color grid cells in the United States and Canada

Question

I would like to color grid cells in the United States and Canada. My goal is very similar to this question: R Plot Filled Longitude-Latitude Grid Cells on Map However, that question only deals with the United States and I cannot figure out how to add Canada.

I was able to draw a map of the U.S. and Canada by modifying code found here: https://groups.google.com/forum/#!topic/ggplot2/KAKhoE0GO4U

library(ggplot2)
library(rgeos)
library(maps)
library(maptools)

PolygonCoords <- function(polygon) {
  polygons <- polygon@Polygons
  coords.list <- lapply(seq_along(polygons), function(i) {
    # Extract the group, sequence, area, longitude, and latitude.
    coords <- polygons[[i]]@coords
    cbind(i, 1:nrow(coords), polygons[[i]]@area, coords)
  })
  coords.df <- as.data.frame(do.call(rbind, coords.list))
  names(coords.df) <- c("order", "seq", "area", "long", "lat")
  return(coords.df)
}

ConvertWorldSimple <- function(mapdata, min.area = 0) {

  coords.list <- lapply(mapdata@polygons, PolygonCoords)
  ncoords <- sapply(coords.list, nrow)
  coords.df <- do.call(rbind, coords.list)
  coords.df$country <- rep(mapdata@data$NAME, ncoords)
  country.group <- factor(paste(coords.df$country, coords.df$order))
  coords.df$group <- as.numeric(country.group)
  coords.df <- coords.df[coords.df$area >= min.area, ]
  return(coords.df)
}

data("wrld_simpl")
world <- ConvertWorldSimple(wrld_simpl, min.area = 0.1)

world <- world[world$country %in% c('United States', 'Canada'),]

na <- data.frame(
  country = c("United States", "Canada"),
  is.north.america = TRUE)

world <- merge(world, na, all.x = TRUE)
world$is.north.america[is.na(world$is.north.america)] <- FALSE

world <- world[order(world$order, world$seq), ]

ggplot(world, aes(long, lat, group = group)) +
  geom_polygon(aes(fill = is.north.america)) +
  geom_path(color = "white", size = 0.1) +
  scale_fill_manual(values = c("darkgray"), guide = "none") +
  scale_y_continuous("", breaks=(-2:2) * 30) +
  scale_x_continuous("", breaks=(-4:4) * 45) +
  coord_equal() +
  theme_bw()

Here is code to create fake attribute data for the grid cells, found here: http://www.numbertheory.nl/2011/11/08/drawing-polar-centered-spatial-maps-using-ggplot2/

set.seed(1234)

xlim = c(-110,-100)
ylim = c(40,60)

dat_grid = expand.grid(x = xlim[1]:xlim[2], y = ylim[1]:ylim[2])
dat_grid$z = runif(nrow(dat_grid))

head(dat_grid)

Here is the ggplot2 code used in the earlier Stack Overflow post to overlay a grid of attributes on a map of the Lower 48:

library(ggplot2)
library(maps)
us_states <- map_data("state")
(ggplot(aes(x=x,y=y,fill=z),data=dat_grid) + geom_tile())+geom_polygon(data=us_states,aes(x=long, y=lat, group=group), colour="black", fill="white", alpha=0)

How can I combine the two ggplot statements to overlay the grid of fake attribute data onto the map of the U.S. and Canada? Thank you for any advice.

Answer 1

This should do the job

library(ggplot2)
library(maps)

us = map_data("state")
# or this if you don't want the states' boundary
# us = map_data("states", boundary=FALSE)
ca = map_data("world", "Canada")

set.seed(1234)
xlim = c(-110,-100)
ylim = c(40,60)
dat_grid = expand.grid(x = xlim[1]:xlim[2], y = ylim[1]:ylim[2])
dat_grid$z = runif(nrow(dat_grid))

p = ggplot(aes(x=x,y=y,fill=z),data=dat_grid) 
p + geom_tile() + geom_polygon(data=us,aes(x=long, y=lat, group=group), colour="black", fill="white", alpha=0) + 
  geom_polygon(data=ca,aes(x=long, y=lat, group=group), colour="black", fill="white", alpha=0)

If you need Alaska:

library(ggplot2)
library(maps)

m = map_data("world2", c("usa", "Canada"))

set.seed(1234)
xlim = c(250,300)
ylim = c(40,60)
dat_grid = expand.grid(x = xlim[1]:xlim[2], y = ylim[1]:ylim[2])
dat_grid$z = runif(nrow(dat_grid))

p = ggplot(dat_grid,aes(x=x,y=y)) + geom_tile(aes(fill=z))
p  + geom_polygon(data=m,aes(x=long, y=lat, group=group), colour="black", fill="white", alpha=0)