GPS comes in two flavors, SPS (standard positioning service, your standard civilian positioning) and PPS (precise positioning service, for the military). The S

GPS comes in two flavors, SPS (standard positioning service, your standard civilian positioning) and PPS (precise positioning service, for the military). The SPS specification[1] says you should expect 95% of reported locations to be within a 9m diameter circle (ref 3.8.3). Realistically, though, you can get much better, and a lot of receivers get within 2-3m. Up until about 2000, there was thil thing called Selective Availability[2] (aka SA). This limited civilian performance to much worse (95% ~20-30m) by some clock dithering techniques. During-SA and post SA accuracies are very different. In order to support airplanes using GPS for precision landing, the FAA rolled out WAAS[3], which is a wide-area augmentation system that broadcasting atmospheric correction data (specialized just for the US; there are other national systems). GPS+WAAS gets you to within a meter or so. [3] If absolute position isn't important, but relative position is (say, if you're measuring the movement of two tectonic plates relative to each other at a fault), there's this neat thing called RTK. This gets you relative positions (in the field) on the order of millimeters[4]. [1] http://www.gps.gov/technical/ps/2008-SPS-performance-standard.pdf http://www.gps.gov/technical/ps/2008-SPS-performance-standar... [2] http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System#Selective_availability http://en.wikipedia.org/wiki/Error_analysis_for_the_Global_P... [3] http://www.navipedia.net/index.php/WAAS_Performances http://www.navipedia.net/index.php/WAAS_Performances [4] http://hemispheregnss.com/DesktopModules/EasyDNNNews/DocumentDownload.ashx?portalid=0&moduleid=522&articleid=28&documentid=197 http://hemispheregnss.com/DesktopModules/EasyDNNNews/Documen...