[R-sig-Geo] SpatialGrid from matrix

[pedro at dpi.inpe.br]

Very nice! Also now the size of the Spatial object is almost the same 
of the kernel's result:

> object.size(g)
[1] 83476
> object.size(x)
[1] 82224

Thanks!

Pedro Andrade

Quoting "Edzer J. Pebesma" <e.pebesma at geo.uu.nl>:

> Michael Sumner wrote:
>
>>> require(splancs)
>>> data(bodmin)
>>> x= kernel2d(as.points(bodmin), bodmin$poly, h0=2, nx=100, ny=100)
>>> require(sp)
>>> g=expand.grid(x$x,x$y,x$z)
>>>
>>> The last line gives the following error:
>>> Error: cannot allocate vector of size 390625 Kb
>>>
>>>
>>>
>>
>> You are expanding your grid for every cell in the matrix, per every grid
>> location - which *really is* a lot of memory.  You really want something
>> like this:
>>
>> g <- cbind(expand.grid(x = x$x, y = x$y), as.vector(x$z))
>> coordinates(g) <- c("x", "y")
>> gridded(g) <- TRUE
>> image(g)
>> contour(x, add = T)  ## just to be sure
>>
>> I'm pretty sure the ordering is right for this case, but be sure to check.
>>
>> Cheers, Mike.
>>
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>>
>>
> Thanks, Mike, that's an excellent solution.
>
> In this way g is of
>
> > class(g)
> [1] "SpatialPixelsDataFrame"
> attr(,"package")
> [1] "sp"
>
> meaning that it stores x and y coordinates. To save this
> space, you could store it as SpatialGridDataFrame:
> > object.size(g)
> [1] 283528
> > fullgrid(g)=T
> > object.size(g)
> [1] 83476
> > class(g)
> [1] "SpatialGridDataFrame"
> attr(,"package")
> [1] "sp"
>
> A direct way to create a SpatialGridDataFrame without
> using expand.grid() would use GridTopology() and
> SpatialGridDataFrame(). You'd have to reorder x$z
> for that, too.
> --
> Edzer
>
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