[STATSGRASS] "Neighbourhood modifier" from r.mapcalc in R?

[Roger Bivand]

On Tue, 16 Oct 2007, Rainer M Krug wrote:

> Hi
>
> I want to do some map calculations which involve binominal distributed random 
> numbers which I can not obtain in r.mapcalc. Therefore I import the raster 
> via readRAST6() into R and I want to do the calculations there. But I am 
> stuck...
>
> In r.mapcalc I use somthing like
>
> OUTPUT=INPUT1[-2,-2]*INPUT2+INPUT1[+2,+2]*INPUT2
>
> How can I translate this into R?
>
> rA <- readRAST6("INPUT1")
> rB <- readRAST6("INPUT2")
> but now?
>
> rA$A gives me all cell values,
> rA[1:2,1:2]$A gives me the cell values from columns and row 1 and 2,
>
> But I have no idea where I should put the offset in.

As designed, the SpatialGridDataFrame objects are intended to have the 
same regions in the g.region sense. They do have grid slots, and you could 
shift the whole grid by changing the cellcentre.offset slot in the 
GridTopology object in the grid slot, and update the bbox. This would give 
grids that are offset in relation to each other, but to get the values 
across all offset grids, you'd need to back the original SpatialGrid out 
to SpatialPoints, and use the overlay() method for SpatialPoints and 
SpatialGridDataFrame objects.

An alternative that would need coding is to look inside the "[" method for 
SpatialGridDataFrame objects (which you refer to above) and see how one 
might write a relative reference function which would not subset on the i 
and j arguments, but shift by row and column.

Probably the first alternative involves less work.

Roger

>
> Rainer
>
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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