What's your true altitude?

What the altimeter shows

The altimeter measures static pressure and displays it as altitude above the pressure level set in the Kollsman window. In the video, the Kollsman window is set to 30.10 and the altimeter indicates 4,500 feet. The correct altimeter setting is 29.80, so the altimeter is set 0.30 inches of mercury too high.

Altimeter adjustments

An inch of mercury equals 1,000 feet of altitude (AIM 7-2-3). In the quiz, we subtract 29.80 from 30.10 and determine there's a 0.30 inch error, which is 300 feet. With the setting too high, the altimeter indicates higher than the airplane really is, so the true altitude is 4,500 - 300, or about 4,200 feet.

High to low, look out below

The mnemonic is "from a high to a low, look out below." Flying from an area of high pressure into an area of low pressure without resetting the altimeter leaves the airplane closer to the surface than the altimeter indicates. A setting that's too high does the same thing. Being lower than you think represents a dangerous situation, as it brings you closer to terrain and obstacles.