How To: Pick an Altimeter

Early altimeters with 19 jewels and Swiss-made movements were price-prohibitive and thus weren’t included in either The Mountaineers navigation courses or the Ten Essentials. However, with new technology emerging, the upcoming  9th edition of Freedom of the Hills  will feature map, compass, and all of the modern tools with altimeters prominent among them.

Since mountains are not two-dimensional...the altimeter is sometimes as helpful as the compass, particularly where topographic maps are available. With altitude known, point-position can often be found with only one visible feature recognized; in any other case, altitude provides a check against map and compass orientation.  - Mountaineering: The Freedom of the Hills, 1st ed. 1960, p. 79

In October, some 70 navigation students took the newly named Wilderness Navigation class (FKA Basic Navigation). The class has been updated by the Seattle Navigation Committee with input from several branches, and guided by the results of the June 2015 Navigation Summit. The course now prominently features the altimeter and lays a solid foundation for GPS use. 

How cheap can you go?

Now that Swiss jewelers are no longer needed to make an altimeter, just how cheap can we go? It’s easy to spend $130 to $350 on a Suunto Core ABC (altimeter, barometer & compass) watch. But how functional is a ~$30 Casio multi-function watch? Or cheaper still, how functional are the many, free cell phone apps?

It was a little awkward to wear three wrist altimeters and more awkward still to ask my spouse and climbing partner to wait in a snowstorm on the Muir snowfield while I recorded data. But the results are in and they are clear. The cheapos work great. I can highly recommend at least one cheap watch and a handful of mostly Android cell phone apps.

The Test

I tested three wrist altimeters and six apps on my Samsung Galaxy S5 smartphone. The S5 has a built-in altimeter chip as do the latest iPhone 6 models. The apps were picked were free and strongly rated.

For wrist altimeters I used my 15+ year old Suunto Observer, a new Suunto Core, and the cheapest altimeter I could find on Amazon—the “Casio SGW300H-1AVCF Twin Sensor Multi-Function Digital Sport Watch.”

In addition to the six apps from the Android Play Store (https://play.google.com), I performed a cursory test of three iOS apps (http://itunes.apple.com) with similar results. See Table 1 for tool details. In the end, tool choice made no material difference.

These tools use three, quite different methods to determine altitude. The traditional method measures atmospheric pressure, as do barometers, but calibrated in feet rather than inches of mercury (or meters vs. millibars). Since weather effects air pressure, and therefore apparent altitude, these tools must be calibrated at the start of a trip and perhaps during. I calibrated them at the trailhead to a known elevation and made no further adjustments.

Tools that determine altitude using GPS use two quite different methods. The first GPS altitude method takes the absolute position in 3D space and compares it to a math model (the ellipsoid) that approximates mean sea level. GPS technology has taught us that the sea isn’t as level as we thought and undulates considerably around the world. Using a built-in table, modern GPS units then correct for the difference between the ellipsoid and local mean sea level. Early GPS units did not make this adjustment. Perhaps this is one reason GPS units have an undeserved bad reputation for determining elevation.

The other way GPS determines altitude is to assume that you are standing on the surface of the earth (a bad assumption for pilots in flight). The tool then uses GPS coordinates to find elevations in a lookup table. An Internet connection is required so this is not practical for wilderness navigation. If the Internet is not available, these apps default to the 3D method.

TABLE 1 - ALTIMETER TOOLS TESTED

Altimeter Apps Tested

Methods

The test was simple. I took all eight tools on four trips: three at in Mt. Rainier NP (a glacier climb to Whitman Crest below Little Tahoma, a climb to Anvil Rock near Camp Muir and a hike to Mildred Point) and one hike to the summit of San Jacinto Peak near Palm Springs, CA. I took measurements along the way and then compared with a gold standard “actual” elevation based on GPS location.

While we might like to know elevation with great precision, mountain navigation is tolerant of several hundred feet of deviation. The bold topographic index lines on a 1:24000 (7.5 minute) USGS map are 200 vertical feet apart so I set two criteria for success:

1. Each individual altitude measurement does not deviate more than 300 feet from actual. As long as all measurements are within 300 feet (150% of the distance between bold lines) of actual, you will not be more than one bold line off from the actual nearest bold line. This is not a potential 600-foot error.
2. The average altitude measurement is within 100 feet. Ideally, the altimeters would, on average, put you on the correct bold map line.

I assumed that some of the apps or the $30 Casio special would produce material errors, especially over large changes in altitude.

Ain't Misbehavin'

For these three trips, I could not get any of the wrist altimeters or apps to misbehave, with two exceptions that I’ll address below. Each device’s worst individual reading was within my criterion of success of +/-300 ft. with the worst individual reading varying by 259 ft. On average, the tools were almost ridiculously accurate with the worst average difference being only 60 ft. This was surprising especially considering one of the trips was over two days with an elevation difference of 5400 feet.

While all the tools performed exceptionally, the GPS apps were more accurate. If the barometric altimeters had been recalibrated, as recommended for a normal trip, they would have been even more competitive.

Using GPS for elevation data tends to raise eyebrows but the data says this reputation is undeserved. The data allowed me to test this question in addition to the main topic of this paper. I compared Gaia GPS, my go-to favorite navigational
tool with actual elevation data. I was able to take 14 measurements in diverse locations from the San Jacinto Mountains of southern California to Calgary, Canada. Table 2 compares the Gaia GPS measurements against good map data.

Table 2: Gaia measures compared to topo map elevations

A few cautions are in order. There were two anomalies that surfaced during testing, both with wrist altimeters. On the summit of San Jacinto Peak it was below freezing. When I took the watch off my wrist to use the built-in thermometer, the temperature plummeted until it hit 35F and then shot up to over 200F. The altitude, which had shown a reasonable 10,800 ft., shot up to 27,400 ft. If anyone can explain this, please let me know. When the altimeter was back on my wrist, readings returned to normal.

The second anomaly was with the Suunto Core. This watch has large buttons that protrude. There is a locking mechanism that I had not engaged and on one trip, the buttons were accidentally pushed and the watch adjusted to the wrong altitude. I left this data out of the results.

Also, remember smartphones are fragile, power-dependent electronics so get a credible case, turn it off (or to airplane mode) between uses and have a map, compass and backup tools. Wristwatch altimeters and apps that rely on atmospheric pressure are subject to changes in weather. Keep that in mind and recalibrate when the altitude is known (map position) or recalibrate against GPS which is unaffected by weather. There were no dramatic changes in weather and I did no recalibration during the tests.

Table 3. Field test results dedicated altimeters and Android apps

Table 3 Note: Average and maximum deviation is the difference between the tool and the actual known elevation as determined by the Google elevation service. Where the tool uses both atmospheric pressure and GPS, each is shown with the atmospheric first followed by GPS to the right of the slash.

Take this study as a strong recommendation to add an altimeter (or two!) to your navigation tools. A cheap Casio wristwatch altimeter will run for years on one battery and if you have a smart phone, add a free app. Or, better yet, add Gaia GPS for $20 and have a world-class navigation tool in your pocket.

Steve McClure sits on the Mountaineers Board of Directors as Treasurer, chairs the Finance Committee, and sits on the Seattle Climbing, Scrambling, and Navigation committees. He is an intermediate climbing student. Contact him at McNorth@Gmail.com.

Editor's Note: The Seattle Navigation Committee invites prospective gear reviewers to submit requests to test new navigation tools. The Committee will reimburse the reviewer for the tool and offer it to the reviewer to keep at a 50% discount. Submit your brief (1 page) review proposal to the editor. Specify your experience, committee(s) affiliation, tool, source, cost and review procedures in your proposal.