We actually have a test course for GPS units under canopy and in open. Many companies send us units to test. We test some phones and have gotten into testing more tablets now as well as standard GPS units. Phones typically suck especially iPhones (+/- 8~10 meters. Some Android devices are better at 3-5meters some are about like iPhone accuracy. I will say be careful of property line info on all apps as they come from the tax office, which is known for their maps to be inaccurate. You can look at the arial image and often see property lines and the tax map over lay is off by 8-10meters as well. Digitizing from an arial photo or satellite imagery will be pretty close. Just use the “property lines” in apps or courthouse as a reference but not exact.