R - how to get a smooth curve between two points?

Question

I'm trying to draw a line between two cities, on a world map, using R (using map and wrld_simpl).

How can I get a smooth line between the two points? If I drew a line between US and Australia, it has three different segments

require(rworldmap)

map("world", col="#f2f2f2", fill=TRUE, bg="white", lwd=0.05)

data(wrld_simpl)

US_lat = wrld_simpl$LAT[wrld_simpl$NAME == 'United States']
US_lon = wrld_simpl$LON[wrld_simpl$NAME == 'United States']

australia_lat = wrld_simpl$LAT[wrld_simpl$NAME == 'Australia']
australia_lon = wrld_simpl$LON[wrld_simpl$NAME == 'Australia']

lines(c(US_lon, US_lat), c(australia_lon, australia_lat))

This draws a line, but it isn't correctly between aus and US. what am I doing wrong?

Answer 1

The method lines takes a vector x of x-coordinates and y of y-coordinates. Therefore your line should look like this:

lines(c(US_lon, australia_lon), c(US_lat, australia_lat))

If you want a great circle, the following will work:

require(geosphere)
gc <- gcIntermediate(c(US_lon, US_lat), c(australia_lon, australia_lat), breakAt=TRUE, n=1000)
invisible(lapply(gc, lines, col='red', lwd=2))

Answer 2

You can also draw a "curve" using grid.curve():

grid.curve(US_lon, australia_lon, US_lat, australia_lat)

... and you can define the "look" of the curve.