This app altimeter came out since the iPhone 6
This app is nice and it works on iPhone 6 7 8 X and more
Not the best
Didn't fully work on iPhone 6s. Worked when set to GPS height but not when set to device.

This app measures your current elevation using GPS or your device's barometer. It offers features like setting a base altitude for difference calculations and calibrating the barometer for improved accuracy, with support for both metric and imperial units.
1.0
November 8, 2016
This app has been updated by Apple to display the Apple Watch app icon.
The app uses GPS for absolute altitude, which is less accurate, and the device's barometer for higher accuracy, especially indoors. Barometer accuracy relies on proper calibration due to changing air pressure.
You can calibrate the barometer altitude by connecting the current air pressure to your current altitude. Calibration can also be done using GPS, standard sea level air pressure, or any desired altitude.
The app is described as ad-supported freeware, meaning it contains advertisements.
You can switch between metric and imperial units within the app to display altitude measurements.
Yes, you can set a base altitude as a starting point and the app will provide the altitude difference, which is useful for activities like hiking or biking.
Altimeter (Barometer) Free is supported on iPhone, iPad, and iPod devices.
The app was last updated on November 8, 2016, with version 1.0.
The app has a rating of 2.7 stars based on 3 user ratings.