Raster manipulation in R

jjcabrera20 wrote on 12/01/2011 04:02:47 AM:

Hello everyone in the forum
For introducing myself I would say I have a basic knowledge of R.
Now I am intended to implement a flood algorithm using R. I have some

MatLab

experience doing these, and as an example to explain more or less what I
want, I have a m code to calculate the slope from a Digital elevation

model:

slope=zeros(row,col);
for i=2:row-1
     for j=2:col-1
        dzdx=(raster(i,j+1)-raster(i,j-1))/(2*res);
        dzdy=(raster(i+1,j)-raster(i-1,j))/(2*res);
        slope(i,j)=sqrt(dzdx^2+dzdy^2);
     end
end;

The question is to know how to do the similar procedure on R. All
suggestions are welcome
Thanks

All best,

Jorge
PD:I am using R on windows system 64 bits

If I am interpreting your code correctly (I don't use MatLab myself),
something like this should give you the same result in R:

# example matrix of heights
raster<- matrix(runif(20, 10, 30), nrow=4, ncol=5)

# example resolution
res<- 8.5

# dimensions of matrix
drast<- dim(raster)

# for every non-boundary point in the matrix,
# calculate the distances between its adjacent columns (dzdx) and rows
(dzdy)
dzdx<- raster[2:(drast[1]-1), 3:drast[2]] - raster[2:(drast[1]-1),
1:(drast[2]-2)]
dzdy<- raster[3:drast[1], 2:(drast[2]-1)] - raster[1:(drast[1]-2),
2:(drast[2]-1)]

# calculate the slope from these distances and the resolution
slope<- sqrt(dzdx^2 + dzdy^2) / (2*res)


Jean
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