[R-sig-Geo] questions on RasterStack/Brick

[Robert J. Hijmans]

On Tue, Sep 21, 2010 at 12:49 AM, Martin <martin_brandt at gmx.net> wrote:
>
> Dear all,
>
> I have some questions concerning stacks and bricks from the raster package.
>
> 1. is it possible to flip the whole stack vertically? I get an error when I
> use the 'flip' function..
>

You can use what Nikhil send you or perhaps (for objects that are not
too big), with RasterStack s

x <- as(s, 'SpatialGridDataFrame')
x <- flipVertical(x)
s2 <- stack(x)  # or   b <- brick(x)

or get raster 1.5.9 from R-Forge in a couple of hours and then do

x <- flip(s, direction='y')


> 2. is it possible to use the calc function (or something else) on a stack to
> get a raster, which shows the regression slope of the pixels through the
> stack (treated like a kind of time series)?

You can do things like this:

myfun = function(v, ...) { d <- data.frame(x=1:length(v), y=v);
lm(y~x, data=d)$coefficients[2] }

a <- calc(stack, fun=myfun)


> And is it possible to compute a
> correlation between two stacks of the same dimensions?

Not directly, I think, but if you have two stacks, s1, and s2, and
some patience, you can do:

 r <- raster(s1)
for (i in 1:ncell(r)) {
    r[i] <- cor(as.vector(cellValues(s1, i)), as.vector(cellValues(s2, i)))
}


Robert


> greetings,
> Martin
>
>
> --
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