Qfyilyi

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)

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")

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: