Dear Jorge,
there are a lot of ways to do this. In straight R, here's a function designed
to work on a matrix of elevation values that I have used for many years. It is
very similar to the code you provide:
# This function calculates slope for a matrix using the Horn (1981)
# method, as reported by Burrough & McDonnell (1998). Slope is not
# calculated for the outermost rows and columns of the input matrix.
# Parameters: z is an input matrix of elevations, and spacing is
# the distance in ground units between cell centers.
# Returns a matrix of slope, in percent.
# written by Ashton Shortridge, 3-30-2001, 3/2004, 3/2008, 11/2009
# References:
# Burrough, P. A., McDonnell, R. A. (1998) Principles of GIS. Oxford Press:
Oxford.
# Horn, B. K. P. (1981) Hill shading and the reflectance map. Proc. IEEE 69(1):
14-47.
calc.slope <- function (z, spacing)
{
# Takes a matrix z of elevations. Returns a matrix of slope, in percent.
rows <- dim(z)[1] - 2
cols <- dim(z)[2] - 2
slope.mat <- matrix(nrow=rows+2, ncol=cols+2)
for(i in 2:rows) {
for(j in 2:cols) {
dzdx <- (z[i+1,j+1] + 2*z[i+1,j] + z[i+1,j-1]) - (z[i-1,j+1] +
2*z[i-1,j] + z[i-1,j-1])
dzdy <- (z[i+1,j+1] + 2*z[i,j+1] + z[i-1,j+1]) - (z[i+1,j-1] +
2*z[i,j-1] + z[i-1,j-1])
dzdx <- dzdx/(8*spacing)
dzdy <- dzdy/(8*spacing)
slope.mat[i,j] <- sqrt(dzdx^2 + dzdy^2) * 100
}
}
return(slope.mat)
}
On 11/28/11, Jorge Cabrera, wrote:
> 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
>
> [[alternative HTML version deleted]]
-----
Ashton Shortridge
Associate Professor ash...@msu.edu
Dept of Geography http://www.msu.edu/~ashton
235 Geography Building ph (517) 432-3561
Michigan State University fx (517) 432-1671
_______________________________________________
R-sig-Geo mailing list
R-sig-Geo@r-project.org
https://stat.ethz.ch/mailman/listinfo/r-sig-geo
