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

Schuyler Erle schuyler at nocat.net
Sat Apr 30 19:43:57 BST 2005


* On 30-Apr-2005 at 11:17AM PDT, Nick Whitelegg said:
> 
> BTW One other experiment I've just tried is converting the GPX coordinates to 
> Airy as well as the transformation from lat/lon to screen - so everything is 
> consistently in Airy. I get exactly the same offset error across the board as 
> I did when I converted everything to WGS84.
> 
> In other words the erroneous mixture of WGS84 and Airy gave me the least bad 
> error while pure WGS and pure Airy both give me the same (worse) error.

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.

SDE




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