Hi all, I have been testing gdalwarp to aggregate (using -r average) an image. In order to better understand what is happening I have created a reproducible example within an R environment and compared it with the aggregate function of the R raster package (see below). There are some differences between the gdalwarp raster (r2) and the aggregated raster (r1).
How is the gdalwarp -r average working? Which pixels are selected for averaging (e.g.the corner pixels, center pixels, or all within the aggregation window)? If there is a gdal aggregate option to average pixels comparable to the aggregate function in the R raster package, it would be great as that would potentially faster, and you could also reproject and aggregate at once. Thanks! Jan http://bfast.r-forge.r-project.org http://goo.gl/1mBC5F ## R script require(raster) filei <- '10by10.tif' fileo <- '5by5.tif' set.seed(123) r <- raster(ncols=36, nrows=18) r <- setValues(r, round(runif(ncell(r))*10)) r plot(r) writeRaster(r, filename=filei, overwrite = TRUE) ## aggregate using R aggregate function using the mean r1 <- aggregate(r, fact=2, fun = mean, na.rm = TRUE) cmd <- paste("gdalwarp -r average -tr 20 20 -te -180 -90 180 90 ", filei, " ", fileo, sep = "") system(cmd) r2 <- raster(fileo) ## compare plot(r1) plot(r2) r1 r2 getValues(r1) getValues(r2) ## plot(r1-r2) sessionInfo() R> sessionInfo() R version 3.0.1 (2013-05-16) Platform: x86_64-pc-linux-gnu (64-bit) locale: [1] LC_CTYPE=en_US.UTF-8 LC_NUMERIC=C LC_TIME=C LC_COLLATE=C LC_MONETARY=C [6] LC_MESSAGES=C LC_PAPER=C LC_NAME=C LC_ADDRESS=C LC_TELEPHONE=C [11] LC_MEASUREMENT=C LC_IDENTIFICATION=C attached base packages: [1] stats graphics grDevices utils datasets methods base other attached packages: [1] rgdal_0.8-10 raster_2.1-49 sp_1.0-11 loaded via a namespace (and not attached): [1] grid_3.0.1 lattice_0.20-23 tools_3.0.1 rgdal: version: 0.8-10, (SVN revision 478) Geospatial Data Abstraction Library extensions to R successfully loaded Loaded GDAL runtime: GDAL 1.10.0, released 2013/04/24 Path to GDAL shared files: /usr/share/gdal/1.10 Loaded PROJ.4 runtime: Rel. 4.8.0, 6 March 2012, [PJ_VERSION: 480] Path to PROJ.4 shared files: (autodetected) _______________________________________________ gdal-dev mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/gdal-dev
