Originally Posted by
noglider
iphones have no altimeters. I believe smartphone apps and Garmins use knowledge of elevation built into the map data. This knowledge is not very accurate yet. I might be wrong, and perhaps some Garmins have altimeters, but not all of them.
You are incorrect. The iPhone 6 and 6s have a barometic altimeter, as do most Garmins Edge units (like the very popular Edge 500).
For my iPhone I use Cyclemeter, which has an option to use the Barometer for elevation over GPS, and I turn this on. Unfortunately Strava corrects all elevation readings from Phones, regardless of the presence of the alitmeter, so my Strava elevation changes do not match my Cyclemeter calculation (although they are usually close). In the case of Garmins, Strava "trusts" the elevation data, leading to rides with vastely different elevations from different riders who road together. They have the option of correcting to Strava's built in elevation maps if they want.