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.

Technorati Tags: Altimeter Watch, altimeter watch guide, Altimeter Watch Review

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.

Technorati Tags: Altimeter Watch, altimeter watch guide, Altimeter Watches

I was sitting down with my Suunto rep on Monday going through their 2010 line. They have some really, really cool stuff coming out, particularly related to altimeter watches. With all of the altimeter watch choices out there, making an intelligent and frugal decision about your next altimeter watch purchase can be a very daunting activity.

#5 - Budget. The plain fact is there are many, many really good altimeter watches in the market. The bad news is like anything else you pay for what you get, particularly if you want add on features like a stopwatch or barometer. If you are in the $100-$150 range, go after some of the High Gear or Nike watches. If you can afford a bit more, go for the Suunto Core or even the Suunto X10.

#4 - Durability. Depending on how you plan to use your altimeter watch, how durable it is should be one of your main concerns. While the Suunto Core models have received very good reviews, we have experienced many returns due to software and band breakage. The High Gear Axis watches have a lot of the same issues. The best of the bunch is the Suunto Vector (my rep confirmed this in our meeting too) - it takes a beating and keeps on ticking. Look for a watch with a solid band and face. You want one that can survive a fall while hiking or a drop while changing. Another consideration is how well the altimeter watch face handles scratches.

#3 - Compass. Many people want an altimeter watch that has it all. Our recommendation is to eliminate the compass as a decision maker when choosing your watch. Digital compasses are a battery drain and just are not that accurate compared to a regular hand held compass. Spend the $20 on a good Suunto or Brunton compass and don't think about it anymore.

#2 - Activity. How will you be using your altimeter watch? Do you need one for hiking, biking, cross country skiing, maybe all of these? Many altimeter watches have some very cool features like vertical rate of ascent and descent, barometer (which warns of coming weather conditions). Consider what you will need your altimeter watch for and choose one that makes the most sense for your needs. Remember, you are buying a watch that compliments your activity, not the other way around.

#1 - Accuracy. After all, the purpose of an altimeter watch is to know your elevation. A common complaint from new altimeter watch owners is that the watch does not show the correct altitude. Do you mind re-calibrating hourly (or every few minutes, depending on changing weather)? If not, then a lower end watch might work for you. Want an altimeter watch that automatically adjusts based on changing weather conditions? The Suunto Core does just that. Decide how accurate you need your watch to be before making your purchase decision.

Technorati Tags: Altimeter Watch, altimeter watch guide, Altimeter Watch Review, Altimeter Watches