gistfile1.rb

class StationLog < ActiveRecord::Base
  
  belongs_to :boat_log
  has_many :rep_logs, :dependent => :destroy
  accepts_nested_attributes_for :rep_logs, :reject_if => lambda {  |a| a[:replicate].blank? }, :allow_destroy => true
  
  EARTH_RADIUS = 6371000 # Earth's radius in meters
  
  # convert degrees to radians
  def self.convDegRad(value)
    unless value.nil? or value == 0
      value = (value/180) * Math::PI
    end
    return value
  end

  def self.coorDist
    stn1 = Station.station_one # Station.station_one == Station.find(1)
    stn2 = Station.station_two # Station.station_two == Station.find(2)
    
    deltaLat = (stn2.latitude - stn1.latitude)
    deltaLon = (stn2.longitude - stn1.longitude)
    deltaLat = convDegRad(deltaLat)
    deltaLon = convDegRad(deltaLon)

    # Calculate square of half the chord length between latitude and longitude
    a = Math.sin(deltaLat/2) * Math.sin(deltaLat/2) +
        Math.cos((stn1.latitude/180 * Math::PI)) * Math.cos((stn2.latitude/180 * Math::PI)) *
        Math.sin(deltaLon/2) * Math.sin(deltaLon/2); 

    # Calculate the angular distance in radians
    c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a))

    distance = EARTH_RADIUS * c

    return distance
  end
end


from console log...
irb(main):032:0> BoatLog.first.station_logs.coorDist
  BoatLog Load (0.4ms)  SELECT "boat_logs".* FROM "boat_logs" LIMIT 1
  StationLog Load (0.2ms)  SELECT "station_logs".* FROM "station_logs" WHERE "station_logs"."boat_log_id" = 4 AND "station_logs"."stn_number" = 1 LIMIT 1
  StationLog Load (0.3ms)  SELECT "station_logs".* FROM "station_logs" WHERE "station_logs"."boat_log_id" = 4 AND "station_logs"."stn_number" = 1 LIMIT 1
  StationLog Load (0.2ms)  SELECT "station_logs".* FROM "station_logs" WHERE "station_logs"."boat_log_id" = 4 AND "station_logs"."stn_number" = 2 LIMIT 1
  StationLog Load (0.2ms)  SELECT "station_logs".* FROM "station_logs" WHERE "station_logs"."boat_log_id" = 4 AND "station_logs"."stn_number" = 2 LIMIT 1
=> 3428.1395884519347