Very hard to say.  A haunted castle aboveground can look feel and function, while in play, just the same as an underground dungeon.  A pocket of air deep underwater can function like a cavern...but isn't.  And so on.
I'm trying to think of a suggestion for a different way to ask this, but not coming up with much.  You might go:
--- % indoor
--- % outdoor
--- % other (e.g. a different plane where indoors and outdoors are effectively the same)
Or, perhaps expand a bit:
--- % aboveground, indoors
--- % aboveground, outdoors (includes airborne encounters where ground is present)
--- % underground, constructed dungeon
--- % underground, natural caverns or similar
--- % underwater
--- % other (e.g. a place where there is no ground such as elemental plane of air)
Otherwise, I'll guess at an answer to the poll and give it but it's a shot in the dark.
Lan-"if I roll natural 20 on that shot in the dark, do I get a cookie?"-efan