[gdal-dev] Resampling a raster with gdalwarp

[Even Rouault]

Giovanni,

gdalwarp takes the absolute value of the second tr value, so -tr 10 -10 
is understood as -tr 10 10

It is expected that gdalinfo reports a negative value for the pixel size 
factor for the vertical axis, as in GeoTIFF when the image is north-up, 
the first line of the image is intended to be the one displayed at the 
top of your screen, and as the line number of your screen increase from 
the top of your screen to the bottom, northings or latitudes increase 
from the bottom to the top, hence the negative sign...

More detailed equivalent explanation at 
https://gis.stackexchange.com/a/229807/218992 (you can ignore the 
paragraph "As to why the image flipped..." which is specific to the 
question in that stackexchange thread)

Even

Le 19/11/2023 à 02:15, Giovanni Anconitano via gdal-dev a écrit :
> Hello,
>
> I have a question about how to properly set the -tr flag when using 
> gdalwarp.
> In particular, I'm using gdalwarp for resampling a GeoTiff from its 
> original resolution to a target resolution.
> Let's consider, for simplicity, the original pixel size equal to (5m, 
> 5m) and the target resolution equal to (10m, 10m).
>
> This is the command I use: gdalwarp -tr 10 -10 input.tif output.tif
>
> I'm using a negative <yres> because when I open the GeoTiff file in 
> QGIS, or if I use gdalinfo, I get a pixel size equal to (5, -5) for 
> the source raster.
>
> Is it correct to use a negative <yres> with gdalwarp? I also tried -tr 
> 10 10, but the result seems to be the same as the case in which I use 
> -tr 10 -10.
> In both cases gdalinfo and QGIS display (10,-10) as the pixel size.
>
> How does gdalwarp treat a negative yres value?
>
> Thank you
>
> Giovanni
>
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