[R-sig-Geo] Basic raster operations

[Michael Sumner]

I would follow this up in the elide() function of maptools.  I thought 
I'd post this as a partial answer, since I'm sure this is the most 
productive way to go given the goals of sp/maptools.

This is a test done only for simple shift on SGDF:

## WARNING: test method only partially implemented (for shift=)
setMethod("elide", "SpatialGridDataFrame",
    function(obj, ...)
    {
    .local <- function (obj, bb = NULL, shift = c(0, 0), reflect = c(FALSE,
        FALSE), scale = NULL, flip = FALSE, rotate = 0, center = NULL)
    {
        #res <- elide(as(obj, "SpatialPoints"), bb = bb, shift = shift,
         #   reflect = reflect, scale = scale, flip = flip, rotate = 
rotate,
          #  center = center)

    warning("TEST METHOD: elide only defined for shift on SGDF!")
    gt <- getGridTopology(obj)

    cc.offset <- gt at cellcentre.offset + shift
    gt <- GridTopology(cc.offset, gt at cellsize, gt at cells.dim)
    p4 <- proj4string(obj)

        df <- slot(obj, "data")
        res <- SpatialGridDataFrame(gt, data = df, proj4string = 
CRS(as.character(p4)))
        res
    }
    .local(obj, ...)
})

## Examples
library(maptools)
x <- image2Grid(list(x = 1:nrow(volcano), y = 1:ncol(volcano), z = volcano))

image(x)
contour(elide(x, shift = c(-15, 10)), add = T)

y <-  readGDAL(system.file("pictures/Rlogo.jpg", package = "rgdal")[1], 
band=1)
image(y)
image(elide(y, shift = c(10, 10)), add = T)
   
I hope that helps, at least for your first question, certainly I 
appreciate you pushing me in the right direction. 

Cheers, Mike.

Barry Rowlingson wrote:
> I'm getting frustrated at what I thought would be simple raster 
> manipulations...
>
>  * How can I shift a SpatialGridDataFrame? I can't just change the 
> coordinates:
>
> > coordinates(compare)=coordinates(compare)+10
> Error in `coordinates<-`(`*tmp*`, value = c(10.5, 11.5, 12.5, 13.5, 
> 14.5,  :
>   setting coordinates cannot be done on Spatial objects, where they 
> have already been set
>
>  Do I have to construct a new GridTopology object, then feed that to 
> SpatialGridDataFrame along with the @data slot from the original? It 
> seems to work, but I was hoping for something simpler, and I always 
> get the jitters when working with slots since I tend to think of them 
> as private object data that I shouldn't mess with. Isn't there a 
> 'getData' method?
>
>  * How can I construct larger SpatialGridDataFrames by tiling smaller 
> ones?
>
>  If they operated like matrices or data frames I could just cbind or 
> rbind them together. I think I'm going to have to do a similar 
> procedure as above - work out the new GridTopology and pass a new data 
> frame constructed from merging the individual data frames together and 
> making sure it's all in the right order. Which is tricky, because the 
> SpatialGridDataFrame constructor expects its data argument to start at 
> top-left and sweep across and then down. To paste another raster to 
> the right of an existing one would mean mingling the data frame rows....
>
>  Or am I just missing something simple with this? Of course it *will* 
> be simple, once I've written the tiling functions:
>
>  foo = tile(bar,west=baz)
>
> will construct a new grid 'foo' which is bar with baz pasted on it's 
> western edge, equivalent to:
>
>  foo = tile(baz,east=bar)
>
>  it'll check the grids have the same number of rows (or for N-S 
> tiling, columns) and that the data frames are similar.
>
> Barry
>
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