[R-sig-Geo] C API for sp classes?

[Edzer J. Pebesma]

Wouter, if you want to use the .C interface, you need to pass
the maps as simple vectors. I would pass one with the map
values, and one with the topology. Below is some example
code how to get them. An R function should be used to do
the coercion to double arrays, and wrap the .C call.

Please note that a SpatialGridDataFrame can actually hold
more than one attribute (map).
--
Edzer

For getting the values:

 > library(sp)
 > data(meuse.grid)
 > gridded(meuse.grid)=~x+y
 > fullgrid(meuse.grid)=T
 > meuse.grid at data@att[[1]] # first attribute
 [1] NA NA NA NA NA NA NA NA NA NA ...
 >

Getting the topology:

 > meuse.grid at grid
                       x      y
cellcentre.offset 178460 329620
cellsize              40     40
cells.dim             78    104
 > class(meuse.grid at grid)
[1] "GridTopology"
attr(,"package")
[1] "sp"
 > unlist(gridtopology(meuse.grid))
Error in unlist(gridtopology(meuse.grid)) :
        couldn't find function "gridtopology"
 > unlist(gridparameters(meuse.grid))
cellcentre.offset1 cellcentre.offset2          cellsize1          cellsize2
            178460             329620                 40                 40
        cells.dim1         cells.dim2
                78                104
 > as.double(unlist(gridparameters(meuse.grid)))
[1] 178460 329620     40     40     78    104
 >


Wouter Buytaert wrote:

>Hi,
>
>I want to write some R functions for calculating hydrological 
>catchment characteristics from a DEM (topographic index, overland flow 
>delay function etc.).
>
>Looks like using a DEM with SpatialGridDataFrame class as an input is a 
>good idea.
>
>Then what is the best way to extract the data matrix from this class to 
>pass it on to a .C() function which does the actual calculations?
>
>cheers,
>
>wouter
>
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