[R-sig-Geo] How to overlay world map using non-standard projection

[ElenaC]

Dear all,

I have a SpatialGrid (covering the entire world) in geographical coordinates
that I'm trying to transform to equal-area cylindrical projection (such as
given by CRS("+proj=cea +lat ts=30")). I also want to center the plot in the
Pacific and overlay a map of the coasts on top. 

I have successfully transformed my SpatialGrid using the functions
"projectExtent()" and "projectRaster()" from the raster package, and I've
also centered the data where I want. 

Problem is that I've been unable to overlay a map of the world. I've tried
to use the map() function, and I've generated the map I want (right
projection and centered correctly) with the call:

library(maps)
world <-map(projection = "cylequalarea", parameter=30, interior=FALSE,
wrap=TRUE, ylim=c(-60,60),xlim=c(-180,180), orientation=c(90,-205,0))

But when I try to overlay this map on the plot of my transformed SpatialGrid
I find that the scale of both maps is completely different. Their
coordinates don't match:

> bbox(mydata)
         min      max
s1 -17352991 17352991
s2  -6291291  6356742

> range(world$x, na.rm=TRUE)
[1] -2.355834  2.355591
> range(world$y, na.rm=TRUE)
[1] -0.8613535  0.8899210

 Moreover, it looks like it is a different scale factor affecting the 'x'
and 'y' axis. Since the projection I'm trying to use dictates the ratio
between the x and y axis, I don't understand how this can be.

Any suggestion on how to proceed would be greatly appreciated..

Thanks,
Elena
 
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