excuse my ignorance ,but as far as i can tell it has always been the case that the drone will geofence at 396 ft directly above the home point,
What do you mean by "geofence"?
It certainly posts a warning message about breaking the 400ft ceiling and I think it posts that warning irrespective of wherever it is when breaching that ceiling but I don't remember the precise height at which that warning first appears nor do I remember with certainty whether or not it halts the climb of the drone but I don't think it does halt the climb. I rarely fly above 200ft relative to the take off point.
what you are saying, is that it will know somehow ,what the elevation of the ground beneath itself is, and prevent the drone from exceeding that limit
No or at least that's not what I meant to convey.
My understanding of the impending EU rule is that it imposes a ceiling of 120m with respect to the take off point .... full stop.
I.e. no hill climbing to heights higher than 120m relative to the take off point.
That I know of there are only two ways the drone could know its AGL when 'high'.
1) Through some sort of altitude sensing 'radar' e.g. some form of VPS sensor but with a 400ft+ range.
2) Carry, in its memory, a data base of the earth's surface elevation at various points and then, using the indicated barometric height and that data base, calculate the AGL. I have no idea of the memory space that would require nor the additional workload for the drone's CPU.
The accuracy and resolution of the data base map would be important in some locations.
I suppose it would be conceivable to load the data for the flight's locale, in the same way that people down load maps to the screen device.
I suppose this could be handed off to the screen device but then what happens in a disconnection? Beside's would such an added work load 'push the boundaries' of what the screen device's cpu is capable of?
Actually the above "hand off" idea has me wondering, could someone write a third party app to actively read live data from the control app and show an AGL height super imposed on the screen? Xrecorder does the 'super imposed' thing with its record/stop button so that aspect shouldn't be problem but reading live data from the app might be.