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

[Kyle Shannon]

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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Without looking too far into it, it seems similar to:

http://trac.osgeo.org/gdal/ticket/5311

-- 
Kyle