One of the perennial points of discussion when talking about GPS elevation profiles is the accuracy of the elevation measurement. Total elevation gain and loss on a given route can significantly deviate between various devices and from what you would calculate on a topographic map.
On our FAQ page we try to delve into this:
Most GPS devices use either the GPS signal (using triangulation), a built-in barometric altimeter or a combination of both to give you an elevation reading. The latter will generally give you the most accurate results, but unfortunately in many cases there will still be significant errors â€“ for instance, the barometric pressure may change during your ride for other reasons than elevation change (i.e. weather changes), or the sensor itself can get out of calibration.
That being said, the devices with barometric sensors will usually give you reasonable results. There is however a problem that typically occurs on technical trails: all the tiny ups and downs of the trail result in minute elevation changes that are being recorded. These accumulate and on longer rides it will generally result in overestimates for the elevation change numbers compared to what youâ€™ll see on a topographic map.
Accuracy is generally measured with respect to what your route mapped on a high resolution topographic map would read – this can be obtained by downloading your GPS track data and mapping the track on for instance National Geographicâ€™s Topo! tool. Of course, the cumulative elevation numbers you will get this way are depending on the spatial resolution of the topographic elevation data – effectively resulting in some averaging. which numbers are best is in a way a mattter of taste: the topo value gives you an averaged and ‘reasonable’ looking number, but on the other hand, you did ride all those tiny bumps up and down (the problem statement is somewhat similar to the ‘length of a coastline’ problem).
We chose to apply some averaging in calculating the total elevation gain and loss from the GPS track data, in order to get fair agreement with values that are typically obtained using topographic data.
The best you get with a barometric sensor is probably a relative accuracy of half a meter or so. ‘Relative’, because in order to obtain absolute accuracy you would certainly need to recalibrate the sensor frequently.
The figure below shows some elevation data recorded on the Tahoe Rim Trail (fairly technical) using a Garmin Edge 305 in the ‘smart recording’ setting (in this setting, the sampling speed is adaptive), and it illustrates some of the minute, sub 1m elevation changes mentioned before.