Height Measurement with GPS and Barometer
Many of us know it: you are hiking all day in the mountains or driving hard mountain biking and in the end you know quite well how many kilometers you have walked or hiked, but not how many heights have been overcome. In the last few years the manufacturers of outdoor hardware have reacted and built appropriate altimeters in clocks, bike computers and GPS devices, which usually also sum up the copied altitude in the ascent and descent.
There are two basic methods of height measurement; the barometric and GPS-assisted height measurement. We take both methods under the magnifying glass and betray you the respective advantages and disadvantages!
BAROMETRIC HEIGHT MEASUREMENT
The basis of this method is air pressure. This is determined by means of a barometer, in order to determine the height from this. The air pressure drops in the normal case with increasing height – how strong, can be determined with calculator for height-dependent conditions.
An advantage of this type of measurement is its accuracy in stable weather conditions and constant temperature. Under such conditions, the error in the measurement is less than in comparison to a measurement by GPS. The disadvantage of this method, on the other hand, is that you need a reference point, that is, a point with a known altitude above normal zero, at which the air pressure is measured. For these pre-necessary calibrations, mountain huts as well as passports are suitable for which the actual altitude is known from the available map material. If you adjust your barometer occasionally in such places, the data are accurate to a few meters.
GPS-BASED HEIGHT MEASUREMENT
As the name suggests, this method works through the American Global Positioning System (GPS). The exact position of the device is determined by means of the signals from different satellites in the orbits of the earth. However, in order to obtain an indication of the current altitude, the receiver requires the signal of at least 4 satellites. The accuracy of this geodetic triangulation of its own position also depends decisively on the quality of the signal. If a lot of satellites are available, a good signal is obtained, since the receiver can rage the best, i.e. the strongest signals. However, if you only receive exactly four satellites, it may be that the data on position and height deviate very much from the actual values.
This is because the GPS signal is physically similar to light. Clouds weaken the signal; deep ravines can even completely isolate the receiver. Even a dense forest can weaken the signal. The signal can also be reflected on walls. Such influences interfere with the result of the position and height determination so strongly that different running times arise from the signal transmitter to the receiver. This may lead to actual positional deviations of up to 100 meters.
WHAT DEVICES ARE NEVER BETTER?
In our opinion, this depends entirely on their application. We would like to introduce some requirements profiles and propose the appropriate method:
You want to know how many heights you can climb during a day hike or a mountain bike trip:
Here, the barometric method is clearly recommended. Your device measures the air pressure in defined time intervals and thus determines differences in altitude and sums them up. At constant weather, the air pressure is a reliable source for altitude calculations and is absolutely suitable for summing up the altitude. The actual altitude is not the most important for such tours, so the device does not necessarily have to be calibrated at the beginning of the tour.
You’ll make for days tours with big differences in altitude (a thousand meters and more) and you want to know at what level you are currently moving:
For such ventures, we would rather advise you to a GPS-based device. These determine the altitude partly with an error of 20-25 meters, however, your position is constantly recalculated and the error thus balanced in most cases. In a barometric measurement, the device may be incorrectly calibrated by 20 meters after the first day, and then each subsequent measurement is incorrectly calibrated by 20 meters. If you do not have a known position where you can recalibrate the device, it quickly happens that the error continues and even increases. Then this measurement is quicker than the GPS.
You are going for adventures where the weather and the temperature play an important role:
As already explained, the weather and temperature have a great influence on the air pressure. If it fluctuates due to the natural influences, it is advisable to rely on the GPS. On the other hand, some devices have integrated a storm warning: when the air pressure drops rapidly, the result is that a gravure area is on the way, which often leads to bad weather. Some devices warn you. In addition, a barometer provides a reliable statement about the anticipated development of the weather.
In summary, it can be said that barometric altimetry is always interesting when you want to know the altitude of a tour. The fluctuations are simply smaller and the accuracy is better. However, in order to obtain precise information, the instruments should be calibrated before each tour. For long trips, it is advisable to adjust the height of the unit between the huts and the maps.
If you are interested in absolute heights, however, GPS is the better choice. Such devices do not have to be calibrated, but can produce calculable measurement errors, particularly in very isolated, isolated (mountain) regions. This is often the case with devices with combined GPS / GLONASS reception. The Russian counterpart to the American navigation satellites, which in fact is called NAVSTAR, helps to offset occasional gaps in the satellite network, especially in Eurasian and Asian regions.
Other GPS devices, in turn, combine the advantages of the different methods by measuring the altitude barometrically, but they always match the data from the GPS.