[FGS] gdal_translate projection query

[Frank Warmerdam]

Eric Legge Smith wrote:
> Hello List
> 
> I have a 20000 x 20000 pixel geo tiff map in AGD66/AMG and I required a
> 1500 x 1500 pixel sub section of this map in WGS84/UTM Zone 55S in order
> to directly plot GPS data.
> 
> To do the conversion, I used gdal_translate -srcwin Xoff Yoff Xsize Ysize
> -a_srs "EPSG:32755" , however the coordinates of the projected map extents
> (and the centre point) are approx 200m out  when compared to reference
> points on the ground and consequently, the GPS data are also out by the
> same 200m.
> 
> Am I doing something wrong in reprojecting from AGD66/AMG to WGS84/UTM
> Zone 55S?
> 
> If so, how should I be reprojecting the original tif to bring it into
> WGS84/UTM Zone 55S ?

Eric,

gdal_translate does not do image reprojection.  The -a_srs switch *assigns*
a coordinate system, but does not do image reprojection.  You want to use
gdalwarp.

Something like:

gdalwarp -t_srs EPSG:32755 -te xmin ymin xmax ymax -ts 1500 1500 in.tif out.tif

The xmin/ymin/xmax/ymax values would be the region you want expressed in
the output coordinate system.

There is a gdalwarp documentation page at:

   http://www.gdal.org/gdalwarp.html

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