Alt-O-Meter uses the barometer (air pressure sensor) in your late-model iPhone or iPad to calculate your precise height above sea level. Great for hiking, skiing, driving through mountain passes, flying, and more! β’ Reads ambient air pressure for a more precise reading than GPS. β’ Realistic, retro style. Looks like a classic airplane's altimeter! β’ Automatically calibrates for local atmospheric pressure. β’ ...also allows precise user calibration (optional; for advanced users) β’ Uses GPS hardware in devices that lack a barometer. β’ Completely free! No ads or in-app purchases.