Get aspect ratio for a small-area lat-long dataset to approximate Mercator projection

map_aspect.R

# ref: http://stackoverflow.com/questions/31745894/get-aspect-ratio-for-lat-long-plots/31746542#31746542
# requires ggplot2

map_aspect = function(x, y) {
  x.center <- sum(range(x)) / 2
  y.center <- sum(range(y)) / 2
  x.dist <- ggplot2:::dist_central_angle(x.center + c(-0.5, 0.5), rep(y.center, 2))
  y.dist <- ggplot2:::dist_central_angle(rep(x.center, 2), y.center + c(-0.5, 0.5))
  y.dist / x.dist
}