# What Is a Geodetic Datum? > A latitude or longitude has meaning only when tied to a defined reference. The same is true of an elevation. A geodetic datum supplies that reference by connecting coordinates to a model of Earth's size, shape and gravity. NOAA describes a datum... Canonical URL: https://www.argo.net/what-is-a-geodetic-datum/ Byline: ARGO.net Editorial Team Published: 2026-09-03T12:27:59+00:00 Categories: Earth, Explainer ![A land surveyor operating geodetic equipment](https://www.argo.net/wp-content/uploads/2026/09/argo-wave21-53581-pexels-29560007.jpg) A latitude or longitude has meaning only when tied to a defined reference. The same is true of an elevation. A **geodetic datum** supplies that reference by connecting coordinates to a model of Earth's size, shape and gravity. NOAA describes a [datum](https://oceanservice.noaa.gov/facts/datum.html) as an abstract coordinate system with a reference surface. Surveyors use it to make positions from different projects agree, much as a common starting point lets separate sets of directions meet. Using the wrong datum can shift a mapped feature by meters. The coordinate digits may look precise while describing a different place or height than the user intended. **Datum identification** is therefore part of the measurement, not optional documentation added afterward for careful mapping users. **Reference metadata** prevents that ambiguity. It records the datum and coordinate units. The metadata often identifies the realization or epoch used for the measurement. Those details let software transform data correctly and allow future surveyors to compare positions after reference systems improve. Without the metadata, even a carefully measured coordinate may be unusable in a precise project. ## A datum anchors coordinates A coordinate without a datum resembles a measurement without a stated starting point. The digits may be precise while the location remains ambiguous. Survey records therefore identify both the coordinate values and the reference frame, allowing another user to reproduce the position instead of guessing which Earth model supplied it. Earth is irregular and constantly changing, so mapping begins with a simplified mathematical model. An ellipsoid approximates the planet's overall shape. A reference frame then defines how coordinates relate to that model at a stated time. Physical survey marks once provided the main access to national datums. Their published coordinates allowed nearby work to connect with the larger network. Modern users often access the frame through continuously operating satellite-tracking stations. The [National Geodetic Survey](https://www.ngs.noaa.gov/datums/index.shtml) defines official U.S. datums within the National Spatial Reference System. Shared control lets engineers combine projects without creating a separate local origin each time. ## Horizontal datums locate points Transforming coordinates between datums is a calculation based on models and control data. Relabeling an old coordinate with a new datum name is incorrect. Professional software can perform the transformation, but users still need the proper source frame, target frame and geographic region. A horizontal or geometric datum defines latitude and longitude, often with ellipsoid height. The current U.S. framework uses realizations of the **North American Datum of 1983**, known as NAD 83. Older maps may use NAD 27. That system was built from ground surveys and fitted to North America. Coordinates transformed incorrectly between NAD 27 and NAD 83 can move a feature enough to affect boundaries or field navigation. WGS 84 is familiar because GPS uses it. It is maintained by the U.S. Department of Defense and is closely aligned with global reference systems. NAD 83 differs in its definition and realization, so high-accuracy work cannot treat the names as interchangeable. NOAA's explanation of [horizontal datums](https://www.ngs.noaa.gov/datums/horizontal/) lists historical realizations and their periods of use. Metadata should identify the complete datum tag rather than saying only that coordinates are in NAD 83. ## Vertical datums define height Elevation requires a zero surface. A geodetic vertical datum uses a gravity-related reference that approximates mean sea level across a large region. The North American Vertical Datum of 1988, or **NAVD 88**, remains the current official U.S. vertical datum during modernization. Orthometric height is the elevation people usually mean when describing land above sea level. It differs from ellipsoid height measured directly by satellite positioning. A geoid model connects the two. Gravity varies because Earth's mass is distributed unevenly. The geoid follows a surface where gravity potential is equal, producing gentle rises and dips relative to the ellipsoid. Better gravity data improves height conversion. Construction and flood mapping depend on consistent heights. A