Format latitude and longitude axis labels in ggplot

Question

I have a ggplot map, for example:

library(ggmap)
ggmap(get_map())

I'd like the axis labels to be automatically labeled as N-S / W-E: in the above case, for example, instead of lon -95.4 it should show 95.4°E.

I have tried to mess with the scales package and using scale_x_continuous and scale_y_continuous labels and breaks options, but I have not managed to make it work.

It would be awesome to have a scale_y_latitude and scale_x_longitude.

EDIT: Thanks to @Jaap 's answer I got to the following:

scale_x_longitude <- function(xmin=-180, xmax=180, step=1, ...) {
    ewbrks <- seq(xmin,xmax,step)
    ewlbls <- unlist(lapply(ewbrks, function(x) ifelse(x < 0, paste(x, "W"), ifelse(x > 0, paste(x, "E"),x))))
    return(scale_x_continuous("Longitude", breaks = ewbrks, labels = ewlbls, expand = c(0, 0), ...))
}
scale_y_latitude <- function(ymin=-90, ymax=90, step=0.5, ...) {
    nsbrks <- seq(ymin,ymax,step)
    nslbls <- unlist(lapply(nsbrks, function(x) ifelse(x < 0, paste(x, "S"), ifelse(x > 0, paste(x, "N"),x))))
    return(scale_y_continuous("Latitude", breaks = nsbrks, labels = nslbls, expand = c(0, 0), ...))
}

Which works pretty well. But for some reason my R doesn't seem to like the degree symbol in front of the cardinal point... It is displayed as a simple dot, e.g. longitude -24 becomes 24..W

Flipflop

Answer 1

Unfortunately, there is no such thing as scale_x_longitude or scale_y_latitude yet. In the meantime here is a workaround in which you specify the labels beforehand:

# load the needed libraries
library(ggplot2)
library(ggmap)

# get the map
m <- get_map(location=c(lon=0,lat=0),zoom=5)

# create the breaks- and label vectors
ewbrks <- seq(-10,10,5)
nsbrks <- seq(-10,10,5)
ewlbls <- unlist(lapply(ewbrks, function(x) ifelse(x < 0, paste(x, "°E"), ifelse(x > 0, paste(x, "°W"),x))))
nslbls <- unlist(lapply(nsbrks, function(x) ifelse(x < 0, paste(x, "°S"), ifelse(x > 0, paste(x, "°N"),x))))

# create the map
ggmap(m) +
  geom_blank() +
  scale_x_continuous(breaks = ewbrks, labels = ewlbls, expand = c(0, 0)) +
  scale_y_continuous(breaks = nsbrks, labels = nslbls, expand = c(0, 0)) +
  theme(axis.text = element_text(size=12))

which gives:

---

To get the degrees in the functions, you can raise the o as superscript (which will circumvent the need for a special symbol):

scale_x_longitude <- function(xmin=-180, xmax=180, step=1, ...) {
  xbreaks <- seq(xmin,xmax,step)
  xlabels <- unlist(lapply(xbreaks, function(x) ifelse(x < 0, parse(text=paste0(x,"^o", "*W")), ifelse(x > 0, parse(text=paste0(x,"^o", "*E")),x))))
  return(scale_x_continuous("Longitude", breaks = xbreaks, labels = xlabels, expand = c(0, 0), ...))
}
scale_y_latitude <- function(ymin=-90, ymax=90, step=0.5, ...) {
  ybreaks <- seq(ymin,ymax,step)
  ylabels <- unlist(lapply(ybreaks, function(x) ifelse(x < 0, parse(text=paste0(x,"^o", "*S")), ifelse(x > 0, parse(text=paste0(x,"^o", "*N")),x))))
  return(scale_y_continuous("Latitude", breaks = ybreaks, labels = ylabels, expand = c(0, 0), ...))
}    

ggmap(m) +
  geom_blank() +
  scale_x_longitude(xmin=-10, xmax=10, step=5) +
  scale_y_latitude(ymin=-10, ymax=10, step=5) +
  theme(axis.text = element_text(size=12))

which gives the following map:

I used geom_blank just to illustrate the desired effect. You can off course use other geom's (e.g. geom_point) to plot your data on the map.

Acridine

The answer's very good, thanks, but I'm not marking it as accepted as I'm looking for an automated approach, such as when doing library(scales); p + scale_y_continuous(labels = percent) (see docs.ggplot2.org/current/scale_continuous.html)... by the way my R doesn't seem to like the degree symbol in "°E"... it is not rendered correctly for some reason. Ideas? – Flipflop

@Flipflop I think I found a solution for the degrees symbol. See the update. – Acridine

Would it be possible to have the axis on the top and right as well? Adding sec.axis = dup_axis() to the return of the function only adds the Latitude/Longitude labels, not the ticks and numbers: imgur.com/a/BcQew – Gladisgladney

BTW, parsing x as -x for the West and South coordinates would be better. – Gladisgladney

Regarding Alfredo's comment, one could potentially also use a modulus to remove negatives (SQRT(x^2) unless someone knows a direct function); I'd have to play with this to check it works properly. – Shimkus

using this answer, is there a way to remove the negative sign? (for W & S) – Jeer

@Jeer wrapping the labels vectors in abs() should work – Acridine

Answer 2

You can now use metR package. It has scale_*_latitude() and scale_*_longitude() functions.

# load the needed libraries
library(ggplot2)
library(ggmap)
library(metR) #new package

# get the map
m <- get_map(location=c(lon=0,lat=0),zoom=5)

# create the map
ggmap(m) +
  scale_x_longitude(breaks = seq(-10,10,5)) +
  scale_y_latitude(breaks = seq(-10,10,5))
#> Scale for x is already present.
#> Adding another scale for x, which will replace the existing scale.
#> Scale for y is already present.
#> Adding another scale for y, which will replace the existing scale.

^{Created on 2022-11-28 with reprex v2.0.2}

Mixedup