How rotate map in R

Question

I would like to rotate only the map, or change the orientation, for example 45 degrees, but not the other elements. It's posible? For example:

• Without rotating the image

• Rotating the image

My initial code without rotating:

library("maps")
library("mapproj")
library("mapdata")

xlon = seq(-1, 7, 0.01)
xlat = seq(34, 42, 0.01)

map(database = "worldHires",
    xlim = c(min(xlon), max(xlon)), ylim = c(min(xlat),max(xlat)),
    mar = c(0, 0, 0, 0))
text(2, 37, labels = "point1", pos = 4)
points(2, 37)

Answer 1

Normally when you call maps::map() it automatically plots the map during the function call, but you can pass plot=F to prevent that. At the same time, you can store the return value from the call in a variable, which will contain the x and y coordinates of the contours of the requested map. You can then use some trigonometry to rotate all the x and y coordinates about a center point, and finally plot the rotated points manually using base R plotting functions.

library('maps');
library('mapproj');
library('mapdata');

xlon = seq(-1,7,0.01);
xlat = seq(34,42,0.01);

md <- map('worldHires',xlim=range(xlon),ylim=range(xlat),mar=c(0,0,0,0),plot=F);
md2 <- md;
rot <- -30*pi/180;
about <- c(2,37);
newangles <- atan2(md$y-about[2],md$x-about[1])+rot;
mags <- sqrt((md$x-about[1])^2+(md$y-about[2])^2);
md2$x <- about[1]+cos(newangles)*mags;
md2$y <- about[2]+sin(newangles)*mags;
par(mar=c(0,0,0,0)); plot(md2,type='l',xlim=range(xlon),ylim=range(xlat),axes=F,ann=F);

text(about[1],about[2],labels='point1',pos=4);
points(about[1],about[2]);

Answer 2

It's 2021 now and this is much easier with the modern sf R package. With this you can rotate the underlying polygons before plotting to achieve the desired affect. This also uses country polygon data from Natural Earth via the rnaturalearth package.

library(sf)
library(rnaturalearth)
library(ggplot2)


# Rotate an sf geom around a center point. If no center is
# specified then it rotates around the center of the geom.
# This is technically an affine transformation: https://r-spatial.github.io/sf/articles/sf3.html#affine-transformations-1
st_ellide_rotate = function(x, degrees, center_coords=NULL){
  if(degrees < -360 | degrees > 360) stop('Degrees must be in the range -360 to 360')
  x = sf::st_combine(x)
  if(is.null(center_coords)){
    center_coords = sf::st_centroid(x)
  }
  radians = degrees * pi/180
  transform_matrix = matrix(c(cos(radians), sin(radians), -sin(radians), cos(radians)), 2, 2)
  
  return((x-center_coords) * transform_matrix + center_coords)
}


countries = rnaturalearth::ne_countries(scale = 10,returnclass = 'sf')
saved_crs = st_crs(countries)

points = st_sf(name=c('point1'), geometry = st_sfc(st_point(c(2,37)), crs = saved_crs))

countries_rotated = countries %>%
  st_ellide_rotate(-20, center_coords = c(2,37))
# applying an affine transformation nulls the CRS for some reason, so reset it here
st_crs(countries_rotated) <- saved_crs

ggplot() + 
  geom_sf(data=countries_rotated) +
  geom_sf(data=points, size=1) + 
  geom_sf_label(data=points, aes(label=name), nudge_x=1) + 
  coord_sf(xlim = c(-1,7), ylim=c(34,42)) +
  labs(subtitle = 'rotated 20 deg')

ggplot() + 
  geom_sf(data=countries) +
  geom_sf(data=points, size=1) + 
  geom_sf_label(data=points, aes(label=name), nudge_x = 1) + 
  coord_sf(xlim = c(-1,7), ylim=c(34,42)) +
  labs(subtitle = 'original')

Answer 3

If you are not tightly integrated with R, you can explore map rotation with emptymap.js here is the article explaining the usage.

Disclaimer: I am developer of the module.