Convert rworldMap to UTM

Question

I would like to use the worldMap from rworldMap, but need this in UTM projection rather than long/lat. I first clip the map to my area of interest. How do I then convert this to UTM projection? My code below does not seem to work:

library(rworldmap)
library(rgeos)
library(raster)


# load land
worldMap <- getMap(resolution = "high")

# polygon holding area of interest in lat/long
CP <- as(extent(-70, -55, -60, -38), "SpatialPolygons")
projection(CP) <- "+proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0"

# clip world map to area of interest
map.cp <- crop(worldMap, CP, byid = TRUE)
plot(map.cp)

# polygon holding area of interest in utm
CP.utm <- as(extent(-1000, 1500000, -6500000, -5000000), "SpatialPolygons")
projection(CP.utm) <- "+proj=utm +zone=20 +datum=WGS84 +units=m +no_defs"
projection(CP.utm)

# make projection of map.cp same as CP.utm
map.cp.utm <- map.cp
projection(map.cp.utm) <- CRS(proj4string(CP.utm))
projection(map.cp.utm)

# this shows that something is not right
plot(CP.utm)
plot(map.cp.utm, add = T)

Answer 1

This does not change the numbers in the data, it only sets the coordinate system metadata:

> projection(map.cp.utm) <- CRS(proj4string(CP.utm))

see how the extent is still in degrees:

> map.cp.utm
class       : SpatialPolygonsDataFrame 
features    : 3 
extent      : -70, -57.52767, -55.89173, -38  (xmin, xmax, ymin, ymax)
crs         : +proj=utm +zone=20 +datum=WGS84 +units=m +no_defs 

because the coordinate values for all the features aren't changed by assigning a new projection or coordinate system. You have to do a transform.

Use spTransform:

> map.cp.utm = spTransform(map.cp, proj4string(CP.utm))

which gives you an object with the correct new coordinate numbers and metadata:

> plot(map.cp.utm)
> map.cp.utm
class       : SpatialPolygonsDataFrame 
features    : 3 
extent      : -114952.6, 980634.2, -6203644, -4219970  (xmin, xmax, ymin, ymax)
crs         : +proj=utm +zone=20 +datum=WGS84 +units=m +no_defs