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

Nick Whitelegg nick at hogweed.org
Sat Apr 30 20:25:04 BST 2005

> The fact that the difference between the two ellipsoids at this
> latitude is so close to the discrepancy you're seeing suggests to me
> that you've got a bug in the works somewhere. The Landsat tiles are
> almost certainly georeferenced to the WGS84 datum and ellipsoid, and
> the GPS traces certainly are (at least as they came out of the GPS
> receiver), so unless you're doing some fairly sophisticated warping of
> the Landsat tiles or I'm completely misunderstanding what you're
> doing, then I suspect the problem is ellipsoid or datum related.
> I think you'll find that the PROJ.4 API is really easy to use. It's
> only practically speaking got two functions: project a point from
> lat/long and unproject to lat/long.

Well I've now converted everything to lat/lon - no grid references - think I 
can get away with that though everything looks a bit squashed. (Can always 
internally re-project the lat/lon later on when I've got this sorted...) 
Still get the same error, and the error is independent of satellite reception 
i.e. a track surveyed on a bus from the New Forest back to Southampton has 
the same error as a track walking out in the open on a hill. The track's 
always a little - perhaps 100m south from its normal position - and the error 
is pretty consistent. Beginning to think this may be something to do with the 
the way I'm grabbing the Landsat...


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