R arctic polar plots

For a project I needed to create an appealing plot of the arctic, showing the location of some field sites. I’ve posted this map on twitter earlier today. Below, I’ll outline a simple routine to recreate this plot, and if need be adjust it to your liking.

First of all I downloaded an appealing background from the Blue Marble dataset as created by NASA. Geotiffs can be downloaded here or by direct download following this link. Alternatively you can download the less realistic and more summary style graphics as produced by Natural Earth.

After downloading the Blue Marble geotiff you trim the data to the lowest latitude you want to plot. Subsequently, I reproject the data to the EPSG 3995 projection (or arctic polar stereographic). All this is done using GDAL. This step could be done using rgdal, but at times this doesn’t play nice. For now I post the command line GDAL code.

UPDATE: the below command line gdal code is not necessary anymore as I call the raster library in R now which works fine in dealing with the reprojection after some fiddling.

gdalwarp -te -180 55 180 90 world.topo.bathy.200407.3x5400x2700.tif tmp.tif
gdalwarp -wo SOURCE_EXTRA=200 -s_srs EPSG:4326 -t_srs EPSG:3995 -dstnodata "255 255 255" tmp.tif blue_marble.tif

The remaining R code ingests this background image and overlays a graticule and some labels. For this I heavily borrowed from the sp map gallery.

# load required libraries

# function to slice and dice a map and convert it to an sp() object
maps2sp = function(xlim, ylim, l.out = 100, clip = TRUE) {
  m = map(xlim = xlim, ylim = ylim, plot = FALSE, fill = TRUE)
  p = rbind(cbind(xlim[1], seq(ylim[1],ylim[2],length.out = l.out)),
            cbind(seq(xlim[1],xlim[2],length.out = l.out),ylim[2]),
            cbind(xlim[2],seq(ylim[2],ylim[1],length.out = l.out)),
            cbind(seq(xlim[2],xlim[1],length.out = l.out),ylim[1]))
  LL = CRS("+init=epsg:4326")
  IDs = sapply(strsplit(m$names, ":"), function(x) x[1])
  m = map2SpatialPolygons(m, IDs=IDs, proj4string = LL)
  bb = SpatialPolygons(list(Polygons(list(Polygon(list(p))),"bb")), proj4string = LL)
  if (!clip)
  else {
    gIntersection(m, bb)

# set colours for map grid
grid.col.light = rgb(0.5,0.5,0.5,0.8)
grid.col.dark = rgb(0.5,0.5,0.5)

# coordinate systems
polar = CRS("+init=epsg:3995")
longlat = CRS("+init=epsg:4326")

# download the blue marble data if it doesn't
# exist
if (!file.exists("blue_marble.tif")) {

# read in the raster map and
# set the extent, crop to extent and reproject to polar
r = raster::brick("blue_marble.tif")
e = raster::extent(c(-180,180,55,90))
r_crop = raster::crop(r,e)

# traps NA values and sets them to 1
r_crop[is.na(r_crop)] = 1 
r_polar = raster::projectRaster(r_crop, crs = polar, method = "bilinear")

# some values are not valid after transformation 
# (rgb range = 1 - 255) set these back to 1
# as they seem to be the black areas
r_polar[r_polar < 1 ] = 1

# define the graticule / grid lines by first specifying
# the larger bounding box in which to place them, and
# feeding this into the sp() gridlines function
# finally the grid lines are transformed to
# the EPSG 3995 projection
pts=SpatialPoints(rbind(c(-180,55),c(0,55),c(180,85),c(180,85)), CRS("+init=epsg:4326"))
gl = gridlines(pts, easts = seq(-180,180,30), norths = seq(50,85,10), ndiscr = 100)
gl.polar = spTransform(gl, polar)

# I also create a single line which I use to mark the
# edge of the image (which is rather unclean due to pixelation)
# this line sits at 55 degrees North similar to where I trimmed
# the image
pts=SpatialPoints(rbind(c(-180,55),c(0,55),c(180,80),c(180,80)), CRS("+init=epsg:4326"))
my_line = SpatialLines(list(Lines(Line(cbind(seq(-180,180,0.5),rep(55,721))), ID="outer")), CRS("+init=epsg:4326"))

# crop a map object (make the x component a bit larger not to exclude)
# some of the eastern islands (the centroid defines the bounding box)
# and will artificially cut of these islands
m = maps2sp(c(-180,200),c(55,90),clip = TRUE)

#----- below this point is the plotting routine
# set margins to let the figure "breath" and accommodate labels

# plot the grid, to initiate the area
# plotRGB() overrides margin settings in default plotting mode
plot(spTransform(gl, polar), lwd=2, lty=2,col="white")

# plot the blue marble raster data
raster::plotRGB(blue_marble, add = TRUE)

# plot grid lines / graticule
lines(spTransform(gl, polar), add = TRUE, lwd=2, lty=2,col=grid.col.light)

# plot outer margin of the greater circle
lines(spTransform(ll, polar), lwd = 3, lty = 1, col=grid.col.dark)

# plot continent outlines, for clarity
plot(spTransform(m, polar), lwd = 1, lty = 1, col = "transparent", border=grid.col.dark, add = TRUE)

# plot longitude labels
l = labels(gl.polar, crs.longlat, side = 1)
l$pos = NULL
text(l, cex = 1, adj = c( 0.5, 2 ),  col = "black")

# plot latitude labels
l = labels(gl.polar, crs.longlat, side = 2)
l$srt = 0
l$pos = NULL
text(l, cex = 1, adj = c(1.2, -1), col = "white")

# After all this you can plot your own site locations etc
# but don't forget to tranform the data from lat / long
# into the arctic polar stereographic projection using
# spTransform()

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