World map overlay on level plot

Question

I am trying to place a world map on my current plots made by level plot. This plot was made as follows:

library(raster)
library(ncdf4)
library(maps)
library(maptools)
library(rasterVis)
library(ggplot2)
library(rgdal)
library(sp)
library(gridExtra)

MFplot4<-levelplot(MFMeaner3,margin=F, at=Fcutpoints4,cuts=11, 
pretty=TRUE,par.settings=mapTheme, main="Historical five-day maximum   
precipitation (mm/day) model mean")

The object "MFMeaner3" has the following attributes:

class       : RasterLayer 
dimensions  : 64, 128, 8192  (nrow, ncol, ncell)
resolution  : 2.8125, 2.789327  (x, y)
extent      : -181.4062, 178.5938, -89.25846, 89.25846  (xmin, xmax, ymin, 
ymax)
coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0 
data source : in memory
names       : layer 
values      : 0.1583802, 164.2064  (min, max)

Here was my attempt to place a world map overlay on the above plot:

world.outlines<-map("world", plot=FALSE) 
world.outlines.sp<-map2SpatialLines(world.outlines,proj4string =   
CRS("+proj=longlat"))
MFplot4 + layer(sp.lines(world.outlines.sp,col="black",lwd=0.5))

However, this leads to the following error:

Error: Attempted to create layer with no stat.

I also tried placing a simple world map using this:

MFplot4 + plot(wrld_simpl)

But I receive this error:

Error in UseMethod("as.layer") : 
no applicable method for 'as.layer' applied to an object of class "NULL"

Why would these errors occur?

Any assistance with this would be extremely appreciated!

Answer 1

The issue is that, by loading ggplot2, you have masked the latticeExtra::layer() function (attached by rasterVis) with ggplot2::layer(). If you must load ggplot2, then you'll want to fully qualify your call to the masked function, writing latticeExtra::layer() in place of layer().

Here is a reproducible example that works for me when ggplot is not loaded, but fails when it is:

library(rasterVis)
library(sp)
library(maps)
library(maptools)
## library(ggplot2)  ## `levelplot() + layer()` fails when this is loaded

## Read in a RasterLayer
tmax <- getData('worldclim', var='tmax', res=10)[[6]]

## Create a SpatialLines object
countries <- map("world", plot=FALSE) 
countries <- map2SpatialLines(countries, proj4string = CRS("+proj=longlat"))

## Overlay lines on levelplot
levelplot(tmax, margin=FALSE) + 
    layer(sp.lines(countries))