[R-sig-Geo] raster directions to vectorial lines (Stream to Feature)

[Gavan McGrath]

Hi Mauricio,

Below is a simple pair of functions for plotting a flow network over an image which should achieve what you were after.  You may want to play around with how your flow direction numbers 1 -9 match up with those in the lookupfidr function. Its a bit cluncky but gets the job done.

The function fdirPlot takes as input a matrix imData the background image, and a matrix fdirs of flow direction integers.

Good luck.


lookupfdir <-function(fdir) {
   switch(paste(fdir,"",sep=""),
       '1' = c(-1,-1),
       '2' = c(-1,0),
       '3' = c(-1,1),
       '4' = c(0,1),
       '5' = c(1,1),
       '6' = c(1,0),
       '7' = c(1,-1),
       '8' = c(0,-1),
       '9' = c(0,0),
           c(-2,-2)
   )
}

fdirPlot <- function(imData,fdirs) {
  dimDat<- dim(imData)
  image(imData,axes=FALSE)
  for (i in 1:dimDat[1]) {
  for (j in 1:dimDat[2]) {
  dxdy <- lookupfdir(fdirs[i,j])
  if (dxdy[1] != -2) {
   lines(rbind(c(i-0.5,j-0.5)/dimDat[1],c(i-dxdy[1]-0.5,j-dxdy[2]-0.5)/dimDat[1]))
   } else {
          if (i==1) {
            lines(rbind(c(i-0.5,j-0.5)/dimDat[1],c(i-1.5,j-0.5)/dimDat[1]))
          } else {
                if (i==dimDat[1]) {
                    lines(rbind(c(i-0.5,j-0.5)/dimDat[1],c(i+1-0.5,j-0.5)/dimDat[1]))
                 } else {
                        if (j==1) { 
                          lines(rbind(c(i-0.5,j-0.5)/dimDat[1],c(i-0.5,j-1-0.5)/dimDat[1]))
                        } else {
                               if (j==dimDat[2]) {
		                 lines(rbind(c(i-0.5,j-0.5)/dimDat[1],c(i-0.5,j+1-0.5)/dimDat[1]))
                               }
                          }
                     }
             }
      }
  }
  }
}

Gavan McGrath