[R-sig-Geo] Subsetting RasterBrick very slow

[Christian Kamenik]

Dear all,

The package 'raster' provides excellent tools to work on geographic 
data. Subsetting a RasterBrick object, however, takes ages.

I have the following RasterBrick object:

     Temp.CRU.cropped

     class:                  RasterBrick
     filename:
     dimensions:        43, 99, 1200  (nrow, ncol, nlayers)
     ncell:                  4257
     projection:         +proj=longlat +ellps=WGS84
     min values:        -14.8 -22.1 -15.0  -8.6  -3.8   4.7   8.7   
6.0   1.6  -2.4 ...
     max values:       2.1 -3.9 -2.1  1.8  6.6 15.7 18.0 14.8 10.7  7.7 ...
     extent:               15, 31.5, 64, 71.16667  (xmin, xmax, ymin, ymax)
     resolution:         0.1666667, 0.1666667  (x, y)

These are temperature readings from northern Fennoscandia, and each 
layer represents a month, thus each grid cell has monthly readings over 
the last 100 years.

Now I want to extract the readings for only July:

     Months <- rep(month.abb,nlayers(Temp.CRU.cropped)/12)
     Temp.CRU.J <- subset(Temp.CRU.cropped,which(Months=='Jul'))

But as I said, this takes ages. It is much faster to do the subsetting 
on an array:

     CRU.values <- getValues(Temp.CRU.cropped)
     Temp.CRU.J <- CRU.values[,which(Months=='Jul')]
     Temp.CRU.J <- 
array(Temp.CRU.J,dim=c(nrow(Temp.CRU.cropped),ncol(Temp.CRU.cropped),nlayers(Temp.CRU.cropped)/12))
     Temp.CRU.J <- brick(Temp.CRU.J)
     Temp.CRU.J <- setExtent(Temp.CRU.J,extent(Temp.CRU.cropped))
     projection(Temp.CRU.J) <- projection(Temp.CRU.cropped)

It is strange that several lines of code run much faster than the 
'subset' command, and I was wondering why 'subset' takes at least a 1000 
times longer.

I would be interested in your opinion.

Many thanks, Christian

-- 
-----------------------------------------------
Oeschger Centre for Climate Change Research, Institute of Geography&  Institute of Plant Sciences
University of Bern

http://www.stomatocysts.unibe.ch/kamenik
mailto: christian.kamenik at giub.unibe.ch

Postal address:
Dr. Christian Kamenik
Institute of Geography
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3012 Bern, Switzerland

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