code: def haversine(self, other):
          
          # convert decimal degrees to radians
          lon1, lat1, lon2, lat2 = map(radians,\
                  [self.lon.low, self['lat'].low, other.lon.low,
                      other['lat'].low])
          # haversine formula
          dlon = lon2 - lon1
          dlat = lat2 - lat1
          a = sin(dlat/2)**2 + cos(lat1) * cos(lat2) * sin(dlon/2) ** 2
          c = 2 * asin(sqrt(a))
          # multiply by radius of the earth
          # km = 6367 * c
          miles = 3961 * c
          return miles
docs: Calculate the great circle distance between two points
          on the earth (specified in decimal degrees)