levee crest measured in one datum cannot be compared safely with a flood elevation in another until both are transformed. The [USGS 3D Elevation Program](https://www.usgs.gov/programs/3d-elevation-program) distributes elevation data with documented reference systems. Users still need to inspect metadata before combining terrain layers. ## Tidal datums follow local water A tidal datum is calculated from water-level observations at a specific place. Mean lower low water commonly serves as the chart datum for depths in U.S. coastal waters, while other tidal levels support shoreline and engineering work. Because tides vary along the coast, a tidal datum is local. It is not a continuous global surface. Connections between tide gauges and geodetic control allow data to move between tidal and land-height references. NOAA's [datum update program](https://tidesandcurrents.noaa.gov/datum-updates/) is revising the National Tidal Datum Epoch used to compute official tidal datums. A newer epoch captures changes in water levels and coastal conditions since the previous analysis period. ## Satellite positioning still needs a frame A GNSS receiver calculates position from signals transmitted by satellites. The result is referenced to the system behind the satellite orbits and ground stations. Satellite accuracy does not remove the need to name that frame. High-precision surveying accounts for antenna height and atmospheric delay. It must also account for station motion. Continuously Operating Reference Stations provide data that connect local observations to the national system. Coordinates also have an epoch, which states when they apply. Tectonic plates move centimeters each year. A stable monument can therefore acquire different global coordinates over time even though it has not moved relative to nearby land. **Coordinate transformations** use models to translate among frames and epochs. Entering a datum conversion as a simple constant offset can fail because differences vary with location. ## The U.S. reference system is changing NOAA is modernizing the National Spatial Reference System because NAD 83 is offset from Earth's center of mass and NAVD 88 contains regional bias. The project replaces ground-mark dependence with GNSS access and a gravity-based height system. The agency's [new datum program](https://www.ngs.noaa.gov/datums/newdatums/index.shtml) defines four plate-fixed terrestrial reference frames. A new geopotential datum will use an updated geoid model for orthometric heights. Plate-fixed frames minimize routine coordinate change within their regions. Deformation models will address motion that a rigid plate rotation cannot capture, such as subsidence or earthquake displacement. Transition tools are essential because existing maps and legal records will remain in older datums. Agencies need documented transformations and testing before changing operational systems. Staff training is another requirement. The modernized system does not make old data useless. It makes the reference difference explicit so past surveys can be converted with an uncertainty appropriate to their original measurements. ## Why the datum name belongs in metadata Datum information is essential when data sets are combined. A road centerline, flood surface and property boundary can appear aligned at a small map scale even when reference differences remain hidden. Those offsets become consequential during construction or precise boundary work, where centimeters may affect a decision. Coordinates copied without a datum invite hidden error. A complete record includes the reference frame and realization, the vertical datum when heights are used and the coordinate epoch for precise dynamic work. Software may apply a transformation automatically, but the result is trustworthy only when the input metadata is correct. Guessing from the file's location or age can place boundaries on the wrong side of a road. Surveyors also preserve units and projection information. A map projection converts curved coordinates to a flat grid; it is related to the datum but performs a different job. Flood models, nautical charts and construction plans all rely on a common spatial foundation. **Reference-system metadata** makes that foundation visible and allows future users to reproduce the intended position. **Related reading:** [why sea level is not the same everywhere](https://www.argo.net/is-sea-level-the-same-everywhere/) and [how land responds after ice loss](https://www.argo.net/what-is-glacial-isostatic-adjustment/). **Related reading:** [why sea level is not the same everywhere](https://www.argo.net/is-sea-level-the-same-everywhere/) and [how land responds after ice loss](https://www.argo.net/what-is-glacial-isostatic-adjustment/). **Explore this topic:** [Why Do We Still Need Lighthouses?](https://www.argo.net/why-do-we-still-need-lighthouses/) and [What Is Latitude?](https://www.argo.net/what-is-latitude/).