[raster] a railroad, a raster with a different value on each side of it

If your railroad is just a single line feature running approx N-S then:

 Create a box polygon for your study area, make sure the railroad just
crosses it at N and S edges

 Use rgeos functions overlaying the RR line with the box polygon to
create the E and W polygons

 Job done?

On Fri, Sep 16, 2011 at 1:11 PM, Mathieu Rajerison

Hi List,


I've got a railroad shapefile and a corine land cover raster.

I'd like to perform a land cover comparative analysis between the west side
of the railroad and the right side.

To do that, I tried first to have a raster with values=1 when on the east
and 0 when on the west.
But my calculation is very slow.

Maybe anyone has a better idea on how to accomplish that?

Here is the code, for the moment:

#* *I create a raster of the same extent and resolution as the corine land
cover raster

track.r <- raster(extent(clc.ov))
res(track.r) <- res(clc.ov)

# I rasterize it

track.r <- rasterize(track, track.r)

# I extract the row-col pairs of cells with value 1

rowCol <- rowColFromCell(track.r, which(track.r[]==1))

# I give the value 1 to each cell which col number is > to that of the
rasterized line cell and of same row

for (i in seq(1,length(rowCol))) {

?track.r[rowCol[i,1], rowCol[i,2]:ncols] <- 1
}
# too slow.....


Any help would be greatly appreciated,

Mathieu

? ? ? ?[[alternative HTML version deleted]]

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On Fri, Sep 16, 2011 at 4:51 PM, Barry Rowlingson

If your railroad is just a single line feature running approx N-S then:

?Create a box polygon for your study area, make sure the railroad just
crosses it at N and S edges

?Use rgeos functions overlaying the RR line with the box polygon to
create the E and W polygons

Actually you dont even need rgeos...

 If your RR goes from the S edge of a rectangular study region to the
N edge, and is a single line, then your W and E polygons are then:

bottom left corner, intersection point of RR with S edge, RR points,
intersection point of RR with N edge, top left corner

bottom right corner, intersection point of RR with S edge, RR points,
intersection point of RR with N edge, top right corner

 - in other words you just trace along the bounding rectangle and the
RR. Get the RR coords from the SpatialLines* object and you're pretty
much set to make polygons out of it. Once you have the polygons you
can overlay them on rasters. Should be pretty quick.

Barry

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