[talk-au] Misaligned streets OSM or Bing wrong? - use survey mark?

[Russell Edwards]

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 <inas66+osm at gmail.com> 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.
>
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