It works, but a little touchy – GPS MegaTape - Tape Measure Review

To be fair, the creators state the app is more accurate at distances longer than about 100 feet. But it wasn't encouraging to see myself rack up 5-20 feet while standing completely still. Be sure to read the info link on the main page before you start, to get the best results.
Review by PScooter63 on GPS MegaTape - Tape Measure.