Originally Posted by
njkayaker
The calibration is necessary for determining altitude. But cyclists don't care too often about that.
How different is the elevation at the start and the end?
Drift in the reading (if it's slow) probably won't change the gain by much.
Small gains over large distances likely have a higher percentage of noise.
The weather here has been pretty stable. No fronts of areas of high and low pressure moving through. Just sometimes rain and a thunderstorm that develop and dissipate in pretty much the same general area. So my last 5 rides over 2 hours had a start finish difference averaging 48 feet. The biggest 62 feet and the smallest 38 feet. Not a lot. However some where back in my history I've seen on the order of 100 feet of change or more during a ride.
I agree the calibration is only needed for those that want to know their elevation. I've never understood the need to know actual elevation for cycling. My house is the normal start point for my rides so I have the driveway as a calibration waypoint. On the topo maps of my area a contour line goes right through it. But even that might be incorrect as the topo maps data set includes data from times before this house or even the subdivision was built. So who knows and why should I really care? I don't!
I'm not suggesting this is the reason for the OP. But this and other things can add up to always make the relative or actual elevation numbers be different from one ride to the next.
There is even an overpass I travel routinely and on days with the wind blowing steady from the soutwest, I'll show a higher elevation going over it than days with no wind. I wonder if that is a localized reduction in air pressure because the wind is being funneled along that area at a higher velocity than air elsewhere. So Bernoulli's Principle might be another reason for errors in elevation or gain/loss calcs. And it can occur in many places.
For me, elevation gain/loss is a lot like Calories. The actual exact number is not important. If someone wants to ride hilly terrain, then ride the routes that show the most gain loss per distance. Doesn't matter if they don't match up from day to day.
Map data isn't correct for total elevation gain, nor are our barometric sensors in our devices and certainly GPS calculate altitude is less trustworthy. So people need to learn to judge by the trends and not the actual exact number they will never have.