[R-sig-Geo] Map of pixel area?

[Michael Sumner]

See ?area in raster for the standard approximation method.

Cheers, Mike

On Sat, 10 Jan 2015 10:15 Forrest Stevens <forrest at ufl.edu> wrote:

> Oops, sorry for the mistake when copying my code in.  Please find the
> working code for the areas here:
>
> r1 <- raster()
> r1[] <- 1:ncell(r1)
> p1 <- rasterToPolygons(r1)
>
> ## Project to cylindrical equal area:
> require(rgdal)
> p2 <- spTransform(p1, CRS("+proj=cea +lat_0=0 +lon_0=0 +lat_ts=30
> +a=6371228.0 +units=m"))
>
> ## Extract areas and assign back to raster:
> areas <- sapply(slot(p2, "polygons"), slot, "area")
> r2 <- r1
> r2[] <- areas
> plot(r2)
>
>
> Sorry for the confusion!
>
> Sincerely,
> Forrest
>
> --
> Forrest R. Stevens
> Ph.D. Candidate, QSE3 IGERT Fellow
> Department of Geography
> Land Use and Environmental Change Institute
> University of Florida
> www.clas.ufl.edu/users/forrest
>
>
> On Fri, Jan 9, 2015 at 6:11 PM, Forrest Stevens <forrest at ufl.edu> wrote:
> > The easiest way I can think of would be to create an "index" raster
> > based on your raster of interest, convert it to polygons, project it
> > to an equal area projection with linear units amenable to you, then
> > extract the polygon areas and assign them back to your index raster
> > for weighting.  Perhaps something like this:
> >
> >
> > r1 <- raster()
> > r1[] <- 1:ncell(r1)
> > p1 <- rasterToPolygons(r1)
> >
> > ## Project to cylindrical equal area:
> > require(rgdal)
> > p2 <- spTransform(p1, CRS("+proj=cea +lat_0=0 +lon_0=0 +lat_ts=30
> > +a=6371228.0 +units=m"))
> >
> > ## Extract areas and assign back to raster:
> > areas <- sapply(slot(p2, "polygons"), slot, "area")
> > r2 <- r1
> > plot(r2)
> >
> >
> > Though, maybe someone else has a better way?
> >
> > Sincerely,
> > Forrest
> > --
> > Forrest R. Stevens
> > Ph.D. Candidate, QSE3 IGERT Fellow
> > Department of Geography
> > Land Use and Environmental Change Institute
> > University of Florida
> > www.clas.ufl.edu/users/forrest
> >
> >
> > On Fri, Jan 9, 2015 at 5:29 PM, Jonathan Greenberg <jgrn at illinois.edu>
> wrote:
> >> R-sig-geo'ers:
> >>
> >> I have an analysis where I need to take a geographic projection map,
> e.g.:
> >>
> >> r1 <- raster()
> >>
> >> And calculate, for each pixel, the square kilometers that each
> individual
> >> pixel covers (since they vary across a geographic projection) -- the
> idea
> >> is to end up with a map of pixel area.  Can anyone think of a clever
> way to
> >> do this?  I'll be using this to do some area weighted summaries, but I
> >> can't simply reproject the input map.  Thanks!
> >>
> >> --j
> >>
> >>         [[alternative HTML version deleted]]
> >>
> >> _______________________________________________
> >> R-sig-Geo mailing list
> >> R-sig-Geo at r-project.org
> >> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at r-project.org
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>

	[[alternative HTML version deleted]]