Grab a couple of pics for you to make it easier to explain;
This first pic I can see cliffs in front of me through the canopy, but because I am backed up to the cliff face the ground in the ground height map display is obscuring them, so I can't see in the display what I can see out of the canopy.
This second pic I climb higher in my ship without moving forward or back, once I am at ground level I can see the cliffs on the ground height map display.
Ideally a night view mode should always show you what you could see through the canopy, in deep canyons the ground height map won't always do that so it's far from ideal for the purpose.