Before my question, I want to say hello to all flightgear developers. I'm new to Flightgear ( nearly 3 months) and also to Linux. The open-source concept is a wonderful way to develop something especially FlightGear. Now, I'm trying to write my own FDM. It works fine for me but I have some problems. 
    These days, I work on calculation of altitude above ground level value. I couldn't transfer JSBSIM or YASIM get_agl functions into my FDM. Then, I decided to use get_agl functions in flight.cxx . But, I couldn't understand the usage of it. I don't know the variables like normal[3] or pt[3]. 
   Can you help me about the get_agl functions? Thank you!