Ruby class to calculate distance in between too lat,long coordinates using the Great Circle algorithm

gps_distance.rb

module GPS
  class Distance
    RAD_PER_DEG = Math::PI / 180
    GREAT_CIRCLE_RADIUS_MILES = 3956
    GREAT_CIRCLE_RADIUS_KILOMETERS = 6371 # some algorithms use 6367
    GREAT_CIRCLE_RADIUS_FEET = GREAT_CIRCLE_RADIUS_MILES * 5280
    GREAT_CIRCLE_RADIUS_METERS = GREAT_CIRCLE_RADIUS_KILOMETERS * 1000
    GREAT_CIRCLE_RADIUS_NAUTICAL_MILES = GREAT_CIRCLE_RADIUS_MILES / 1.15078

    attr_accessor :great_circle_distance
    attr_accessor :point1
    attr_accessor :point2

    def initialize(great_circle_distance = 0)
      @great_circle_distance = great_circle_distance
      @point1 = [0,0]
      @point2 = [0,0]
    end

    def to_miles
      calculate
      @great_circle_distance * GREAT_CIRCLE_RADIUS_MILES
    end
    alias_method :to_mi, :to_miles

    def to_kilometers
      calculate
      @great_circle_distance * GREAT_CIRCLE_RADIUS_KILOMETERS
    end
    alias_method :to_km, :to_kilometers

    def to_meters
      calculate
      @great_circle_distance * GREAT_CIRCLE_RADIUS_METERS
    end
    alias_method :to_m, :to_meters

    def to_feet
      calculate
      @great_circle_distance * GREAT_CIRCLE_RADIUS_FEET
    end
    alias_method :to_ft, :to_feet

    def to_nautical_miles
      calculate
      @great_circle_distance * GREAT_CIRCLE_RADIUS_NAUTICAL_MILES
    end
    alias_method :to_nm, :to_nautical_miles
    
    private

    # Radians per degree
    def rpd(num)
      num * RAD_PER_DEG
    end

    def calculate
      # Accept two arrays of points in addition to four coordinates
      if point1.is_a?(Array) && point2.is_a?(Array)
        lat2, lon2 = point2
        lat1, lon1 = point1
      elsif
        raise ArgumentError
      end
      dlon = lon2 - lon1
      dlat = lat2 - lat1
      a = (Math.sin(rpd(dlat)/2))**2 + Math.cos(rpd(lat1)) * Math.cos((rpd(lat2))) * (Math.sin(rpd(dlon)/2))**2
      @great_circle_distance = 2 * Math.atan2( Math.sqrt(a), Math.sqrt(1-a))
    end
  end
end