Barometric and GPS-based altimeters

Barometric and GPS-based altimeters

4. January 2018
Barometric and GPS-based altimeters

Air pressure calculator at Alpinetrek

Isn’t it irritating to have trekked through the mountains all day or mountain-biked your way over some tough single track only to find at the end of the day that you have know idea how much elevation you’ve gained? Fortunately, those days are basically over. In recent years, manufacturers of outdoor hardware have been incorporating altimeters into watches, cycle computers and GPS devices that usually calculate the elevation gain and loss you’ve accumulated over the course of your ride, run or walk.

There are two types of altimeters: GPS-based and barometric altimeters. We’re going to take a closer look at both of these technologies and tell you what the pros and cons of each are!

Barometric altimeters

Barometric and GPS-based altimeters

Barometric altimeter

The basis for this method of measurement is air pressure. The barometer measures the air pressure and figures out the altitude out based on that. Atmospheric pressure drops as you gain altitude – if you want to know how much, you can use our handy altitude conditions calculator.

One advantage of this kind of measurement is its accuracy in stable weather conditions and constant temperatures. In conditions such as these, measurement errors are not as drastic those made by GPS-based device. The disadvantage of this method is that a point of reference is required, meaning a pre-determined location above sea level at which the air pressure is measured. Both mountain huts as well as passes are good references points because more often than not you can find the actual altitude by looking at a map. If you recalibrate your barometer in such places from time to time, the information you receive will be accurate within a few metres.

GPS-based altitude measurement

Barometric and GPS-based altimeters

The Garmin Montana 610 with a barometric altimeter

As the name suggests, GPS devices use the American Global Positioning System (GPS). The exact position of the device is determined by means of the signals from various satellites in the earth’s orbit. However, in order to receive information on the current altitude, the receiver requires the signal from at least 4 satellites. The accuracy of this geodetic triangulation of your location also depends to a large extent on the quality of the signal. If there are several available satellites, the receiver will be able pick and choose, giving you the best or strongest signals. However, if your device only receives four satellites, it is possible that both your position and altitude information will strongly deviate from the actual values.

This is due to the fact that a GPS signal behaves physically similar to light. Clouds weaken the signal, and deep canyons can even isolate the receiver completely. Even a dense forest can weaken the signal. The signal can also be reflected off walls. All these things can have such a negative impact on the determination of your position and altitude that they can even result in deviations of up to 100 metres.

Which device is better

In our opinion, that depends entirely on what you plan on using it for. Here are some examples of possible uses and the best device for those particular activities:

Example 1: You’re a mountain biker or hill walker and would like to know how much elevation you’ve gained over the course of your outing:

For this purpose, a barometric altimeter would clearly be your best choice. Your device would measure the air pressure in defined time intervals, thereby determining differences in elevation and subsequently adding them together. When the weather conditions are relatively stable, atmospheric pressure is a reliable source for elevation calculations and perfect for calculating elevation gain and loss. The actual altitude is not usually the most important factor for such excursions, so you don’t have to calibrate your device beforehand.

Example 2: If you’re going on day-long trips with major differences in altitude (a thousand metres or more) and would like to know the altitude of your current position:

For this purpose, we would recommend using a GPS-based device. A GPS-based device may only be able to determine the elevation with an accuracy of 20 to 25 metres, but your position is constantly recalculated and the error will be balanced out in most cases. With a barometric measurement, it’s possible that the device was calibrated incorrectly after the first day, resulting in a deviation of 20 metres for every subsequent measurement. If you don’t have a known position at which you can recalibrate the device, the error could continue and the deviation could even increase. In such a case, a barometric altimeter would be even more inaccurate than GPS.

Example 3: You tend to go on adventures in places where the weather and temperature play a major role:

As was already touched upon, weather and temperature can have a major impact on air pressure. If the air pressure varies as a result of these factors, it’s better to use GPS. However, it’s important to mention that some devices have integrated storm warning systems: if the air pressure drops rapidly, this usually means that a low pressure area is approaching, which often leads to bad weather. Some devices warn you in advance. A barometer can make reliable short-term weather predictions that can be incredibly useful for mountaineers.

In sum, barometric altimeters are great if you’d like to know the elevation profile of your route. The fluctuations are much smaller and the accuracy is better than GPS-based devices. However, as mentioned above, it is imperative to calibrate the devices beforehand in order to receive precise data. For longer trips, we recommend verifying the elevation data at huts or on maps to get the best results.

If you are more interested in absolute elevation, GPS is the better choice. Even though they don’t need to be calibrated, GPS devices may not be as accurate in remote, isolated (mountainous) regions. However, devices with both GPS and GLONASS can often remedy this. The Russian counterpart to the American navigation satellites, which is actually called NAVSTAR, fills the occasional gap in the satellite network, especially in Eurasian and Asian regions.

Other GPS devices combine the advantages of the different methods by measuring altitude barometrically and repeatedly comparing it with the GPS data.

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