[R-sig-Geo] Intersection of polygons with raster

[Michael Sumner]

Hi Kent, this is pretty straightforward with raster::extract. If speed is
an issue you can burn the polygon id into the grid and then group values by
that.

library(raster)
r <- raster(volcano)

## very simplistic polygon layer example
r2 <- raster(r)
res(r2) <- res(r2) * 20
p <- sf::st_as_sf(as(r2, "SpatialPolygonsDataFrame"))

## a dummy threshold value
onefoot <- 150
## grab the first column from extract (it gives multiple columns for
multi-layer rasters)
p$onefoot <- extract(r, p, fun = function(x, na.rm = TRUE) any(x >
onefoot))[,1]

plot(sf::st_geometry(p), col = p$onefoot + 1)
contour(r, levels = onefoot, col = "white", add = T)


## if speed is an issue, use fasterize for a more abstract workflow
library(fasterize)
p$rownum <- 1:nrow(p)
idgrid <- fasterize(p, r, field = "rownum")
p$onefoot <- tapply(values(r), values(idgrid), function(x, na.rm = TRUE)
any(x > onefoot))
plot(p)

FWIW it's always useful to provide a reprex, since that does take time and
might miss the mark for your situation. It's also trickier to provide two
objects that are relevant to each other, so any upfront work you can do in
an example helps the answerer.

HTH

On Sat, 19 May 2018 at 12:20 Kent Johnson <kent3737 at gmail.com> wrote:

> Hi,
>
> I have a raster object `flood` containing projected flooding levels and a
> simple features object `parcels` containing MULTIPOLYGONs representing
> property parcels. I would like to find all the parcel polygons for which
> there is any flood > 1 foot. What function(s) can do this? Something like
> raster::intersect(parcels, flood >= 1)
>
> I'll put together a reprex if that helps. Hoping someone can point me to
> the right function to do this, I haven't found anything promising.
>
> Thanks,
> Kent
>
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-- 
Dr. Michael Sumner
Software and Database Engineer
Australian Antarctic Division
203 Channel Highway
Kingston Tasmania 7050 Australia

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