Shift central meridian - fails concave polygon & closure of split polygons, Greenland example.R

Shift central meridian - test, Greenland example.R

# ===================================================================================
# Test case - Shift central/prime meridian
# Fails concave polygons and closure of split polygons
# Show case example for Greenland
# ===================================================================================

# __________ Load needed libraries __________ #
library(data.table)
library(ggplot2)
library(maps)
library(maptools)

# __________ Load some data __________ #
load(url("https://github.com/valentinitnelav/RandomScripts/blob/master/NaturalEarth.RData?raw=true"))

# __________ Prepare data for ggplot and shift coordinates __________ #
# shift central/prime meridian towards west – positive values only
center <- 180+20

# transform map in a data table that ggplot can use
XY <- data.table(map_data(NE_countries))

# Shift coordinates (data.table way)
# inspired from the answer of @Joris Meys at:
# http://stackoverflow.com/questions/5353184/fixing-maps-library-data-for-pacific-centred-0-360-longitude-display
# and code of Claudia Engel at: https://gist.github.com/cengel/2165374#file-greatcircleflights-r
# or at: http://web.stanford.edu/~cengel/cgi-bin/anthrospace/great-circles-on-a-recentered-worldmap-in-ggplot
XY[, long.new := long + center]
XY[, long.new := ifelse(long.new > 180, long.new-360, long.new)]

# split up coordinates of polygons that differ too much (got split across the globe)
XY[, to.split := sum(diff(long.new) > 300, na.rm=TRUE) > 0, by=group]
XY[, gr.split := ifelse(to.split & long.new < 0, paste0(group, ".", 1), group)]

# __________ plot shifted map __________ #
ggplot() + 
    geom_polygon(data=XY, 
                 aes(x=long.new, y=lat, group=gr.split), 
                 colour="black", 
                 fill="gray80", 
                 size = 0.25) +
    coord_equal()

# __________ Case example Greenland __________ #
# the second Greenland polygon
XY[gr.split=="117.1"] 
ggplot() + 
    geom_polygon(data=XY[gr.split=="117.1"], 
                 aes(x=long.new, y=lat, group=gr.split), 
                 colour="black", 
                 fill="gray80", 
                 size = 0.25)

# the first Greenland polygon
XY[gr.split=="117"]
ggplot() + 
    geom_polygon(data=XY[gr.split=="117"], 
                 aes(x=long.new, y=lat, group=gr.split), 
                 colour="black", 
                 fill="gray80", 
                 size = 0.25)

# Greenland - original polygon
ggplot() + 
    geom_polygon(data=XY[group==117], 
                        aes(x=long, y=lat, group=group), 
                        colour="black", 
                        fill="gray80", 
                        size = 0.25)

# Greenland - split polygon (together)
ggplot() + 
    geom_polygon(data=XY[group==117], 
                 aes(x=long, y=lat, group=gr.split), 
                 colour="black", 
                 fill="gray80", 
                 size = 0.25)