if "corrected to sea level". Mathematically adjusted to give the correct pressure if the elevation were equal to zero. What does elevation have to do with it? The air pressure drops off quickly as you climb uphill and this effect cannot be ignored when interpreting data. Aneroid barometers - a sealed container cannot let air escape. When you have one of these with flexible walls, the walls move when the air pressure outside changes. And we can measure how much they move. How to make a barometer improve aviation safety? Airplanes use an aneroid barometer to help pilots figure out how high they are above the ground. They call this device an altimeter. See more on the Weather Equipment web page.