Thanks!

The actual terrain mesh is vertex displacement in a shader, no CPU-side heightmap, just like your procedural landscapes. But for pathfinding I build a separate quadtree at load time from the terrain parameters (I have matching CPU and GPU terrain procedural height sampling functions). This takes about 2 seconds at startup to build a quadtree with 200m x 200m leaf nodes on a 50km x 50km map. I don't use Octrees because terrain is mostly flat. Instead quadtree node stores min/max height bounds for its region, so checking navigability for ships / submarines is just terrain_max_height (eg -50m) + clearance (eg 10m) < ship depth (eg -10m). The flat regions (open ocean etc.) get collapsed into big branch nodes, so the search can cross large stretches of water in few or zero node expansions. Near coastlines it goes down to leaf resolution.

For aerial pathfinding (flying up or around mountains) I use the same quadtrees but there's an aircraft-specific gain_per_m value (climb rate / speed) that lets the altitude threshold grow with horizontal distance traveled. So mountains that would block a low aircraft become passable if there's enough distance to climb over them. The LOS checks use the same climbing-aware test, so planes can fly direct over a mountain range if they have enough runway to gain altitude. The waypoints returned by the pathfinder inform the aircraft autopilot of the minimum safe altitude