My GIS certificate is ancient at this point and I have never managed to work in the field I trained for, but my recollection is that they are accurate to within only about 10 meters, whether personal or professional, unless you spend quite a lot of time gathering data from multiple satellites (on the order of several hours, depending upon the level of accuracy you are seeking). EDIT: In other words, "I just turned mine on" shouldn't get any reading on any system that is more accurate than about 10 meters. It takes time to get accurate readings with GPS.

Someone with a more recent credential/professional experience want to confirm or deny that recollection -- or even update it with more recent facts? (For reference to my question of "more recent", IIRC, I got my certificate in 2002.)

According to http://en.wikipedia.org/wiki/Wide_Area_Augmentation_System#C... normal GPS systems will be accurate to within 2.5 m 95% of the time. Better accuracy can be achieved using differential GPS systems and things like WAAS, but they rely on ground-based systems and thus might not be a good way to measure how much the ground shifted.