Daily Dose · Maps / Sep 17, 2026 / Geodesy & the Geoid

Sea Level Is Not Level

Every height on every map is measured from “sea level.” But the surface we call zero is lumpy — a warped, invisible skin pulled around the planet by the uneven mass inside it. Drain the oceans and the leftover shape would swing through 191 metres, with its deepest dent sitting in the Indian Ocean, south-west of Sri Lanka.

191 m

Vertical spread of “zero”
−106 m to +85 m · EGM2008

That is the range of geoid height — the gap between the geoid (where the sea would settle if only gravity and Earth’s spin acted on it) and the smooth mathematical ellipsoid that GPS actually measures against. Near Iceland the sea bulges 85 m above the ellipsoid. South-west of Sri Lanka it sags 106 m below it. Both places are, by definition, at sea level.

−106 m−75−50−250 · ELLIPSOID+25+50+85 m

Geoid height relative to the WGS 84 ellipsoid · diverging scale, teal below / brass abovePattern schematic · values: EGM2008

01 · Two Earths

The Earth we calculate with, and the Earth water actually finds

Geodesy keeps three surfaces in play at once, and confusing them is how bridges end up at the wrong height.

Ellipsoid · geoid · terrain — vertical exaggeration extremeH = h − N

6,378.137 km

Equatorial radius · WGS 84

The reference ellipsoid: a spinning, flattened, perfectly smooth idealisation. GPS reports your height above this.

6,356.752 km

Polar radius · WGS 84

Flattening of 1 / 298.257 — the equator sits 21.4 km further from Earth’s centre than the poles.

2,159

Degree of EGM2008

The harmonic model behind the map, built from satellite, marine, airborne and ground gravity on a 5-arcminute grid.

02 · The Gravity Hole

A dent the size of India, and nobody could explain it

The Indian Ocean Geoid Low is the deepest sag in the planet’s gravity field — and it was found from a submarine.

−106 m

Depth of the low

Strip away tides and currents and the sea surface here sits 106 m below the ellipsoid — the lowest geoid on Earth. Centred near 2°N, 76°E.

~3M km²

Area affected

Roughly the size of India itself. Local gravity runs about 50 milligal weak — around 0.005% below normal.

1948

Found by Vening Meinesz

The Dutch geophysicist measured gravity at sea from a submerged submarine, the only stable platform available, and hit an anomaly nobody expected.

The 2023 answer: a ghost ocean

Running 19 mantle-convection simulations back to 140 million years ago, Pal & Ghosh found the only models that reproduce the low are ones where cold slabs of the vanished Tethys Ocean sink to the base of the mantle, disturb the African “superplume” (the African LLSVP), and throw off hot, low-density plumes that spread beneath the Indian Ocean. Less mass below ⇒ weaker pull ⇒ lower sea surface.

Roughly 20 million years old

In those models the low only begins to take its modern shape about 20 Ma, as the plumes reach the upper mantle. India’s northward flight across the Tethys — the same journey that raised the Himalaya — left a hole in the ocean behind it.

03 · The Scale

How far “sea level” can move

From the planetary to the surveyor’s: every one of these is a real vertical disagreement about where zero is. Note the logarithmic axis — the top two bars are two orders of magnitude above the rest.

Geoid low · SW of Sri Lanka

106 m

Geoid high · near Iceland

85 m

US coordinate shift · NSRS 2022

up to 4 m

NAVD 88 tilt · coast to coast

~1 m

NAVD 88 bias · nationwide

~0.5 m

Panama · Pacific over Atlantic *

~0.20 m

GOCE geoid accuracy · 100 km

0.015 m

1 CM10 CM1 M10 M100 M

* The ~20 cm Pacific-over-Atlantic step at Panama is an approximate, widely reported figure for mean sea level; it comes mostly from ocean dynamics (warmer, fresher, less dense Pacific water plus winds and currents) rather than from the geoid itself. It is included here to show the scale of the ocean’s departure from the gravity surface.

04 · Measuring It

Three centuries of arguing about the shape of zero

The geoid is the one map you cannot draw by looking. It has to be weighed.

1687

Newton predicts a flattened EarthThe Principia argues rotation must bulge the equator — the first claim that the planet is not a sphere.

1736–37

The Lapland arc settles itMaupertuis’ expedition to the Arctic measures a degree of latitude and confirms the poles are flattened.

1873

Listing names the “geoid”A word for the equipotential surface that mean sea level follows — the true figure of the Earth.

1948

Vening Meinesz finds the Indian Ocean lowShipboard gravity measurement, made from a submarine to escape wave motion, reveals the planet’s deepest geoid dent.

2002

GRACE starts weighing the Earth monthlyTwin NASA/DLR satellites track the distance between themselves; gravity changes as ice, water and rock move.

2009–13

GOCE flies low and maps the geoid to 1–2 cmESA’s gradiometer satellite orbits at 255 km — later dropped to 224 km — resolving the geoid to 1–2 cm at 100 km scales before re-entering on 11 Nov 2013.

2018

GRACE Follow-On takes overLaunched 22 May 2018 by NASA and GFZ: two satellites ~490 km up, 220 km apart, now ranging by laser interferometer as well as microwave.

2026

The United States retires its old sea levelNOAA’s modernised National Spatial Reference System — NATRF2022 with the NAPGD2022 / GEOID2022 geoid — is expected to replace NAD 83 and NAVD 88, with adoption anticipated this year.

05 · Stakes

Everything vertical hangs off this surface

Get the geoid wrong and water appears to flow uphill — on paper, which is where floods are planned for.

~0.5 m

NAVD 88 bias

America’s current vertical datum is biased about half a metre, and tilted roughly a metre from coast to coast, against the best modern geoid models. Every elevation in the country inherits that error.

4 m

Maximum coordinate change

NOAA warns that fixing it will move existing latitudes, longitudes, ellipsoid heights and orthometric heights by as much as four metres.

8,848.86 m

Everest, agreed 8 Dec 2020

The China–Nepal joint survey ended a decades-old dispute — but the number is only meaningful because both sides finally agreed which sea level to measure from.

Melting ice makes local sea level fall

Ice sheets are massive enough to pull the ocean toward them. When Greenland loses ice, its gravitational grip weakens and the sea near it drops; beyond about 2,000 km the water redistributes and sea level rises faster than the global average. In 2022 Coulson and colleagues detected this fingerprint in satellite data around Greenland — a gravity map you can read in a tide gauge.

GPS does not know about sea level

A receiver gives you height above the ellipsoid. Turning that into a height above sea level requires subtracting a geoid model — which, near Sri Lanka, means subtracting −106 m. Aviation, flood mapping, dam engineering and drainage all depend on getting that one number right in the right place.

What to notice on the map

Read it as a landscape, not a country map

The lows and highs ignore the coastlines. The Indian Ocean dent sits across open water, Sri Lanka and the Horn of Africa alike. This is a map of what is below the crust, not on it — mass in the mantle, thousands of kilometres down.
The two extremes are not opposites. The North Atlantic high (+85 m) and the Indian Ocean low (−106 m) are 191 m apart vertically but only a quarter of the planet apart horizontally — the geoid’s features are broad and gentle, hundreds of kilometres per metre.
Nothing here is topography. No mountain range shows up; the Himalaya barely register. The geoid answers a different question than a relief map: not “how high is the rock,” but “where would the water stop.”
It is a map of a surface you can stand on and never see. Every “above sea level” figure you have ever read is measured from this thing.