Nice idea, but why not wait for the release of ARKit? The demos Apple showed off were amazing, this is rather "meh" in comparison. Hopefully the author is already working on ARKit support, otherwise the app will be surpassed by other apps in months. We've already seen that fantastic ruler apps are coming in iOS 11.
I also do not understand the ruler on the left in "lengths" mode, which becomes huge and takes up too much screen when you switch to imperial units.
EDIT: It's also hard to maintain accuracy with the way it works currently. Should have the ability to set up a measurement and tweak it.