[Gdal-dev] Warp / rotation

Richard Greenwood Rich at GreenwoodMap.com
Tue Aug 19 22:49:16 EDT 2003

At 03:56 PM 8/19/2003 -0600, you wrote:

>I do mean grid convergence of the projection.
>It sort of is lat / long except it isn't. Everything is projected to UTM, 
>then localized to the center of the area and rotated by the grid 
>convergence. Possibly all that I need to do is rotate the image, which 
>again, I am not sure what would be the most efficient method.

Warp and the other gdal tools accept (expect) coordinate system definitions 
for the input and output images. If you are converting between 'standard' 
systems you simply specify them with -s_srs and -d_srs. If you have a local 
system i.e. non-standard, then you can create a definition, or specify its 
parameters on the command line in proj format, or reference a .prj file.

But I am not aware of a way to specify an arbitrary rotation which is 
independent of a spatial reference system. Photoshop allows you to rotate 
an image by an arbitrary amount. I think it rotates it around the center of 
the image. I have actually re-projected images in this way but I don't 
recommend it if you have more than one image to re-project (or you have a 
lot of time on your hands).

Also, gdalwarp and gdal_translate allow you to clip the image, which I am 
guessing is what you are also wanting to do.

You might look at GRASS, which re-projects images, and you might find to be 
one click easier to use than gdalwarp.

And as Ed said, if you can clearly explain exactly what you are wanting to 
do, maybe someone can offer a better solution.


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