What is Sea Level?

TL;DR

Sea level is defined using gravity, not as a simple average of ocean surface height.

Briefing Cornell Notes

Briefing

Sea level isn’t a single “ocean average” number—it’s a carefully defined reference tied to gravity, and it has to account for Earth’s shape, uneven mass, and the gravitational pull of land. That’s why a height like “Mt. Everest is 8850 m above sea level” can be meaningful even though there’s no ocean beneath it for hundreds of kilometers.

A simple approach works only in idealized worlds. If Earth were flat, sea level would just be the average ocean height. If Earth were a perfect sphere, sea level could be defined as a constant distance from Earth’s center. Real Earth is neither: it spins, so centrifugal effects push material outward near the equator and slightly squash the poles. The result is a planet about 42 km wider at the equator than from pole to pole. Under that spherical assumption, the ocean surface at the equator would sit roughly 21 km above sea level defined at the north pole—showing that geometry alone already forces a more nuanced definition.

Even an ellipsoidal model still falls short because Earth’s interior isn’t uniform. Density varies from place to place, which makes gravity slightly stronger or weaker across the globe. Oceans respond to that uneven gravitational field by “puddling” more near denser regions. The consequence is large enough to matter: sea level can differ by as much as 100 m from what a smooth, uniform ellipsoid would predict.

Land adds another complication. Continents are dense rock masses that protrude from the ellipsoid and tug extra water toward themselves, raising sea level nearby. Ocean-floor valleys have less mass, so water tends to flow away from them, lowering local sea level. This creates the core conundrum for mountain measurements: a mountain’s presence changes the nearby gravitational field, so the sea level “around” it depends on whether the reference should include the mountain’s mass or exclude it.

Geodesists resolved this by defining sea level in terms of gravity rather than direct ocean height. They built an extremely detailed model of Earth’s gravitational field—called the Earth Gravitational Model—and used it to establish a consistent sea-level reference surface. Modern GPS receivers incorporate this model so altitude readings don’t drift by tens of meters due to regional gravity differences; the model enables geodesists to predict the average ocean level to within about a meter worldwide. With that framework, “sea level under mountains” is computed as it would be if the mountains weren’t there geometrically, but their gravitational influence were still included—making Everest-style elevations comparable across the planet.

Cornell Notes

Sea level can’t be defined as a simple average ocean height because Earth’s rotation and gravity vary by location. Spinning makes Earth wider at the equator and squashed at the poles, so an ellipsoidal shape is needed even before considering gravity. Uneven density inside Earth changes local gravity, causing oceans to “puddle” and shift sea level by up to ~100 m compared with a uniform model. Continents further distort sea level by their mass, raising water near land and lowering it near low-mass regions. Geodesists therefore define sea level using gravity via the Earth Gravitational Model, which is also built into GPS to keep altitude references consistent worldwide.

Why does Earth’s rotation force a different sea-level reference than a simple “average ocean height”?

Rotation creates centrifugal effects that push Earth outward at the equator and squash it near the poles. Earth is about 42 km farther across at the equator than pole-to-pole, so a sea-level surface based on a sphere would be badly wrong. Under a spherical assumption, the ocean surface at the equator would sit roughly 21 km above a sea-level definition tied to the north pole.

How can local gravity differences change sea level by amounts as large as 100 m?

Earth’s interior density isn’t uniform, so gravity is slightly stronger over denser regions and weaker over less dense ones. Oceans respond to this by flowing toward stronger gravity (“puddling” more near dense spots). Those shifts are not trivial: sea level can vary by up to about 100 m from what a uniform ellipsoid would predict.

What role do continents play in determining sea level near mountains?

Continents are dense rock masses that protrude from the ellipsoidal reference surface. Their extra mass increases gravitational attraction, pulling water toward them and raising sea level nearby. In contrast, ocean-floor valleys have less mass, so water tends to flow away and sea level drops in those areas.

Why is defining “sea level under a mountain” tricky?

A mountain changes the local gravitational field because it adds mass. That means the sea level around it depends on whether the reference surface should treat the mountain as absent entirely or absent geometrically but still present gravitationally. The choice affects the computed height above sea level.

How does the Earth Gravitational Model make sea level consistent enough for GPS and mountain elevations?

Geodesists created an extremely detailed model of Earth’s gravitational field—the Earth Gravitational Model—and used it to define a gravity-based sea-level reference surface. GPS receivers incorporate this model so altitude readings don’t mislabel positions by large amounts caused by regional gravity differences. With it, geodesists can predict the average ocean level to within about a meter everywhere on Earth, and compute sea level beneath mountains using the same gravity-based standard.

Review Questions

If Earth were perfectly spherical and not rotating, what would sea level correspond to geometrically, and why does that fail for the real Earth?
Explain how variations in Earth’s internal density lead to ocean “puddling” and measurable sea-level differences.
Why does the gravitational influence of a mountain matter when converting its height into “meters above sea level”?

Key Points

1
Sea level is defined using gravity, not as a simple average of ocean surface height.
2
Earth’s rotation makes it an ellipsoid, with the equator about 42 km wider than the pole-to-pole distance.
3
Non-uniform density inside Earth changes local gravity, shifting sea level by up to roughly 100 m compared with a uniform model.
4
Continents raise nearby sea level through their gravitational pull, while low-mass seafloor regions lower it.
5
Mountains complicate “height above sea level” because their mass alters the gravitational field that shapes the reference surface.
6
The Earth Gravitational Model provides a consistent gravity-based sea-level reference and is used in modern GPS to prevent large altitude errors.
7
Geodesists use the gravity-based definition to compute what sea level would be under mountains if the mountains weren’t there geometrically, but their gravity were still included.

Highlights

Earth’s spin reshapes the sea-level reference: the equator bulges, and a spherical assumption would place the equatorial ocean about 21 km above a north-pole-based sea level.

Uneven density inside Earth can make sea level vary by as much as ~100 m across the globe.

Continents don’t just sit above sea level—they tug water toward themselves, raising sea level nearby.

GPS relies on the Earth Gravitational Model so altitude readings don’t drift by tens of meters due to regional gravity differences.

Sea level under a mountain is computed using gravity-based conventions, resolving whether the mountain’s mass should be treated as absent or still gravitationally active.

Topics

Sea Level Definition
Earth Shape
Gravity Variations
Geodesy
GPS Altitude