GoKawiiliPhones since the 6 include a hidden barometer sensor, usable via apps
Apple has built a barometer into iPhones starting with the iPhone 6 and 6 Plus in 2014, with the exception of the first-generation iPhone SE. The sensor measures barometric pressure to estimate elevation changes, since air pressure drops as altitude increases. Because weather also affects pressure readings, the data is typically combined with GPS altitude and local weather station information for accuracy.