RE: flying at an airport
An excellent question. A few people can judge the 400' altitude to within a few yards fairly accurately, many people would miss it by 500 or 600'. One reccomendation has been to take the plane to a known distance (our runway is approximately 450' and see what it looks like. But, it will look different once airborne, with a vertical and horizontal component thrown in to confuse the issue. Also, the angle of the sun, clouds, haze can affect the visual perception. Even how the temperature and humidity affect the people trying to guess.
At the club I am a member of, I've been part of a couple guessing games where a plane with a recording altimeter device is flown at a constant (hopefully) altitude for a bit, then those of us standing around guess what the altimeter is going to say. Once, the plane was a 78" powered glider flown to where it was starting to fade into the clouds. Guesses were from 600' to 4800'. I guessed 2300', actual recorded altitude was closer to 2100'. Another time, it was done with a Kadet MkII. Guesses were from 20' to 800'. Actual altitude was 88'. Trying to estimate the number of wingspans, I had guessed about 60'.
About all you can do if you're flying at or near an airport is use best judgement, always have a spotter keeping watch, and if a full sized plane starts coming over, get out of it's way, FAST.
In my club's situation, the 400' altitude entertains a question of interpretation. Is that 400' measured from the top of the hill, or about 60' down at the base, when the plane is out away from the runway?