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Heightmap highest elevation has been confirmed 8848 meters. Ice said he made mount Everest terrain which worked fine. Helicopter (AH-6 Little Bird) could not climb to that altitude but cars worked there just fine.
Note: this was highest confirmed elevation I've heard anyone talking about, it doesn't mean that elevations above this stop working, who knows how high mountains you can make in ArmA 3.
Have you created terrain with higher elevations, please let me know in ArmA 3 Forum, share the knowledge. BTW deepest ocean depths would be fun to figure out as well, I remember PMC 51km Desert heightmap elevations (at least in one point) had -600 meter deep ocean, that was a scary place to swim.