[R-sig-Geo] plotting latlon axes of sp object of different projection

[Roger Bivand]

On Mon, 4 Jul 2011, Horacio Samaniego wrote:

> I need to add a lat/lon axes to an equal area plot of a
> SpatialLinesDataFrame object currently in units of meters. Is that
> achievable using sp/maptools?
>
> The .prj file where the object comes from is:
> PROJCS["Lambert_Azimuthal_Equal_Area",GEOGCS["GCS_unnamed
> ellipse",DATUM["D_unknown",SPHEROID["Unknown",6370997,0]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Lambert_Azimuthal_Equal_Area"],PARAMETER["latitude_of_origin",45],PARAMETER["central_meridian",-100],PARAMETER["false_easting",0],PARAMETER["false_northing",0],UNIT["Meter",1]]
>
> I'ld hate to install arcinfo to generate 1 plot! (plus the plot is
> already nice as it is!!! just need the axes!)

No, the axes are in the same coordinate system as the plot, and would be 
curves here. You can overplot a long-lat grid on a projected plot:

library(sp)
data(meuse)
coordinates(meuse) <- ~x+y
proj4string(meuse) <- CRS("+init=epsg:28992")
plot(meuse)
library(rgdal)
meuse_ll <- spTransform(meuse, CRS("+proj=longlat +datum=WGS84"))
grd <- gridlines(meuse_ll)
grd_x <- spTransform(grd, CRS("+init=epsg:28992"))
plot(grd_x, add=TRUE, lty=2)
grdat_ll <- gridat(meuse_ll)
grdat_x <- spTransform(grdat_ll, CRS("+init=epsg:28992"))
text(coordinates(grdat_x), labels=parse(text=grdat_x$labels),
   pos=grdat_x$pos, offset=grdat_x$offset)

Hope this helps,

Roger

>
> thanks,
> I'll summarize!
>
> H
>
>

-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no