This rangefinder(telemeter) measures the Distance, Height, Width and Area of a target with your phone by trigonometry.
Usage is simple: just stand up and press the shutter. The important point is aiming the camera at the GROUND, NOT the object. (i.e. In order to measure the distance from someone, aim at his shoes.)
If it is inaccurate, please read the instructions in this app or see the checklist diagram in the blog. The sensors on your phone can be changed by OS version, firmware and battery status.
You can calibrate them with the calibrate menu and settings for yourself.