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I have a doubt about GDAL reprojection. I use "gdalwarp" utility to reproject an image in a specific projection system (for example WGS84 UTM49). "gdalinfo" on original image displays this corner coordinate:

Lower Right ( 123.0220110, -54.6797900) (123d 1'19.24"E, 54d40'47.24"S)

and projected image in UTM49:

Lower Right ( 1845275.980,-6159877.836) (131d34' 2.85"E, 53d48' 4.36"S)

I suppose that degrees (131d34' 2.85"E, 53d48' 4.36"S) are degrees coordinates of the pixel in lower right in the projected image, different from the originals.

I read about "gdaltransform" utility that it reprojects coordinates from one system to another, but running this:

$ gdaltransform -s_srs EPSG:32649 -t_srs EPSG:4326

1845275.980 -6159877.836

(when 1845275.980 -6159877.836 are projected coordinates in utm49) it displays geographic coordinate not in original image ( 123.0220110, -54.6797900), but in the projected image(131.56745835871 -53.8012102381326). From this result I suppose that gdaltransform doesn't reprojects from one system to another, but it simply "translates" coordinates. Is that affirmation correct?

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  • My initial thought is that there was something wrong with the GDAL Warp operation. The gdalinfo command should always return the same location for the lower right. In order for that much shift to have occurred, something seems very off. What type of image are you reprojecting? How much area does it cover? Could you edit your question and include the command string you entered for gdal warp?
    – Get Spatial
    Commented May 15, 2013 at 16:00

2 Answers 2

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The mistake you made is assuming that the reprojected rectangle of border points still is a rectangle. Outside of the definition of your UTM zone, it might rather be distorted and squezed.

This is what a rectangle of 20 to 10 degrees looks like in UTM 49N:

enter image description here

By the way, your projection should rather be UTM 49S.

So you can not assume that the cormers of the extent still represent the same points as before the reprojection.

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I think your lower right is outside of the UTMZone. If you read the overview of this utm zone at http://www.spatialreference.org/ref/epsg/32649/ you will see that the WGS84 bounds for utm49N are 108.0000, 0.0000, 114.0000, 84.0000 (lon_min, lat_min, lon_max, lat_max). Once you get outside of this area you may end up getting some funky results for your projection. Some projections begin to behave erratically once you get outside of their intended area.

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