[R-sig-Geo] Calculating descriptive stats from many maps

[Roger Bivand]

On Mon, 9 Feb 2009, Tomislav Hengl wrote:

>
> Dear Rainer,
>
> This is of course possible in R, and can be done in several ways:
>
> 1) for example, you can derive the average value using the rowSums function:
>
>> maps$Nsum <- rowSums(maps at data, na.rm=T, dims=1)
>> maps$avg <- maps$Nsum/(length(names(meuse.grid at data))-1)
>
> You could also loop the sd, mean and quantile function over a range of cells:
>
>> for(i in length(names(maps at data))) {
>> maps at data$sd[i] <- sd(maps at data[i,])
>> maps at data$mean[i] <- mean(maps at data[i,])
> ...
>> }
>
> This could take a lot of time!

Tom, Rainer,

Yes, using sapply(slot(maps, "data"), summary) or similar, you get the 
map-wise statistics. But have I misunderstood, or are the statistics in 
question cell-wise? This would involve stacking subset areas for all 25' 
maps, wouldn't it? Brutally, a loop in readGDAL() from rgdal with offset= 
and region.dim= shifted? Is there a canned way to do this in the R-forge 
raster package (by the way, regularly one of the R-forge packages showing 
most developer activity)?

Roger

>
> 2) if your maps are rather large, try also using the SAGA function:
>
>> rsaga.get.usage(lib = "geostatistics_grid", module=5)
> SAGA CMD 2.0.3
> library path:   C:/Progra~1/saga_vc/modules
> library name:   geostatistics_grid
> module name :   Statistics for Grids
>
> This is probably the fastest method you can use.
>
> HTH
>
> T. Hengl
>
>> -----Original Message-----
>> From: r-sig-geo-bounces at stat.math.ethz.ch [mailto:r-sig-geo-bounces at stat.math.ethz.ch] On Behalf
>> Of Rainer M Krug
>> Sent: Monday, February 09, 2009 4:58 PM
>> To: R-sig-Geo at stat.math.ethz.ch
>> Subject: [R-sig-Geo] Calculating descriptive stats from many maps
>>
>> Hi
>>
>> I have 25000 maps, generated by simulation predictions, covering the
>> same area, and would like to calculate some descriptive stats, like
>> mean, standard deviation, median, quartiles of all cells, to create a
>> "variability map".
>>
>> Is there an easy way of doing this in R?
>>
>> Thanks,
>>
>> Rainer
>>
>> --
>> Rainer M. Krug, PhD (Conservation Ecology, SUN), MSc (Conservation
>> Biology, UCT), Dipl. Phys. (Germany)
>>
>> Centre of Excellence for Invasion Biology
>> Faculty of Science
>> Natural Sciences Building
>> Private Bag X1
>> University of Stellenbosch
>> Matieland 7602
>> South Africa
>>
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>
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-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no