On 13 September 2013 12:55, Chris Hill <o...@raggedred.net> wrote: > Why do you suppose OS Streetview is correct? I find that compared to multiple > GPS tracks it is not always well aligned and more recent Bing imagery is > often better.
I did consider that possibility, but I did a search for that and it came up suggesting StreetView was mostly good to about 1m (although I can't re-find that reference). On the other hand, that is a reason for requiring more evidence before correcting things which assume zero offset for Bing. As noted elsewhere on this thread, Bing clearly shows parallax errors, and it looks like it hasn't been corrected for height variations, at least not on a local scale. I don't think I would trust commercial GPS much below 5m unless it was averaged over at least an hour, with a clear view of the sky (reflections off builidngs could systematically distort the position solution - see http://wiki.openstreetmap.org/wiki/Accuracy#Systematic_errors). Where I've looked into calibrating against GPS I've tended to find that there are good places that are accurately locatable on Bing where you can sit with GPS for a long time without looking suspicious, and which are near to points of interest and have a clear view of the sky. On the other hand, pre-OpenData specifications for StreetView suggest worst case errors of 4.1m <www.centremapslive.co.uk/files/street_view_userguide.pdf>, so, maybe I'm still going to have to do long averaged GPS calibrations. That still doesn't mean that unsourced BIng tracings, with zero offsets are good things. Fortunately, OSM hasn't been around enough for continental drift to become a severe problem. Even the error since 1984 should be less than about one foot. _______________________________________________ Talk-GB mailing list Talk-GB@openstreetmap.org https://lists.openstreetmap.org/listinfo/talk-gb
