How to remedy a path that crosses the international dateline with R

Question

Aloha! I'm trying to plot the paths of multiple ships on a world map using R and a series of Lat/Lon points.

All is well until the ship crosses the international dateline (-180/180) when the path jumps across the map.

I tried applying the st_wrap_dateline() function in R but it looks like it only works on datasets with two points, a start & end coordinate.

My R code is below as well as a screenshot of the plot produced - any help is much appreciated!

# Download background Blue Marble Globe image
download.file("https://www.researchvessels.org/images/nasa_base_v2.png", "nasa_base_v2.png", mode="wb")
# Download ship lat/lon data
download.file("https://www.researchvessels.org/images/shipdata.RDATA", "shipdata.RDATA", mode="wb")
load("shipdata.RDATA", envir = parent.frame(), verbose=TRUE)

# Ensure that the required packages are installed
list.of.packages <- c("ggplot2", "sf", "dplyr", "png", "grid")
new.packages <- list.of.packages[!(list.of.packages %in% installed.packages()[,"Package"])]
if(length(new.packages)>0) {install.packages(new.packages)}


library(ggplot2)
library(sf)
library(dplyr)
library(png)
library(grid)

bluemarble_bg <- png::readPNG("nasa_base_v2.png")

xlim = c(-180.0,180.0)
ylim = c(-90.0,90.0)

shiptracks$Date.Time <- as.POSIXct(shiptracks$Date.Time, tz="GMT", origin="1970-01-01")

pal <- c("ShipA" = "#488f31", 
         "ShipB" = "#FF00FF", 
         "ShipC" = "#fff1a9",
         "ShipD" = "#f19d61", 
         "ShipE" = "#de425b")

shiptracks %>% group_by(Vessels.Name)

shiptracks <- st_as_sf(shiptracks, coords=c("Lon", "Lat")) %>% st_set_crs(4326)

shiptracks <- cbind(shiptracks, st_coordinates(shiptracks)) 

shiptracks <- shiptracks %>%
  st_sf() %>%
  st_wrap_dateline(options = c("WRAPDATELINE=YES", "DATELINEOFFSET=180"),
                   quiet = FALSE) %>%
  sf::st_sf(crs = 4326)

shipmap <- ggplot(shiptracks, aes(x=X, y=Y, group = Vessels.Name)) +
  coord_sf(xlim = xlim, ylim = ylim, expand = FALSE, 
           crs = 4326,
           datum = sf::st_crs(4326), label_graticule = waiver(),
           label_axes = waiver(), ndiscr = 100, default = FALSE,
           clip = "on") +
  annotation_custom(rasterGrob(bluemarble_bg, 
                               width = unit(1,"npc"), 
                               height = unit(1,"npc")), 
                    -Inf, Inf, -Inf, Inf) +
  geom_path(data = shiptracks, group = shiptracks$Vessels.Name, color=pal[shiptracks$Vessels.Name],
            aes(x=X, y=Y), cex=1, show.legend = TRUE) +
  geom_point(data = shiptracks, group = shiptracks$Vessels.Name, color=pal[shiptracks$Vessels.Name], aes(x=X, y=Y), cex=1, show.legend = TRUE) +
    theme(axis.line = element_blank(),
        axis.text.x = element_blank(),
        axis.text.y = element_blank(),
        axis.ticks = element_blank(),
        axis.title.x = element_blank(),
        axis.title.y = element_blank(),
        panel.background = element_blank(),
        panel.border = element_blank(),
        panel.grid.major = element_blank(),
        panel.grid.minor = element_blank(),
        plot.background = element_blank(),
        plot.margin = unit(c(0, 0, 0, 0), "mm"),
        panel.spacing = unit(0, "mm"),
        legend.position = c(-120, -20),
        legend.title = element_text(size = 7), 
        legend.text = element_text(size = 5), 
        legend.key.size = unit(0.01, "npc"))

shipmap

Answer 1

At the point at which you run st_wrap_dateline you have a data frame of POINT geometries. Points can't cross the dateline, and so nothing changes.

You need to create a data frame of LINESTRING or MULTILINESTRING geometries by grouping and construction, so you end up with one row per ship track, and not one row per ship location with a grouping variable.

Then st_wrap_dateline will split those at the date line into MULTILINESTRING objects.

Then you can use geom_sf in your ggplot call to draw the lines, which will have been split into parts at the dateline.

Step 1, create lines from tracks:

tracks = shiptracks %>% group_by(Vessels.Name) %>% 
    summarise(do_union=FALSE) %>% st_cast("LINESTRING")

ggplot() + geom_sf(data=tracks,aes(col=Vessels.Name))

Now wrap it and plot:

tracks = st_wrap_dateline(tracks)
ggplot() + geom_sf(data=tracks,aes(col=Vessels.Name)) 

(Tip: please try and keep your code to a minimum - my first efforts at this failed because you must be using an updated version of something over what I've got, and one of those numerous options in your plots (waiver() I think) wasn't recognised. Simplifying everything sometimes also reveals where the problem is, or at least makes it quicker to find it. It also encourages people to answer. When I first saw this and its 60 lines of code I was disinclined to look further.)