GPS altitude is notoriously bad via consumer devices. It's better to store altitude as less than 2 bytes with the range from being a few km below sea level to the maximum altitude consumer devices will report (3,000 km -- though maybe less because there is also a speed limit at which they will stop reporting too so you can't just buy a GPS device and build a guided missile).

Lat long and assuming at ground gets you most of the way there.

"Add it all up, and the US has around 340 billion square feet of building stock[3]. This is about 12,200 square miles, or 0.00032% of US land area"

Judging by that, you need a negligible increase in the number of locations you can represent to handle everywhere stable and off the ground someone could be. Much less than one bit per person.

If you want to deal with people currently in airplanes then you could give them an extra couple bytes. It's less than a million people so it won't affect your total storage at all.

Most people don't live in caves or in the sky city, 2 is enough.

And in places you care (multi-floor buildings) you aren't getting GPS signal inside anyway...