Haversine distance (http://www.movable-type.co.uk/scripts/latlong.html)

lldist.rb

!/usr/bin/env ruby
#
class GeoDist
  def self.haversine(lat1, lon1, lat2, lon2)
    r = 6371 ## km
    dLat = self.toRadians(lat2-lat1)
    dLon = self.toRadians(lon2-lon1)
    lat1 = self.toRadians(lat1)
    lat2 = self.toRadians(lat2)

    a = Math.sin(dLat/2) * Math.sin(dLat/2) +
        Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1) * Math.cos(lat2)
    c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a))
    d = r * c
    return d
  end

  def self.toRadians(degrees)
    return degrees*(Math::PI/180)
  end
end


puts GeoDist.haversine(42.1234, 72.1234, 42.1230, 72.1200)