Surveyor Tape Measure gives you the ability to measure distance, height, and area by just pointing your phone to the places you want to measure.
In the distance mode, tap the screen to place a marker, and the distance between 2 markers is displayed instantly.
In the height mode (requires in app purchase), tap the screen to place a marker on the floor and then raise your phone until the yellow line reaches the height you want to measure and tap again to view the result.
In the area mode (requires in app purchase), you can place multiple markers and the floor and the area surrounded by the lines between markers is displayed on the bottom of the screen. You can clear your markers by tapping on the "clear" button.
You can also switch between imperial and metrics units by tapping the toggle on the top right.
Surveyor was my first mobile app published over 7 years ago long before any AR concept exists on mobile platforms. It was able to do distance measurements with simple 3DOF tracking and a bit of guesswork. Now with the 6DOF tracking from ARKit it has reached a whole new level.