[gdal-dev] Resampling a raster with gdalwarp

[Giovanni Anconitano]

Thank you very much.

Just to be sure, gdalwarp takes the absolute value of the second tr value
independently from the units of the pixel size.
It can be either meters or degrees depending on the coordinate reference
system. Is that correct?

Thanks again,
Giovanni

Il giorno sab 18 nov 2023 alle ore 17:27 Even Rouault <
even.rouault at spatialys.com> ha scritto:

> 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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>
>
>
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-- 
**Fai crescere le giovani ricercatrici e i giovani ricercatori*

**con il 5 
per mille alla Sapienza
*Scrivi il codice fiscale dell'Università 
*80209930587
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