R ggmap center around 180 degrees longitude

Question

I am mapping points falling around 180 degrees longitude using R ang ggmap (see example below). The map is mostly empty. Is there a way to center the map around longitude = 180 and limit the extent to where the points are? In other words, I'm interested in limiting the range on the x-axis to 180±14 degrees.

library("ggmap")
x <- structure(list(Bias = structure(c(5L, 4L, 3L, 3L, 4L, 6L, 6L, 
                                       3L, 3L, 4L, 3L, 5L, 2L, 3L,
                                       3L, 4L, 4L, 4L, 4L, 4L, 4L,
                                       6L, 5L, 4L, 3L, 6L, 5L, 4L,
                                       5L, 4L, 4L, 3L, 6L, 4L, 6L,
                                       4L, 5L, 4L, 4L, 4L, 3L, 3L,
                                       4L, 2L, 4L, 3L, 3L, 6L, 4L,
                                       5L),
                                     .Label = c("(-Inf,-5]",
                                                "(-5,-2]",
                                                "(-2,0]",
                                                "(0,2]",
                                                "(2,5]",
                                                "(5, Inf]"),
                                     class = "factor"), 
                    lat = c(-1.35, -13.24, -14.31, -16.13, -17.15,
                            -17.35, -17.75, -18.05, -18.23, -20.67,
                            -29.24, -34.43, -35.13, -35.9, -37.01,
                            -37.56, -37.67, -38.66, -38.74, -39.01,
                            -39.45, -39.47, -40.32, -40.54, -40.9,
                            -40.9, -41.3, -41.33, -41.73, -41.74,
                            -42.42, -42.71, -43.49, -44.3, -45.02,
                            -45.22, -45.93, -46.16, -46.41, -47.28,
                            -50.49, -52.55, -43.86, -18.15, -12.5,
                            -19.05, -52.55, -44.53, -38.17, -17.23),
                    lon = c(176, -176.19, -178.12, -179.98, 176.9,
                            178.22, 177.45, 178.57, -178.8, -178.72,
                            -177.93, 172.68, 174.02, 175.12, 174.81,
                            178.31, 176.2, 177.99, 176.08, 174.18,
                            175.66, 176.86, 175.61, 173, 174.99,
                            176.21, 173.22, 174.81, 174.28, 171.58,
                            173.7, 170.98, 172.53, 171.22, 168.74,
                            166.88, 170.2, 166.61, 168.32, 167.46,
                            166.3, 169.15, 169.01, 177.42, 177.05,
                            178.17, 169.13, 169.89, 174.7, -178.95)),
               .Names = c("Bias", "lat", "lon"),
               class = "data.frame",
               row.names = c(NA, -50L))
# Map extent
xy <- c(left = min(x$lon), bottom = min(x$lat),
        right = max(x$lon), top = max(x$lat))
# Download the base map
gg <- get_stamenmap(xy, zoom = 5, maptype = "toner-lite")
ggmap(gg) +
  geom_point(data = x, aes(x = lon, y = lat, col = Bias),
             size = 1, alpha = 0.9) +
  scale_color_viridis(discrete = T)

Answer 1

It's more an issue with stamen and data structures than with anything else. Not a problem with Google Maps and a bit of data wrangling.

Code:

# Using your data as basis
x[x$lon < 0, ]$lon <- 2*180 + x[x$lon < 0, ]$lon

# Get map with lon center opposite the Meridian
g <- ggmap(get_googlemap(c(180, 0), zoom = 1), extent = "panel")

# Plot map with boundaries
g + scale_x_continuous(limits = c(0, 360), expand = c(0,0)) +
    scale_y_continuous(limits = c(-70, 70), expand = c(0,0)) + 
    # Plot data points
    geom_point(data = x, aes(x = lon, y = lat, color = Bias),
               size = 1, alpha = 0.9) +
    # Add custom color scheme
    scale_color_viridis(discrete = T)

Explanation:

If you use zoom = 1 with the current Google Maps API it gives you a bit of longitude overlap left and right which you can cut to your liking.

> g$data
        lon       lat
1 -44.64844 -87.75631
2 405.35156 -87.75631
3 -44.64844  87.72862
4 405.35156  87.72862

If you check the boundaries of the raw map above you see that we have total 450 degrees of longitude (44.64844 + 405.35156) that we can use. Therefore, we can cut down our map to lon = c(0, 360) and lat = c(-70, 70) (using scale_x_continuous and scale_y_continuous, respectively) which will leave the center point at exactly lon = 180 and lat = 0.

Your data, however, uses the right longitude values of 0:-180 (West) and 0:180 (East). Therefore, we also need to update the longitude values that are below zero to the corresponding value in a 0:360 system. This is achieved by 180 + 180 + lon (so longitude = -172 will turn into longitude = 188 for the purpose of plotting).

Visualization:

Answer 2

Could not figure out how to do this using ggmap. However, here is the solution using the ggplot and maps packages (modified version of the code here: World map with ggmap).

library(maps)
library(ggplot2)

# x is the data.frame defined in the question

# Recenter points
center <- 180
# shift coordinates to recenter x
x$long.recenter <- ifelse(x$lon < center - 180 , x$lon + 360, x$lon)

# shift coordinates to recenter worldmap
worldmap <- map_data ("world", wrap = c(0, 360))

# Plot worldmap using data from worldmap.cp
ggplot(aes(x = long, y = lat), data = worldmap) + 
  geom_polygon(aes(group = group), fill="#f9f9f9", colour = "grey65") + 
  scale_y_continuous(limits = c(-60, 0)) +
  scale_x_continuous(limits = c(180 - 20, 180 + 15),
                     breaks = seq(160, 190, 10),
                     labels = c(160, 170, "180/-180", -170)) +
  coord_equal() +  theme_bw() +
  geom_point(data = x,
             aes(x = long.recenter, y = lat, col = Bias),
             pch = 19, size = 3, alpha = .4) +
  xlab("Longitude") + ylab("Latitude")