I currently have my maximum altitude set just below 400 feet to satisfy Federal rules.  Just for fun after setting this I flew straight up to see what would happen. The drone stopped at my preset values and would not go any higher.

I am going on a trip to the ocean where there are cliffs with drops well over 400 feet.

If I am flying 100 feet over the mainland then go out over the cliff (that has a drop well over 400 feet) to over the sea the get video from there, what happens to the drone’s altitude?

Will it dive to get below 400 feet or stay at the altitude it was when over the mainland?

The drone will never drop on it's own (unless under the RTH function), you're suppose to decrease the altitude yourself to stay within the regs.
The app will only show the altitude from where it took off and does not know the land's elevation below it. Yes it is a bit of a guessing game unless you have elevation maps on hand, so don't worry too much.
Just use common sense.

Thanks Cobberwebb.

Not sure how Just use common sense applies in your response.

I main concern is getting the drone back to the landing pad after its flight over the ocean.  In my example I am flying 100 feet over the mainland.  Flying out past the cliff with a drop of 400 feet and over the ocean the drone now has an altitude of 500 feet.  From what you are saying the drone will  tell me in that app that I am still at 100 feet.  I hope you are correct.  I will see what happens in a month's time.

As long as you do not drop, then thats correct but i think you're meant to drop to the 400ft after say 400ft** distance from the drop.

** that amount of distance is an educated guess but i bet its not far off that amount, therefore, you must drop to the legal 400ft or lower

Thanks Bashy, just corrected that error in my second post.

Ontario Drone Posted at 9-24 03:54
Thanks Cobberwebb.

Not sure how Just use common sense applies in your response.

when you go over the cliff, you are still 100 feet above your starting point.

the "maximum altitude setting" is designed to limit the maximum number of feet the drone will fly over the starting point.  it is not intended to be used to satisfy federal rules.  where common sense would come into play if you wanted to cross over a canyon that is 500 feet deep and 200 feet across, the safest way to do that would be to simply fly across from one side of the cliff wall to the other while maintaining vlos.  it would be unsafe to try to dive down into the canyon in order to try to stay within 400 feet above ground level.  if you are half a mile from the canyon, how would maintain vlos?

Now I get it.  Thank you everyone

Ontario Drone Posted at 9-24 05:17
Now I get it.  Thank you everyone

If you are 100' high from your take off point, when you fly beyond the edge of a cliff, if you were to descend into the valley the height would change to a negative number when you descend past your home point height of 0, i.e. if you continued further down, say 50' then the reading would indicate -50'. If you had set the RTH to say 100' then the drone will fly from -50' up through 0' on up to 100' before flying home. Hope that makes sense.

Very interesting questions, and opinions.  Thanks one-and-all.

Just incase you are thinking of it, when out over the sea you can descend below the take off point but keep an eye out for gulls and keep an eye on the number of GPS satellites. Gulls may not like the drone and losing GPS could give serious problems and may introduce height restrictions. I recollect several threads in various places where people struggled to get their drone back into GPS lock.

My lowest flight  so far from a sea cliff top is in 'excess' of  -100m