If you are interested in this tool, you may also be interested in Photographer's Field Tools. It has a distance ruler using map and many other cool features.
A screen ruler. It can set the origin to the phone edge or the screen edge. It also supports sliding scale for length that is several times longer than the phone length. Supports both phones and tablets. It has self-calibration feature. The more people using it, the more accurate the scale will be.
And the best of all, absolutely free, there is no ads to crowd the already busy screen space.
Note: this app has no camera related feature. Advanced Ruler Pro has camera rulers.
1. Internet access permission is for crash report
v3.2 fixed a minor bug when showing the help. only on android versions 2.x.
v3.1 further tuning to the auto-calibration
v3.0 use a new way to auto-calibrate the ruler
v2.2 fixed the crash in v2.1
v2.1 added phones, fixed the text size issue on larger screen phones
v2.0 support tablet resolution