Calculates the distance in meters between 2 GPS coordinates using the haversine formula

haversine_distance.rb

# Calculates the distance in meters between 2 GPS coordinates
# http://en.wikipedia.org/wiki/Haversine_formula

# Arguments p1, p2 are instances of the class Point below:
# class Point
#   attr_accessor :lat, :lon
# end
  
def haversine_distance(p1, p2)
  lat1 = p1.lat
  long1 = p1.lon
  lat2 = p2.lat
  long2 = p2.lon

  dtor = Math::PI / 180
  r = 6378.14 * 1000

  rlat1 = lat1 * dtor
  rlong1 = long1 * dtor
  rlat2 = lat2 * dtor
  rlong2 = long2 * dtor

  dlon = rlong1 - rlong2
  dlat = rlat1 - rlat2

  a = (Math.sin(dlat / 2)**2) + Math.cos(rlat1) * Math.cos(rlat2) * (Math.sin(dlon / 2)**2)
  c = 2 * Math.atan2(Mat.sqrt(a), Math.sqrt(1 - a))
  d = r * c

  d
end