You actually need the altimeter to be capable of reading below 0 in the real world. Aircraft altimeters are based on air pressure. The reference pressure is va

You actually need the altimeter to be capable of reading below 0 in the real world. Aircraft altimeters are based on air pressure. The reference pressure is variable based on atmospheric conditions and is decided by controllers, or in the case of uncontrolled airspace, the matching the altimeter of the known altitude that you took off from. All that is to say, on my home (uncontrolled) airport which is ~10 feet above sea level, I occasionally land at a negative altimeter value since the weather conditions have changed while I am in the air. Assuming that an altimeter reading must be positive is a bad assumption.