[Gdal-dev] GeoTiff pixel resolution interpretation with gdalinfo

[Frank Warmerdam]

mattia wrote:
> Coordinate System is:
> PROJCS["HDFLook: LINEAR",
>     GEOGCS["unnamed",
>         DATUM["unknown",
>             SPHEROID["unnamed",6370997,0]],
>         PRIMEM["Greenwich",0],
>         UNIT["degree",0.0174532925199433]],
>     PROJECTION["Equirectangular"],
>     PARAMETER["latitude_of_origin",0],
>     PARAMETER["central_meridian",0],
>     PARAMETER["false_easting",0],
>     PARAMETER["false_northing",0],
>     UNIT["metre",1,
>         AUTHORITY["EPSG","9001"]]]
> Origin = (8912408.472916,7090227.412053 )
> Pixel Size = (694.49452741,-1000.15538912)
...
> Is my interpretation correct? The pixel resolution as indicated is in 
> meters... Accordingly 694 meters horizontally (X res) , 1000 meters 
> vertically (Y res)?!?!

Mattia,

That is correct.

> What is the minus sign i front of the Y res for???

It means that as you step "down" the image from the top left corner you
subtract 1000m per pixel step from the y coordinate.  This is normal.

> What is the UNIT["degree",0.0174532925199433]], factor in this output?? 
> can I use it to define the resolution?? Or what is it needed for??

This indicates that one degree on the underlying geographic
coordinate system is 0.017... radians.  It is standard and does
not help you for what you want to do.

> Final question,
> 
> Is there a stright forward way of changing the image's resolution so as 
> to have a quadratic one whereby resX=resY ????

I didn't follow that I'm afraid.  Are you just wanting to resample
your image to have square pixels?  You could do something like:

gdalwarp -rb -ps 694 694 Area1.tif Area1SquarePixels.tif

This would use a bilinear resampling kernel to convert your
image from 694x1000 meter pixels to 694x694 meter pixels.

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org