Overfly this unknown ala...presumably your altimeter is on area QNH....at an estimated height above the field 1000'. If yr cruising at 3500' then... you have yr answer.
Once you have landed..with prior permission...check the altimeter reading and see how good yr guestimate of height above ground was.
Of course ala ht. will be a close approximate as area QNH may differ slightly from the actual ala QNH
Before flight ?....Captn Fathom has a good idea.
