Definition
Elevation above sea level is a number showing how high a spot rises above the average height of the ocean, which is set as 0 meters. It's a shared yardstick that lets us compare heights fairly across Earth's bumpy surface. One catch: each country picks its own sea and its own observation period, so the exact position of 0 meters differs a little from place to place. In Korea, for example, the national zero point is based on the average sea level off Incheon.
Why measure from the sea instead of the ground?
Imagine comparing heights with friends. If one person stands on a staircase and another stands on the floor, you can't tell who is really taller. Everyone has to stand side by side on the same flat floor.
Measuring land works the same way. Ground height varies from place to place, so if you just start from the dirt under your feet, you can't compare anything. A 5-story building on a high plateau might actually sit much closer to the sky than a 10-story building at the beach.
So scientists turned to the ocean that wraps around the whole planet. Water flows downhill and levels itself out, so the sea can act as one giant shared floor. Once we agreed to call the sea surface 0 meters, we could measure mountains and cities all over the world with the same yardstick.
Finding zero in the middle of moving waves
But the sea never sits still. Wind makes waves, and the pull of the moon and sun brings high tide and low tide twice a day, so the water level keeps changing.
That means you can't use the sea level at a single moment as your baseline. Instead, people install tide gauges (instruments that measure water height) and carefully record the rise and fall of the tides for years or even decades. Then they average all those readings mathematically to get the mean sea level over a long period of time.
You can't go out to sea every time you need to check that zero line. So the zero is copied onto solid land as a marker called a leveling origin, or benchmark. Korea bases its zero on the average sea level off Incheon and placed its benchmark stone on the campus of Inha Technical College in Incheon. The stone's height is set at 26.6871 meters, and elevations on the Korean mainland are measured from it. Islands that are hard to connect to the mainland by surveying, such as Jeju Island, use a separate reference point.
A little more precisely: Earth isn't a perfect ball
To be more precise, Earth isn't a perfect sphere, so the sea surface isn't a perfectly smooth ball either. The rock inside Earth has different densities, so gravity varies slightly from place to place. Add in the centrifugal force from Earth's spin, and the ocean surface has invisible bumps and dips.
Geodesists call the ideal sea surface calculated from this gravity balance the geoid. Modern GPS and mapmaking use geoid models to correct elevation values and make them even more accurate.
It also helps to separate elevation from the everyday idea of a mountain's height. If a mountain is 1,000 meters above sea level, that doesn't mean you hike 1,000 meters from its base to its peak. If it sits on a plateau already 800 meters above sea level, the mountain itself is only 200 meters tall.
Common misconceptions
Elevation above sea level is measured from the water level at low tide or high tide.
It isn't based on a single moment of high or low tide. Zero meters is the mean sea level, calculated mathematically from many years of observed water-level changes.
Where you meet it
Hallasan, South Korea's tallest mountain, is 1,947 meters above sea level, meaning its peak rises 1,947 meters above the sea surface.
Every airport publishes an elevation for its runways, and pilots use that number for takeoffs and landings.
In one sentence
Elevation above sea level is a height measured from the average ocean level, set as 0 meters, so we can compare heights anywhere using the same baseline.