code: public double getIntensity( double dlat, double dlong, double year, double altitude )
      {
              calcGeoMag( dlat, dlong, year, altitude );
              return  ti;
      }
docs: Returns the magnetic field intensity from the
  Department of Defense geomagnetic model and data
  in nano Tesla.
  
  @param	dlat	 	Latitude in decimal degrees.
  @param 	dlong		Longitude in decimal degrees.
  @param	year		Date of the calculation in decimal years.
  @param	altitude	Altitude of the calculation in kilometers.
  
  @return  Magnetic field strength in nano Tesla.