DIY: Reprojection...Re: [Gdal-dev] raster image preprocessing problem.

[Frank Warmerdam]

Charles Wivell wrote:
> Hi Robert;
> 
> I really couldn't tell you what GDALWarp can do, or is for. I'm a control freak.... I write the stuff myself. I only use GDAL to read and write data from/to the really icky file formats.
> 
> But warp to me says it uses a polynomial to warp from one projection to another (but maybe I'm wrong), where as the correct (more accurate way) is to use a projection library, like I outlined.
> 
> I think if it is doing a warp it might have problems (what is the degree of the polynomial used in the warp...?) and like was pointed out by another replier, since the Mercator projection is not defined at the poles, a polynomial might go off into lala land (technical term) quicker than using the projection library....?
> 
> But I'm guessing about GDALWarp....

Charles / Robert,

Gdalwarp normally uses exact reprojection of the center point of each pixel
when doing reprojection using PROJ.4.  It also has an option for a
per-scanline linear approximation within some error threshold though I don't
believe that is used by default.  (see -et switch).

Gdalwarp also supports polynomial warping when given GCPs are input.

I certainly respect Charles' desire to "do it himself" though this level
of effort is clearly not practical for everyone.  I do aim for gdalwarp to
be generally useful for end users though there are a few things it doesn't
do.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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