A GPS unit may be accurate to 12 feet but that doesn't necessarily mean it can locate a property corner with that kind of accuracy. It would depend more so on the map that you're using, or the coordinate you're using for that particular corner. Most maps used in GPS units are based off county assessor's maps, and plenty of times I've seen where their property corners and/or lines are off by three to four hundred feet. So I sure wouldn't count on a GPS getting you within 12 feet of the actual property corner.