[R-sig-Geo] How to draw grid contours within a map

[Robert Hijmans]

>  In order to illustrate the spatial domain of the General Circulation Model
(GCM) output data I am using, I 
> need to display some grids within a map. They need to be 2.5°x2.5° lat-lon
> over specific coordinates. 

>  Following, I create the "empty" grids to draw: 

>library(raster) 
>rprecip<-raster(xmn=-60,xmx=-50,ymn=-35,ymx=-25) 
>res(rprecip)<-2.5 

Thiago, 

You can do this

rprecip[] <- 1
# (set cells you do not want to NA)
p <- rasterToPolygons(res)
plot(p)

Robert
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