How to Choose and Use a Wrist Altimeter

A wrist altimeter—also known as an altimeter watch, altimeter/barometer or ABC (altimeter/barometer
/compass) —is a handy asset in the outdoors. It estimates your elevation and changes in the weather based on changes in barometric pressure. It's helpful to know the basics of how a wrist altimeter works so you can understand its strengths and limitations.

How Does a Wrist Altimeter Work?

A wrist altimeter is a multifunctional digital wristwatch equipped with a barometric sensor that measures air (or atmospheric) pressure. In simple terms, air pressure is the weight of the atmosphere at a certain location.

A wrist altimeter converts this air pressure reading into an elevation figure. This elevation figure is a close estimate of the elevation where you stand—very useful data for backcountry travel.

We say "close estimate" because whenever weather changes, air pressure changes. A change in air pressure can cause the elevation reading on your wrist altimeter to change, sometimes by more than 100 feet. Yet with proper calibration, a wrist altimeter 1) provides reasonably accurate readings of your elevation, and 2) keeps this information at your fingertips in a watch-sized tool equipped with other useful functions.

Knowing your altitude is advantageous as you navigate mountainous terrain. It can help you:

• determine your location on a topo map.
• track your progress.

Weather's Effect on an Altimeter

As noted above, a wrist altimeter is a barometer that measures the weight (pressure) of air above its location. This reading is calibrated in inches of mercury, or its metric equivalent, millibars. Altimeters convert this reading into an elevation figure indicating feet or meters above sea level.

When weather changes, air pressure typically changes as well. This can cause a wrist altimeter's elevation reading to change even though your actual altitude has not.

An approaching storm can lower (reduce) air pressure, causing an altimeter's elevation reading to rise. Fair weather typically involves high pressure, so your reading may drop. Even in fair weather, an unmoved altimeter's elevation reading may vary as much as 100 feet due to minor air pressure changes.

What to do? The trick to successful altimeter use is to adapt to such fluctuations by adjusting your altimeter at known elevations.

When to Calibrate Your Altimeter

When starting a trip, calibrate your altimeter to the correct altitude when you are at a location with a known elevation, such as a trailhead. You can determine your correct altitude from landmarks or elevations on your topo map or in a guidebook. Be sure to occasionally check and recalibrate your altitude at other locations with known elevations—lakes, trail junctions, passes or peaks—as indicated on your map.

It's also wise to note your elevation before going to bed and recalibrating it to that elevation (as needed) when you wake up. Barometric pressure can change quite a bit overnight and impact the accuracy of your altitude reading.

Frequent checking will help familiarize you with the type of measurement fluctuations that routinely occur with this instrument. Over time, your confidence in it will grow as you become fully acquainted with its performance.

The Altimeter Learning Curve

On most models, the task of obtaining and calibrating an elevation reading is relatively simple. Before using it in the wilderness, plan on devoting some time to reading the owner's manual to make sure you understand these basic steps.

Keep in mind that a wrist altimeter is a complex tool that typically offers many other useful features, too. These will require additional time with the manual and some practice to master. (Warning: Some owner's manuals can be quite extensive.) Once you fully comprehend its operations, the payoff is greater confidence in navigation.

Wrist Altimeter Features

Virtually all wrist altimeters available today offer a backlight feature, an altitude range (up to 29,500', or 8,990m), a calendar, daily alarms, and readings for temperature and barometric pressure.

When shopping for a wrist altimeter, here are the other main features and options to compare:

Altitude resolution: The increment at which altitude is measured. This is typically either 1m (3.2') or 5m (16').

Rate of ascent/descent: This optional feature provides your actual rate of ascent/descent in feet/hour. This helps you gauge the time and effort needed to reach a location. If you know your current elevation and your final elevation, you can use the ascent/descent rate to determine your approximate arrival times at known elevations. On a day trip, this information could help you decide when it might be wise to turn around.

Total ascent/descent: Calculates cumulative elevation gains and losses during a trip. This optional feature is good for record-keeping (and bragging rights).

Altitude alarm: Signals you when a preset elevation has been attained. This option can help you locate, for example, a predetermined starting point for cross-country travel to off-trail spots.

Altimeter/barometer lock: When stationary for an extended time (e.g., overnight), you can lock the altimeter or barometer function with this optional feature. With the altimeter locked (say at 4,000') when you go to bed at night, you'll wake up the next morning at that same elevation even if barometric pressure has changed.

Weather alarm/forecast: When stationary for an extended period, you can set an alarm (linked to the rate of ascent/descent reading) to alert you to a pressure change significant enough to impact the weather. Most wrist altimeters come with this feature. If the pressure is dropping rapidly, it is a good indication that a low-pressure storm is coming.

Compass: This is another commonly included feature. Electronic compasses included with these units all have a bearing resolution of 1 degree as well as declination settings. All require calibration.

Atomic clock: An extremely accurate clock based on an atomic oscillator at a remote location. In the U.S., atomic clocks receive their signal from the U.S. Naval Observatory in Fort Collins, Colo., over a radio frequency. This feature is offered on a few wrist altimeter models.

Watch materials (housing/strap): Materials are mostly a matter of personal style. Watch housings are typically stainless steel, composites, polycarbonate or a resin. For rigorous outdoor use, keep in mind that nonmetallic materials tend to show wear less. Wrist straps are either metal, polyurethane or a resin.

Consumer replaceable battery: REI and most manufacturers recommend professional installation of new batteries. Batteries for certain models can be replaced by consumers. The manufacturer will state in the owner's manual whether or not a consumer should replace the batteries. On average, batteries in wrist altimeters last about 1 year.

Tips for Successful Use

To improve the odds of happiness with your altimeter, we offer these suggestions:

• Practice using your altimeter on day hikes, especially where known elevations are often encountered. This will speed up your familiarity with your altimeter's inevitable fluctuations.
• Explore with a friend who has experience using an altimeter. Ask; observe; practice.
• Read a how-to book devoted to navigation. Shop REI.com's selection of navigation books.
• Since an electronic compass operates on batteries, never rely solely on it. Always carry a traditional air- or liquid-filled compass.

For an overview of related electronic devices, see the REI Expert Advice article on How to Choose a Fitness Monitor.

Contributor: Mike McCarty, REI product manager.

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