,400 feet and gain the summit ridge, make a note of that elevation. On the way back, descend the ridge to that elevation and you will easily find the couloir again.
Guidebook descriptions sometimes specify a change in direction at a particular elevation. If it's on an open snowfield or a forested hillside, good luck in making the turn at the right place without an altimeter. The route you have worked out on a topographic map also may depend on course changes at certain elevations, and again the altime
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ter will keep the party on target. An altimeter obviously helps in mapping, providing elevations of key points along routes included on the map.
Last but not least, an altimeter will reveal if you're on the real summit when the visibility is too poor to be able to tell by looking around.
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