[Openstreetmap-dev] Re: [Openstreetmap] Landsat update

Schuyler Erle schuyler at nocat.net
Sat Apr 30 18:44:19 BST 2005


* On 30-Apr-2005 at 10:32AM PDT, Nick Whitelegg said:
> >
> > no, apart from the usual GPS error factor it shouldn't make a difference.
> >
> > > i.e. it uses Airy coordinates even though the GPS uses WGS84.
> > > to screen display, the error becomes worse!

I knew this sounded strange, since JPL's Landsat tiles have in my
experience mostly been georectified quite well. So it still sounds
like you have a projection problem. Have a look at this:

    $ proj +proj=utm +zone=30 +ellps=airy
    0 51.5
    708196.62       5709273.47
    $ proj +proj=utm +zone=30 +ellps=WGS84
    0 51.5
    708216.87       5709696.98

So, according to PROJ.4, 51.5N, 0E differs when projected into UTM on
the WGS84 ellipsoid versus the Airy ellipsoid by about 424 meters.
That would account for the displacement you observe. My sincerest
recommendation is to ditch the OSGB coordinates entirely, since this
is where your problem is coming from. If that's undesirable or
infeasible, I would be glad to offer help integrating your app with
PROJ.4.

SDE




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