by davekeiller » Fri Jan 24, 2020 3:47 pm
Strictly speaking, you're confusing error and uncertainty in that last comment.
There are two ways to measure your altitude. One way is to use GPS to determine your location in 3D space, the other is to use air pressure.
The weakness of using air pressure is that it's strongly affected by the weather, so you have to re-calibrate regularly otherwise you'll get calibration drift resulting in erroneous calculated altitude and hence ascent and descent. The trouble with GPS is that it's only as accurate as the GPS calculation of your position - it is possible for it to be tens or even hundreds of metres out in areas where the signal is poor.
When using either method to calculate ascent and descent, you also have to consider sampling frequency. A device will generally calculate your altitude once every x minutes and calculate altitude by drawing straight lines between the points. Clearly if you go from the bottom of the hill to the top and then back down again, sampling frequency won't make a big difference beyond possibly missing the very top of the hill. If, however, a route goes up and down a lot (a ridge, say), then a low sampling frequency will smooth out the undulations and underestimate the ascent and descent.
Long story short, pretty much any GPS or GPS equipped device will do what you want. However, not all devices are created equal and there will always be a slight difference between what the device says and your true ascent and descent so take the readings with a pinch of salt.