Knowing all the facts about an altimeter—the pluses and minuses—will make it as valuable as possible in the wilderness. First of all, keep in mind that temperature and weather are always working their will on an altimeter's accuracy. A high-pressure weather area will tend to cause a lower elevation reading than a low-pressure area. Warmer, lighter air will tend to result in a higher elevation reading than colder, heavier air.
There's no need to be surprised if an elevation reading of 5,200 as you go to sleep turns into 5,300 when you wake up the next morning, even though the tent appears to be in the same spot. The elevation hasn't changed, but the weather has, and with it, the air pressure that is the basis for an altimeter's determination of elevation. That's just the way altimeters are, and as with any good friend, you've got to accept them despite their bad habits. The best way to keep them relatively honest is to check the reading at every known elevation point and reset the altimeter accordingly. Topographic maps give the correct elevations of many of the features you encounter on a trip, such as trailheads, lakes, and summits.
Try to keep the temperature of the altimeter relatively constant, perhaps by carrying it in your pocket. The altimeter expands and contracts due to variations in its temperature, causing changes in the indicated elevation. A bimetallic element in temperature-compensated altimeters adjusts for this effect of temperature when there is no actual change in elevation. The element counterbalances the effect on other parts of the instrument. When you are gaining or losing elevation, however, this adjustment sometimes is not enough, resulting in errors even in altimeters that are temperature-compensated.
The altimeter can help in predicting weather. The altimeter and barometer scales operate in opposition to each other. When one goes up, the other goes down. An altimeter reading showing an increase in elevation when no actual elevation change has taken place (such as at camp overnight) means a falling barometer, which generally indicates deteriorating weather. A decreasing altimeter reading, on the other hand, means increasing barometric pressure, generally associated with improving weather. This is an oversimplification, of course, as weather forecasting is complicated by the wind, local weather peculiarities, and the rate of barometric pressure change. Stay observant on climbing trips if you want to figure out the relationship between weather and altimeter readings in your area.
Because even the most accurate and costly altimeters bow to the weather, don't be misled into trusting them to accuracies greater than are possible. A typical high-quality altimeter may have a scale of resolution (smallest marked division) of 20 feet and a stated accuracy of plus-or-minus 30 feet. This doesn't mean the altimeter will always be that close to the truth. Get to know your own altimeter, use it often, check it at every opportunity, and note differences of opinion between it and the map. You'll soon know just what accuracy to expect, and it will then be a dependable aid to roving the wilds.
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