Planes dont measure height relative to the ground. How would that even work? Their sensor is air pressure, which is treated as a function of elevation.
They do sometimes! Via radar altimeters, when relatively close to the ground. And sometimes to keep autopilots from freaking out, we have to build radar reflectors to make the ground look level to radar even when it's really not. https://lustublog.com/2017/02/17/artificiel-mais-pas-superfi...
