With all due respect David, you don't know what you are talking about. Survey grade GPS units can reliably produce sub centimeter accuracy, if you have 10-20K to dump on that kind of setup. You will never get anything even close to that with a consumer grade unit. I have two high end consumer grade GPS units that vary in accuracy from 6 to 20+ feet ...and that knowledge comes from plotting collected coordinate data on a 6" HRO. Bottomline, consumer grade units are not that accurate. Also, just because a GPS unit reports speed to 0.1 units doesn't mean it is accurate, nor that your speedometer is inaccurate. Your vehicles speedometer will always be more accurate unless it is broken. There are numerous ways you can get accurate square footage without using a GPS unit. Just because you are not aware of them does not mean they don't exist.