code: public double getDeclination( double dlat, double dlong, double year, double altitude )
      {
              calcGeoMag( dlat, dlong, year, altitude );
              return  dec;
      }
docs: Returns the declination from the Department of
  Defense geomagnetic model and data, in degrees.  The
  magnetic heading + declination = true heading.
  (True heading + variation = magnetic heading.)
  
  @param	dlat		Latitude in decimal degrees.
  @param 	dlong		Longitude in decimal degrees.
  @param	year		The date as a decimial year.
  @param	altitude	The altitude in kilometers.
  
  @return  The declination in degrees.