[gdal-dev] Re: Require Help Transforming into Equirectangular

[Sjur Kolberg]

Scott,

Sorry to say I'm at the limit of my knowledge here.

Somehow I remember that GDAL/OGR is re-implementing some (but not all) of proj.4's features, and that for some difficult cases (I had some data generated from a rotated sphere) you need to pay extra attention. Have a look in the archives at Frank Warmerdam's answer from June 7th 2007, but I don't know if this solves your case.

Sjur  :-)




-----Original Message-----
From: gdal-dev-bounces at lists.osgeo.org [mailto:gdal-dev-bounces at lists.osgeo.org] On Behalf Of ssobieraj
Sent: 1. desember 2010 21:36
To: gdal-dev at lists.osgeo.org
Subject: [gdal-dev] Re: Require Help Transforming into Equirectangular


Hi Sjur,

Matching the ellipsoids in cs2cs seems to work, but I really need to be able
to use the PRJ information in gdalwarp.  Shouldn't GDAL be using the PRJ
information to correctly transform between sphere and ellipsoid?

This also doesn't explain why the Sinusoidal/WGS84 transformation had such a
large error.  It matches the ellipsoid properly and still exhibits issues.

Any idea as to why gdalwarp would exhibit such an error even when the
ellipsoids match?

Scott


Sjur Kolberg wrote:
> 
> 
> Ssobieraj,
> 
> If the 16 km offset is in N-S direction, and the E-W error is small, it
> might be a sphere/ellipsoid problem. From your RT90 I am guessing you're
> in Sweden, at which latitude you would expect approximately this error.
> The effect is beyond projection, latlong/sphere to latlong/WGS84 gives
> equally long offsets. Try using cs2cs and specify ellipsoids for both
> input and output. 
> 
> There were some discussions on the gdal-dev list about this in June 2007.
> 
> It is only when one of the ellipsoids is a sphere you get these large
> errors, otherwise the error resulting from messing up ellipsoids is much
> smaller.
> 
> Hope this helps,
> 
> Sjur K :-)
> 
> 

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