Why doesn't a drone use GPS for legal height rather than home point height

[scubadesflier]

As it must use GPS for distance from the home point. This would avoid the problem discussed elsewhere of flying at an illegal height when flying from a hill over a valley or indeed apparently exceeding the legal height when trying to fly over rising land from a valley. Surely with all the other amazing functions programmed into the firmware this should be a doddle. Am I missing something or is this not desirable?

 

[Meta4]

As it must use GPS for distance from the home point. This would avoid the problem discussed elsewhere of flying at an illegal height when flying from a hill over a valley or indeed apparently exceeding the legal height when trying to fly over rising land from a valley. Surely with all the other amazing functions programmed into the firmware this should be a doddle. Am I missing something or is this not desirable?

1. GPS wouldn't give you the height above the ground below the drone.
That's not something that GPS can do.
It would give an (inaccurate) height above sea level
2. GPS altitude isn't very good anyway.
There is no sensor that's small enough or cheap enough to do what you are asking for.

The pilots of most real general aviation aircraft don't have agl height either and use a barometric sensor just like your drone.

Given a good idea of the drone's height relative to the home point it's not hard to estimate agl height.
If it's really an issue where you are, do a little research of local altitudes with a topo map or Google Earth to help even more.

 

[mavic3usa]

One day when AI becomes a real thing in drones, there won't be a single spot on the planet where your drone can exist that cannot be determined how high about the ground below your drone is flying. At that point, obstacle avoidance will be obsolete.

 

[MS Coast]

One day when AI becomes a real thing in drones, there won't be a single spot on the planet where your drone can exist that cannot be determined how high about the ground below your drone is flying. At that point, obstacle avoidance will be obsolete.

Height above ground is not a good indicator of the presence of obstacles.