GDA94 to WGS84 is approx 45 cm from what I read but it will be affected by continental drift. GDA is fixed to points on the Australian plate so GDA coordinates will not change (much) but I guess OSM is really meant to be in WGS84.
Just an update to this thread in case anyone is interested, I re-did the survey mark coordinate conversion and found it within 1 m of the bing image. So it seems bing is correctly aligned and numerous streets in my town are misplaced in OSM, with nearmap marked as source. Pretty handy resource: http://services.land.vic.gov.au/landchannel/content/SurveyMarkMarkSearch Russell On Sun, Sep 2, 2012 at 9:53 AM, Ian Sergeant <[email protected]> wrote: > >> But an even bigger error can be caused by using different projections. I > >> forget which one OSM uses, but using different projections can move a > given > >> point 20m quite easily, and a survey marker may well be on a different > one. > > > Can we get a definitive answer on this? > > I think the terminology is a little confused. The projection is the > way that a the 3D shape is projected onto our 2D representation. It > shouldn't "move" a given point on an elipsoid, but the 2D > representation of it may be quite different. That really shouldn't > effect what we are doing here. > > Perhaps we are talking about the conversion from AGD to GDA. Last > time I looked there were still some maps of some areas using the old > datum, which can result in this kind of separation. Just separated > enough to make you think you know what you are doing, and at the same > time get very lost. > > However, I very much doubt that is the case here. I think the survey > mark is using the GDA. > > As to how accurate the location of the survey marks is, I have no > idea. I think it would depend on the technology available at the time > the mark was last surveyed. Lets face it, once we are talking about > 2m errors, the sources are numerous, from parallax on the photo, to > surveying errors, to GPS errors. > > Ian. > > _______________________________________________ > Talk-au mailing list > [email protected] > http://lists.openstreetmap.org/listinfo/talk-au >
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