[gdal-dev] how to aggregate an image to a lower resolution using gdal (gdalwarp?)

[Verbesselt, Jan]

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)