Plotting routes that cross the international datel

问题:

I am interested in plotting the route from Waterloo to Taiwan using the leaflet library for R. The coordinates for the route was obtained using gcIntermediate function from geosphere. However, the route involves the crossing of the international dateline and hence the route gets cut-off at the edges of the map and is joined at the top with a straight line.

Plot that I get: wrong map

Plot that I want: ideal map

There is a possibility that this problem could be solved using Google maps but is there a way to solve this using leaflet? How should I modify the code below to get the ideal route? Thank you!

Code:

library(dplyr)
library(leaflet)
library(geosphere)

# Source
latWaterloo <- 43.46687 
lngWaterloo <- -80.52464

# Destination
latTaiwan <- 23.5983
lngTaiwan <- 120.8354


m <- leaflet() %>% addTiles()
m <- addCircleMarkers(m, lng=c(lngWaterloo, lngTaiwan) , lat=c(latWaterloo, latTaiwan), popup=c("Waterloo","Taiwan"), radius=5, opacity=0.5)
geo_lines <- gcIntermediate(c(lngWaterloo, latWaterloo), c(lngTaiwan, latTaiwan), n=100, addStartEnd=T, sp=T, breakAtDateLine=F)
m <- addPolylines(m, data=geo_lines, color="blue")
m

m <- leaflet() %>% addTiles()
m

回答1:

As Jean-Claude said in his comment, your hand drawn path is longer than the one given by gcIntermediate. To get the correct path, just call gcIntermediate with breakAtDateLine=TRUE:

geo_lines <- gcIntermediate(c(lngWaterloo, latWaterloo), c(lngTaiwan, latTaiwan), n=100, addStartEnd=TRUE, sp=TRUE, breakAtDateLine=TRUE)
m <- addPolylines(m, data=geo_lines, color="blue")
m

which produces

I don't know if there's a way to tell leaflet to duplicate the sections of the path as it crosses the date line so that it goes right to the edge of the plot.

回答2:

The trick is to add 360 degrees to the negative longitudes returned by gcIntermediate. This produces a continuous plot as shown below.

Code:

library(dplyr)
library(leaflet)
library(geosphere)

# Source
latWaterloo <- 43.46687 
lngWaterloo <- -80.52464

# Destination
latTaiwan <- 23.5983
lngTaiwan <- 120.8354


m <- leaflet() %>% addTiles()
geo_lines <- gcIntermediate(c(lngWaterloo, latWaterloo), c(lngTaiwan, latTaiwan), n=100, addStartEnd=T, sp=F, breakAtDateLine=F)
geo_lines <- data.frame(geo_lines)

# Add 360 degrees to lngWaterloo and all the negative longitudes returned by gcIntermediate
lngWaterloo <- lngWaterloo + 360
lonn <- vector(length=nrow(geo_lines))
for (j in 1:nrow(geo_lines)) {
  if (geo_lines[j,]$lon < 0) {
    lonn[j] <- geo_lines[j,]$lon + 360
  } else {
    lonn[j] <- geo_lines[j,]$lon
  }
}

m <- addCircleMarkers(m, lng=c(lngWaterloo, lngTaiwan) , lat=c(latWaterloo, latTaiwan), popup=c("Waterloo","Taiwan"), radius=2, opacity=0.5)
m <- addPolylines(m, lng=lonn, lat=geo_lines$lat, color="blue", weight=2)
m

There may be a way to simplify the code, but I'm not too sure how.