Question

I have some spatial data I am mapping in R using the maps library. Specifically, I am mapping circles on a US map. I have lat/long center coordinates and a radius for each circle, and am mapping them like so:

library(maps)
library(plotrix)
map('state')
draw.circle(x=-73.76275, y=41.04181, radius=2)

This particular circle needs a radius of 205 km. I have been able to match this up and plot accurately in the graphics window, but zooming or exporting to png changes the circle size in relation to the map.

Is there any way to a) automate the circle size, and b) preserve that scaling when exporting?

Answer 1

Try this:

map('state')
x <- -73.76275
y <- 41.04181
radius <- 205 # in kilometers
s <- seq(0,2*pi,length=1000)
yradius <- optimize(interval=c(0,3),f=function(z){
  abs(pointDistance(c(x,y),c(x,y+z),longlat=T) - radius*1000)})$par
xradius <- optimize(interval=c(0,3),f=function(z){
  abs(pointDistance(c(x,y),c(x+z,y),longlat=T) - radius*1000)})$par
lines(cos(s)*xradius+x,sin(s)*yradius+y)  

This gives you a circle where the northern most point is 205km north of the center and the eastern most point is 205km east of the center.