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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchElevation is the vertical position of a point measured from a defined reference surface, or datum. In everyday speech it is often described as height above sea level, but a useful elevation figure also needs context: its units, location, datum and how it was determined. It is related to altitude, but the terms are not interchangeable in every discipline.
What does elevation mean?
The Federal Geographic Data Committee defines elevation as “the vertical distance from a datum to a point or object on the earth’s surface.” The term can also describe measurement of terrain height or deep-water depth. A vertical datum is the reference coordinate system to which those height measurements relate.
“Above sea level” is a convenient everyday shorthand, not a complete technical specification. For precise mapping, sea level is represented by a modelled reference surface, and an elevation value is meaningful only in relation to the datum used to produce it. The NOAA National Geodetic Survey describes the U.S. vertical reference system as known heights above or below a surface approximating mean sea level: NOAA National Height Modernization Program.
How is elevation different from altitude?
In ordinary conversation, elevation and altitude can both mean height above sea level. Their use varies by field. The U.S. Geological Survey notes that elevation may also mean uplift in geology: a geologist might use “altitude” for a location’s height above sea level and “elevation” for the process or amount of uplift, while engineering and topographic work often uses elevation for height above a reference surface.
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Because usage is not uniform, reports and maps should define their terms and use them consistently. The distinction matters most when a number is being compared, interpreted or used in a calculation—not in casual descriptions.
What reference surface does an elevation use?
The geoid: a model related to mean sea level
The geoid is a gravity-related model of global mean sea level used as a reference surface for precise elevations. It is irregular rather than a perfect sphere or a uniformly smooth surface: variations in Earth’s gravity affect its shape. Surveyors extend this model across land to establish heights. NOAA’s explanation, updated September 23, 2026, describes the geoid and its role: What is the geoid?
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Orthometric height and ellipsoid height
Two height values can describe the same point but refer to different surfaces:
- Orthometric height is measured relative to the geoid. It is the kind of height commonly used for topographic mapping and what people generally mean by elevation above sea level.
- Ellipsoid height is measured relative to a reference ellipsoid, a mathematical model of Earth’s shape.
These are different quantities; do not treat a GNSS-derived ellipsoid height as if it were a mapped orthometric elevation. Converting between them requires an appropriate geoid model. NOAA notes that the resulting orthometric height depends on both the model and the accuracy of the vertical control data used: NOAA National Height Modernization Program.
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Which elevation datum applies?
A datum defines the reference for a measurement, so two elevations should not be compared as if they share the same zero surface unless their datums are known to match or have been properly converted. Keep the datum name and units attached whenever you copy or publish a figure.
For U.S. civilian federal surveying and mapping, NOAA lists the North American Vertical Datum of 1988 (NAVD 88) as the current official vertical datum. That is a U.S. institutional reference, not a worldwide standard. NOAA’s datum table identifies the reference and its scope: Datums and Reference Frames (page modified December 7, 2023). NOAA’s height modernization page recounts that NAVD 88 succeeded NGVD 29 for this role.
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How is elevation measured or read from a map?
Surveying and positioning
NOAA’s National Geodetic Survey describes differential, trigonometric and barometric leveling. Of these, it identifies differential leveling as the most accurate method. Traditional differential leveling uses a leveling instrument and calibrated staffs to establish height differences. Modern workflows may combine satellite-based GNSS observations with a geoid model and established vertical control to derive orthometric heights; the result depends on the model and control quality. See NOAA Geodetic Leveling.
Topographic maps and contour lines
Topographic maps show selected point elevations and contour lines. Each contour connects locations of equal elevation; the spacing between contour lines indicates how quickly terrain rises or falls, according to the map’s contour interval. If a particular spot has no printed elevation, USGS guidance describes estimating its height by interpolating between surrounding contours. The estimate is limited by the map’s contour interval and the terrain represented: USGS Circular 1011.
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Why does accurate elevation matter?
The required precision depends on what the information will be used for. A broad terrain overview does not demand the same level of certainty as a structure designed around water levels or a flood boundary. NOAA identifies uses of accurate heights that include:
- Planning bridges, dams, levees and other infrastructure.
- Designing emergency evacuation routes and mapping floods or inundation.
- Monitoring crustal motion, subsidence, erosion and sea-level change.
- Supporting agricultural equipment and fertilizer application.
- Natural-resource projects and geographic information systems (GIS).
For a project, check that the data’s datum, units, geographic coverage and accuracy suit the task. A dataset’s spatial resolution—how finely it samples terrain—is not the same as the accuracy of its elevations.
How to compare two elevation values
Before treating two figures as comparable, check each value’s reference, method and context:
- Datum and reference surface: Is it a geoid-based orthometric height, an ellipsoid height, or a height in another named vertical datum?
- Units: Are both values expressed in the same unit?
- Method and control: Was the height surveyed, derived from GNSS with a geoid model, or read from a map or digital terrain source? What control data support it?
- Accuracy and resolution: How precise is the measurement, and how densely does the map or dataset sample the terrain? There is no single accuracy figure that applies to all methods and products.
- Place and time: Do the values refer to the same location and area? Benchmarks can be disturbed, destroyed or affected by crustal movement, so an older reference point may need checking.
If any of this information is missing, the number may still be useful as a rough description, but it should not be treated as a fully specified survey value.
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