At 03:57 PM 10/9/2007, you wrote: >Rick Collins wrote: > > I very much disagree with this idea. Existing maps and images are > > not likely to be consistently off by a significant amount. > >What makes you assert that? I think it's eminently possible that the >orthorectification or alignment of the Yahoo images is out by 10m. 10m >is not very far, when you have to figure out exactly where a speeding >plane or satellite which took the photos is.
So how does this mean that GPS signals are more accurate? The last time I checked, GPS calculations were done by explicitly calculating both the position and the speed of multiple satellites in a very high orbit, which means a very, very high speed compared to an aircraft or low earth orbit. >They are also much more likely to be *consistently* off within a certain >area than they are to be *inconsistently* off. I won't dispute that. My point is that I have observed GPS receivers to be inaccurate on numerous occasions. I have even seen GPS receivers to read differently by 10 meters just by the simple act of turning them off and back on. Clearly there is much room for error in a GPS reading or set of readings. I also see significant variation between successive trips along the same route. The distortions caused by variable ionization in the upper atmosphere is a significant source of error and often introduces a bias which varies slowly with time and little over a short path. >My GPS on the other hand, is an OpenStreetMap recommended model, and I >was walking up and down the centre of the roads (not cycling or driving) >with a clear view of the sky. I have no idea what is meant by "OpenStreetMap recommended model". Are you trying to say that your model of GPS receiver is more accurate and consistent than others? How was that determined? What model is it? How about checking your route by comparing to some other data rather than just making an assumption about which is more accurate and which is less accurate? If you don't want to use the USGS images, why not try measuring the same route on multiple days? If your data overlays itself well, then I would agree that it is likely that the imagery you are using is less accurate. There is a site somewhere that provides a "solar weather" report. I can't find my link to it, but I have seen a very strong correlation to actual GPS accuracy and solar activity. _______________________________________________ newbies mailing list [email protected] http://lists.openstreetmap.org/cgi-bin/mailman/listinfo/newbies
