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

[Sjur Kolberg]

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 :-)






-----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 17:52
To: gdal-dev at lists.osgeo.org
Subject: [gdal-dev] Re: Require Help Transforming into Equirectangular


Hi Jean-Claude,

Thanks for the help.

To test the effect that I was getting, I created a total of four images. 
Two in Sinusoidal projection, using WGS84 and RT90, and another pair using
Lambert Equal Area (again with both WGS84 and RT90 datum).  I then compared
these to an unaltered Equirectangular projected image in WGS84.

The results seemed odd and so I've provided them below:

The Sinusoidal projections were both 16km from my source, regardless of
whether I used RT90 or WGS84.
The Lambert projection using WGS84 transformed back perfectly.
The Lambert projection using RT90 transformed back with a ~900m error,
similar to what was seen when I used Vincent's method above.

So it's possible that the +towgs84 parameters are at fault, but then why
would the Sinusoidal/WGS84 be as bad as the Sinusoidal/RT90?


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