[gdal-dev] Fwd: gdalwarp -r average bug?

[Kyle Shannon]

On Thu, Sep 25, 2014 at 8:10 AM, Kyle Shannon <kyle at pobox.com> wrote:
> Stephen,
>
> On Wed, Sep 24, 2014 at 2:21 PM, Stephen Roecker
> <stephen.roecker at gmail.com> wrote:
>> Out of curiousity the other day I compared the results of gdalwarp (-r
>> average) against  the raster R package aggregate(fun=mean) function
>> for aggregating a raster to a coarser resolution. I was suprized how
>> different the results of gdalwarp were from raster. When zooming in
>> and manually averaging the overlapping cells, the results of gdal were
>> off.
>>
>> The elevation difference between the raster and gdal aggregated
>> rasters only averaged 0.66 meters, but had a max of 12 meters. Also
>> when subtracting the raster and gdal aggregated rasters, the resulting
>> subtracted layer looked like a hillshade, suggesting the GDAL
>> aggregated raster was shifted. However the raster and GDAL rasters
>> overlapped perfectly, so I can only assume the shift occured during
>> the aggregation process.
>>
>> Can someone explain gdal's behavior to me? Why the difference, is this
>> a bug in gdal? gdalwarp claims it's averaging all the overlapping
>> cells except the NA. That doesn't seem to be the case. FYI I'm using
>> GDAL 1.10.1
>>
>> See a reproduceable R example below.
>>
>> Stephen
>>
>> library(gdalUtils)
>> library(raster)
>>
>> src_dataset <- system.file("external/tahoe_lidar_bareearth.tif",
>> package="gdalUtils")
>> test <- raster(x=src_dataset)
>>
>> writeRaster(test, "test.tif", overwrite=T)
>> gdal_setInstallation(search_path="C:/ProgramData/QGIS/QGISDufour/bin",
>> rescan=T, verbose=T)
>> gdalwarp(srcfile=src_dataset, dstfile="test_gdal.tif", of="GTiff",
>> r="average", ot="Float32", tr=res(test)*3, overwrite=TRUE,
>> verbose=TRUE)
>>
>> aggregate(x=raster(src_dataset), fact=3, filename="test_raster.tif",
>> format="GTiff", NAflag=-99999,
>>   progress="text", overwrite=T)
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>> gdal-dev at lists.osgeo.org
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>
> Without looking too far into it, it seems similar to:
>
> http://trac.osgeo.org/gdal/ticket/5311
>
> --
> Kyle

Apologies, it does appear to be different, my mistake.

-- 
Kyle