What Is Sea Level? Elevation vs Altitude vs GPS Height Explained

In maps and surveying, "sea level" means mean sea level (MSL): the average height of the ocean surface at a place, with tides, waves and weather averaged out. NOAA defines it as the arithmetic mean of hourly water heights observed over the National Tidal Datum Epoch, a fixed 19-year period that is currently 1983 through 2001. Nineteen years is long enough to average out the slow cycles in the Moon's orbit that change the size of the tides from year to year.

Mean sea level is one of several tidal datums that NOAA computes from the same tide gauge records:

So "sea level" is not one surface even at a single beach: the gap between MLLW and MHHW is the tidal range, a few feet on most U.S. coasts and much more in places such as Alaska's Cook Inlet.

Sea level also differs from place to place and changes over time. Winds, currents, water temperature and small differences in gravity make the ocean surface uneven, so local mean sea level at one tide station is not the same height as at another. Tide stations measure sea level relative to the land next to them, while satellite altimeters measure the ocean's height from space; together they show that sea level is rising, and that at stations where the land itself is sinking or rising, the local trend differs from the global one.

Under the continents, "sea level" becomes the geoid. NOAA describes the geoid as a model of global mean sea level used to measure precise surface elevations: the shape the sea would take under gravity alone, extended under the land to serve as the zero for elevations. When a mountain is "14,000 feet above sea level", the reference is a geoid-based datum, not a tide gauge on the nearest coast.

Elevation is the height of the ground at a place, measured from mean sea level; altitude is the height of something above the ground, such as an aircraft, also usually measured from mean sea level. The terms are often used loosely, but surveyors and pilots keep them apart:

In everyday use, elevation describes a place and altitude describes something above a place. A hiker on a 3,000 m summit is at an elevation of 3,000 m; a plane 1,000 m above them is at an altitude of 4,000 m.

Aviation adds one more wrinkle: a pilot's altimeter measures air pressure and is set to the local pressure reported at sea level, so its "altitude" is a pressure-based estimate of height above mean sea level, while the height above the terrain below is what matters for clearing a ridge.

Vertical measurement depends on which surface you measure from:

1. The reference ellipsoid (WGS84): a smooth, mathematically defined oval used by GPS. It is a convenient model of Earth's shape, not a physical surface.
2. The geoid: the surface of equal gravitational potential that mean sea level would follow if the oceans could flow freely under the continents. Because Earth's mass is unevenly distributed, the geoid is lumpy, rising and falling relative to the ellipsoid by up to roughly 100 meters worldwide.
3. The topographic surface: the actual ground, rock, snow or pavement.

The three are linked by a simple relationship:

H = h − N

• H is orthometric height: elevation above the geoid, which is what topographic maps and benchmarks report as "above sea level".
• h is ellipsoidal height: what a GNSS receiver computes directly.
• N is geoid undulation: the height of the geoid above the ellipsoid at that location, taken from a geoid model.

In the contiguous United States the geoid lies below the ellipsoid, so N is negative and raw ellipsoidal heights read lower than map elevations by tens of meters.

A vertical datum fixes the zero point and the geoid model used for elevations:

• NAVD 88 (North American Vertical Datum of 1988) is the official datum for most of the United States. Surveyed benchmarks and many U.S. topographic maps use it. It is a geodetic datum fixed by leveling, not a tidal one, and the National Geodetic Survey states that it is biased by about half a meter and tilted by about 1 meter from coast to coast. That is why NAVD 88 zero and local mean sea level at a given tide station can differ by a foot or more. NGS plans to replace NAVD 88 with a gravity-based geopotential datum realized with an updated geoid model; until that happens, NAVD 88 remains the official vertical datum.
• EGM96 and EGM2008 are global geoid models. Many GPS receivers use one of them internally to convert ellipsoidal height into an approximate sea-level altitude before displaying it.
• Local datums exist in many countries, tied to a historical tide gauge.

Differences between datums are usually small compared with the ellipsoid-geoid separation, but they can amount to a meter or more. When two elevations disagree slightly, check which datum each one uses before assuming one is wrong.

When a web tool estimates the elevation at a coordinate, it reads a Digital Elevation Model (DEM): a grid where each cell stores one height value. DEMs come in two main kinds:

• Digital Surface Model (DSM): records the top of whatever covers the ground, including tree canopy and buildings in some cases. The Copernicus DEM is a surface model derived from radar measurements.
• Digital Terrain Model (DTM, bare earth): removes vegetation and structures to estimate the ground itself. In the United States, the USGS 3D Elevation Program (3DEP) publishes bare-earth elevation data, much of it from lidar.

Resolution matters as much as type. A 90 m grid stores one value for an area roughly the size of a city block, so narrow peaks, ridges, cliffs and gullies are smoothed out. In a dense forest a surface model may report a value closer to the treetops than to the forest floor.

The elevation tools look up your coordinates in the Copernicus GLO-90 DEM through the Open-Meteo elevation API, with Terrarium elevation tiles as a fallback if that service is unavailable. GLO-90 has a grid spacing of about 90 meters and reports heights relative to the EGM2008 geoid, so values are comparable to "above sea level" figures, with the surface-model caveats above.

Typical workflows:

1. Your current ground elevation: What Is My Elevation uses your device location, so first check its accuracy (see How GPS Location Accuracy Works).
2. Any point by address or coordinates: the elevation finder accepts a search or a map click; it is quicker than reading contour lines, which is the only option in Google Maps (see how to find elevation on Google Maps). If you already have precise coordinates, get them from the latitude and longitude finder and paste them in.
3. A route or hike: the elevation profile tool samples heights along a line so you can see climbs and descents. Short steep features may be flattened by the 90 m grid.

For construction, flood insurance or legal purposes, use surveyed elevations or an official elevation certificate rather than a global DEM.

It is normal for four sources to give four different numbers at the same spot:

If you need to compare, convert everything to the same datum, and prefer the source that actually measured your exact point. Elevation also feeds other calculations: the horizon distance calculator uses observer height, and a common approximation for distance to the geometric horizon is d ≈ 3.57 × √h kilometers with h in meters, slightly more when atmospheric refraction is included.