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

[Forrest Stevens]

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]]
>>
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