[gdal-dev] Resampling a raster file with GDAL C#

[Even Rouault]

Le lundi 18 avril 2016 19:59:32, Didier Bernard a écrit :
> Hello,
> 
> 
> I am trying to resample a .tif raster in C# GDAL in a way that every other
> cell in a row and column is removed.
> 
> Something similar to the lower GDAL command:
> 
>     gdal_translate -outsize 50% 0 input.tif output.tif
> 
> But this command creates a new resampled .tif file.
> While I would like to avoid saving a new .tif file, and if possible just
> resample the dataset.
> 
> It looks like this can be achieved by the use of VRT
> file<http://www.gdal.org/gdal_vrttut.html>.
> 
> Here is one example of a .vrt file, which resamples the .tif file by
> removing the every other cell:
> 
> 
> <VRTDataset rasterXSize="42" rasterYSize="32">
>   <GeoTransform>14.3779166666, 0.000833333333333, 0.0, 40.8404166667, 0.0,
> -0.000833333333333</GeoTransform> <VRTRasterBand dataType="Byte" band="1">
>     <ColorInterp>Gray</ColorInterp>
>     <SimpleSource resampling="bilinear">
>       <SourceFilename relativeToVRT="1">vesuvius_wgs84.tif</SourceFilename>
>       <SourceBand>1</SourceBand>
>       <SrcRect xOff="0" yOff="0" xSize="85" ySize="65"/>
>       <DstRect xOff="0" yOff="0" xSize="42" ySize="32"/>
>     </SimpleSource>
>   </VRTRasterBand>
> </VRTDataset>
> 

It is not clear how you produced this VRT file. If you for example create it 
with gdal_translate -of VRT, you'll see that the resolution is doubled.

> 
> What I noticed is that resolution of the resampled dataset does not change
> in comparison to the original dataset.
> 
> In this case it remains the 0.00083 degrees.
> The same thing happens if I reproject the .tif file to some projected
> coordinate system (UTM for example): resolution remains the same (70
> meters for example).
> 
> Why is this the case?
> 
> Shouldn't the resolution of the resampled dataset be twice of the
> resolution of the original (non-resampled) dataset, based on the fact that
> every other cell is removed (original dataset: 85x65, resampled dataset:
> 42x32)?
> 
> 
> I would be very grateful if I could get any kind of reply.
> 
> Thank you in advance.
> 
> With kind regards,
> Didier

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com