[R-sig-Geo] resampling grids

[Tomislav Hengl]

A quick way to rescale a set of maps is to use the grid tools of SAGA GIS (see
rsaga.get.usage("grid_tools", 0)). First, download all ASCII file maps to a working directory, then
obtain the list of maps in the folder, and then automate:

> ascmaps <- list.files(getwd(), pattern="\\.asc$", full=F)
> levels(ascmaps) <- list.files(getwd(), pattern="\\.asc$", full=F)
> ascmaps500 <- ascmaps
> levels(ascmaps500) <- paste("500m",levels(ascmaps),sep="")
> library(RSAGA)
> rsaga.esri.to.sgrd(in.grids=levels(ascmaps),
out.sgrds=set.file.extension(levels(ascmaps),".sgrd"), in.path=getwd())
> rsaga.get.usage("grid_tools", 0)
> for (i in 1:length(ascmaps)) {
     rsaga.geoprocessor(lib="grid_tools", module=0,
param=list(INPUT=set.file.extension(levels(ascmaps)[i],".sgrd"),
     GRID=set.file.extension(levels(ascmaps500)[i],".sgrd"), METHOD=0, DIMENSIONS_CELLSIZE=500,
SCALE_UP_METHOD=1))
}
> rsaga.sgrd.to.esri(in.sgrds=set.file.extension(levels(ascmaps500),".sgrd"), 
    out.grids=set.file.extension(levels(ascmaps500),".asc"), out.path=getwd(), prec=1)
> unlink("*.hgrd") # delete the temporary files (SAGA)
> unlink("*.sgrd")
> unlink("*.sdat")

Note that you only need to set the resolution of the output maps and/or the rescaling method:

-SCALE_UP_METHOD:<num>        Interpolation Method
        Choice
        Available Choices:
        [0] Nearest Neighbor
        [1] Bilinear Interpolation
        [2] Inverse Distance Interpolation
        [3] Bicubic Spline Interpolation
        [4] B-Spline Interpolation
        [5] Mean Value
  -SCALE_DOWN_METHOD:<num>      Interpolation Method
        Choice
        Available Choices:
        [0] Nearest Neighbor
        [1] Bilinear Interpolation
        [2] Inverse Distance Interpolation
        [3] Bicubic Spline Interpolation
        [4] B-Spline Interpolation


Mechanical down-scaling from 20 to 10 m is certainly doable but you will be 'cheating' right?

More stuff on this topic (downscaling) in e.g.:

Marc F. P. Bierkens, Peter A. Finke, P. de Willigen, 2000. Upscaling and Downscaling Methods for
Environmental Research. Springer, ISBN 0792363396, 9780792363392, 190 pages.
http://www.amazon.com/gp/reader/0792363396/ 


Tom Hengl
http://spatial-analyst.net/wiki/index.php?title=Software 




-----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
Jason Gasper
Sent: Wednesday, December 10, 2008 7:23 PM
To: Dave Depew
Cc: r-sig-geo at stat.math.ethz.ch
Subject: Re: [R-sig-Geo] resampling grids

Hello David,

I am not sure I completly understand what you want to do, but Hawths 
tools (a free add in for arcMAP) has a  resampling function that may be 
of use:  http://www.spatialecology.com/htools/rndselss.php.  Spatial 
analyst also has some resampling functions.  If you have access to ESRI 
you could read the data into R using the maptools. package.






Dave Depew wrote:
> Hi all,
> I apologize in advance, I haven't the time today to comb the help list 
> for tips and hints, but does anyone know how to resample a 20 by 20m 
> grid so the operational spacing is down to a 10 by 10m grid?
> Any assistance is MUCH appreciated!
>

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