On Fri, Aug 29, 2008 at 2:52 PM, Tom Hughes <[EMAIL PROTECTED]> wrote: > Matthew Gates wrote: > >> Further to Ed's post, are there really GPS devices with one inch accuracy? >> >> I could imagine letting a device sit for some time, and then averaging the >> position, which might lead to increased accuracy at the cost of >> long "exposure" time... is that method used for surveying and so on? > > He was referring to a combine using a DGPS system which has a base > station at a fixed point on the farm whose location is well known. It > then compares that known location to one calculated from the satellites > in the normal and broadcasts the difference to the mobile received on > the tractor/combine which uses the difference to correct it's own > calculated position. > > What that allows you to do is to compensate for inaccuracy caused by > local atmospheric conditions as you are generating a correction based on > a local base station.
... and then the final stage is carrier-phase enhancement, which gets you down to the one-inch accuracy. It tells you what fraction of a wavelength of the GPS carrier signal you are out by compared to the base station, but not how far away you are. So if you can use DGPS to get down to a 20cm location, the CPGPS will tell you whereabouts in that particular box you must be. The guy from the OS at SOTM did a fairly good job of explaining it the difference between DGPS and CPGPS - I don't think wikipedia helps much. http://en.wikipedia.org/wiki/Global_Positioning_System#Accuracy_enhancement Cheers, Andy _______________________________________________ Talk-GB mailing list Talk-GB@openstreetmap.org http://lists.openstreetmap.org/listinfo/talk-gb
