For those that own an altimeter watch that gives altitude based on GPS readouts, your altitude will be as accurate as the map datum the watch is using to generate altitude. We'll discuss why in a minute. The majority of altimeter watch owners have an altimeter watch that calculates altitude using barometric pressure. In the following paragraphs I'll discuss the difference between GPS and barometric pressure altimeter watches and give examples of each.

GPS altimeter watches use latitude and longitude coordinates to identify your position on the grid and read out an altitude. This data is delivered to the watch from the manufacturer satellite and usually is based on USGS datum. If you have a GPS enabled altimeter watch you never need to recalibrate your watch since the information being displayed comes from a satellite and not from "on ground" conditions. Examples of GPS enable altimeter watches include the Forerunner series from Garmin and the Suunto X10.

Almost every non-GPS altimeter watch on the market uses barometric pressure to determine altitude. The watch has a function in it's built in software that takes the barometric pressure and calculates altitude. Because of this, the altitude readout accuracy needs to be carefully scrutinized, particularly if the weather has changed significantly since you last calibrated your watch. Examples of barometric pressure altimeter watches are the High Gear Axis and the Suunto Core.  As already discussed, barometric pressure altimeter watches are subject to the outdoor weather conditions and as such should be calibrated at regular intervals.

The price points on GPS and barometric pressure watches are varied, with GPS enabled altimeter watches generally being more expensive. An altimeter watch using barometric pressure (such as the HighGear Axis or Suunto Vector) can be found for as little as $100. The GPS enabled altimeter watches are generally more expensive, with an entry price of around $250.

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So a simple lesson on how an altimeter watch calculates altitude:

In laymans terms, an altimeter watch measures atmospheric pressure. The higher you go, the less atmospheric pressure there is (this is why airplanes have pressurized cabins and climbers on Everest wear oxygen). Essentially, what you have on your wrist is a barometer with a function that uses a simple calculation to spit out altitude.

HighGear's Ryan Hamsho said it like this:

An altimeter does not actually measure altitude directly, but rather just atmospheric pressure. So an altimeter is actually a barometer created for a specific purpose. The altimeter uses the changes in atmospheric pressure to determine the changes in altitude. Why does pressure change with altitude? Simply put, the pressure at any given point in the atmosphere around the earth is a result of the weight of the atmosphere above it (pulled down by gravity). For this reason, the higher in the atmosphere you are, the less atmosphere you have above, and the less pressure exerted on you.

Again, the calculation is simple. What it means in the field is you need to calibrate at least daily from known positions if you want the altimeter watch to be accurate. With stable weather, once a day is probably enough. When weather changes, you need to recalibrate at known points as often as once an hour. Use your paper map or Google Maps to record and set known points on your altimeter watch and make sure to set your watch when you get there. You User Manual on your watch will show you how to do this.

Keep in mind as you purchase an altimeter watch that it is a tool to assist you with calculating altitude, but it is not an exact scientific instrument. Used correctly with appropriate supplemental information (maps, etc). your altimeter watch can be a valuable tool in getting you to your destination.

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