The barometric altimeter on iPhones is not exposed as a built-in app or other info feature, such as on the home page of the phone, so I didn’t realize that there is a barometric altimeter on the iPhone 6s Plus (and iPhone 6 and 7).
GPS elevation can be subject to errors of hundreds of feet or more, so I don’t like to use it. As shown below, the error is 28% as the actual altitude is 504 feet yet GPS says it is 358 feet—GPS can be much more in error in the mountains. Also, a good GPS signal may not be obtainable under tree cover or in a tight canyon. Finally, a barometric altimeter is an excellent cross-check.
When I hike, I often know the exact altitude at certain points, at least when I start a hike. So a barometric altimeter is a reliable way of tracking altitude, at least for some hours. And frequently I know the altitude of key points, such as lakes or summits, so exact recalibration is possible.
The Pro Altimeter app for iPhone can be calibrated from GPS automatically, or most critically, one can enter the known altitude. Perfect for my needs (manual entry) and the GPS altitude is a plus for when there is a good constellation of satellites, as shown below.
At fixed altitude, the barometer is useful for showing air pressure, which can hint at weather changes. Too bad the app doesn’t graph the pressure every 30 minutes or so. It has two choices of display as shown, one better for day and one better for night.
Update: I’ve used Pro Altimeter in the mountains now, and it is has one major bug: it doesn’t say when the GPS altitude was last obtained. So it will show an altitude a day or a week old, with no indication of current status. The only clue that the reading is wrong is the barometric reading. The app sometimes has to be left on for several minutes to get a GPS reading (and in canyon it might never get one), yet there is nothing that indicates that the GPS reading is hours or days old and completely utterly wrong.