Vertical Datums Explained

This article continues the Elevation & Vertical Datums sub-hub from /learn/mean-sea-level-explained. Where the MSL article covered the foundational measurement, this one covers how vertical datums are formally defined, realized, and used in modern surveying, engineering, and GIS. Cross-links into the existing /learn/horizontal-vs-vertical-datum support in the Datums sub-hub.

The two height types

Heights in modern geodesy come in two fundamentally different forms.

Ellipsoidal height (h)

Height above the reference ellipsoid — the mathematical idealized shape used as Earth's reference (typically the WGS 84 ellipsoid; see /learn/wgs84-explained). GPS naturally produces ellipsoidal heights because GPS positioning is purely geometric (satellite distances relative to the Earth-centered Earth-fixed frame).

Ellipsoidal height is:

• Easy to measure: any GPS gives it directly.
• Mathematically well-defined: no ambiguity in the reference surface.
• Useless for water flow: water doesn't care about the ellipsoid; it flows along the gravity field.

Orthometric height (H)

Height above the geoid — the equipotential surface of Earth's gravity that the ocean would assume in static equilibrium. Orthometric heights have intuitive physical meaning:

• Two points at the same orthometric height have water that won't flow between them.
• Building foundations and water-level structures reference orthometric heights.
• Maps showing “elevation” (USGS topo maps, hiking maps) use orthometric heights.

The geoid undulation N

The difference between the two:

h = H + N

Where N is the geoid undulation (also called geoid height) — the height of the geoid above the ellipsoid at a given location. N varies smoothly across Earth's surface, ranging from about -106 m (south of India) to +85 m (over Indonesia/Papua New Guinea), with most land areas in the ±50 m range.

Computing N requires a geoid model — a mathematical representation of Earth's gravity field. The dominant global models are EGM2008 and EGM2020 (see below).

For GPS-derived heights: compute h directly from GPS, look up N from a geoid model, derive H = h - N. This is the standard workflow for GPS-based surveying that wants orthometric heights.

The North American Vertical Datum of 1988

NAVD88 is the current official vertical datum for the United States, Canada, and Mexico. Adopted in 1991, replacing the National Geodetic Vertical Datum of 1929 (NGVD29).

Realization

NAVD88 was realized through approximately 70,000 km of precise spirit leveling — surveyors with optical levels measuring height differences between benchmarks. The leveling network covers the contiguous US, Canada, and parts of Mexico.

The datum is anchored at the Father Point / Pointe-au-Père tide gauge near Rimouski, Quebec. This single point defines “zero” for the entire NAVD88 system; all other heights are measured relative to it through the leveling network.

Known issues

NAVD88 has known biases that motivate its replacement:

• Continental tilt: NAVD88 is tilted about 50 cm from mean sea level across the continental US — lower in the west, higher in the east. This is because the realization process didn't weight multiple coastal tide gauges; the single Quebec anchor introduces systematic error elsewhere.
• Single tide-gauge anchor: tying the datum to one tide gauge makes it sensitive to local vertical land motion at that gauge. Father Point has moved (post-glacial rebound) since the 1988 realization.
• Out-of-date geoid: the geoid model used in NAVD88 has been superseded by more accurate models.

Geoid models for NAVD88

The official NGS geoid models for NAVD88 work:

• GEOID96 (1996): the original.
• GEOID99, GEOID03, GEOID09, GEOID12B: successive refinements.
• GEOID18 (2019): the current model, which gives the conversion between GPS-derived ellipsoidal heights and NAVD88 orthometric heights with ~2 cm accuracy across the continental US.

NAPGD2022: the planned replacement

The North American-Pacific Geopotential Datum of 2022 (NAPGD2022) is the planned replacement for NAVD88. Originally scheduled for 2022; rollout has been delayed multiple times due to data-processing complexity and is now expected in the mid-to-late 2020s.

Key features:

• Purely gravimetric: no tide-gauge anchor; the datum is defined by Earth's gravity field itself.
• No traditional leveling required: the realization comes from the GRAV-D airborne gravity survey program (see below).
• Higher accuracy: target ~2 cm accuracy across the continental US, comparable to NAVD88's best regional accuracy but uniform.
• Self-consistent globally: no continental tilt or single-anchor biases.
• Integrated with horizontal datum: NAPGD2022 is designed to work with the National Spatial Reference System 2022 (NSRS 2022) — the planned replacement for NAD 83.

GRAV-D

GRAV-D (Gravity for the Redefinition of the American Vertical Datum) is the airborne gravity survey program that provides the data for NAPGD2022.

• Started: 2007.
• Aircraft: NOAA Twin Otter and Cessna Citation-class.
• Sensors: airborne gravimeters measure local gravity acceleration to ~0.1 mGal precision.
• Survey lines: flown at ~10,000 m altitude, spaced ~10 km, covering all of the continental US, Alaska, Hawaii, US territories, and parts of Canada and Mexico.
• Completed: 2022 for continental US; ongoing in remote areas.
• Output: a high-resolution geoid model accurate to ~2 cm over the survey area.

The airborne approach is dramatically faster than traditional surveying: GRAV-D mapped all of the continental US in about 15 years; the original NAVD88 leveling campaign took decades.

European Vertical Reference Frame

The European Vertical Reference Frame (EVRF) is the official vertical datum for the European Union, with multiple realizations:

• EVRF2000: the first version.
• EVRF2007: revised with additional data.
• EVRF2019: the current realization, incorporating the latest United European Levelling Network (UELN) data.

EVRF is anchored at the Normaal Amsterdams Peil (NAP) — the Amsterdam tide gauge — which has been a European height reference since the Dutch infrastructure built it in the 17th century.

EVRF is the standard for trans-European projects (railways, highways, infrastructure that crosses borders). Individual countries also maintain national vertical datums (UK Ordnance Datum Newlyn, German DHHN 2016, etc.) that are tied to EVRF with known offsets.

Other regional datums

Australian Height Datum (AHD)

The official vertical datum for Australia, adopted in 1971 and based on tide-gauge fixings at multiple ports. The realization is through ~200,000 km of leveling. Heights in AHD are sometimes denoted AHD71 to indicate the realization year.

Ordnance Datum Newlyn (ODN)

The UK's official vertical datum, anchored at the Newlyn tide gauge in Cornwall (replacing the earlier Liverpool reference in 1921). Mean sea level 1915–1921 at Newlyn defines “zero.”

Russian Baltic Height System

Used in Russia and most former Soviet states. Anchored at the Kronshtadt tide gauge near St. Petersburg.

Tokyo Peil

The Japanese vertical datum. Anchored at a tide gauge in Tokyo Bay.

Many others

Most countries have their own vertical datum, typically anchored at a national tide gauge with local realization through leveling. The patchwork is the focus of IHRS modernization (below).

Earth Gravitational Models (EGM)

Global gravity-field models are the foundation of ellipsoidal-to-orthometric height conversion. They predict the geoid undulation N at any latitude/longitude.

EGM96

Earth Gravitational Model 1996, produced by NIMA (now NGA). Spherical-harmonic expansion to degree and order 360 (corresponding to ~50 km resolution). The first widely used global geoid model. Still embedded in some legacy GPS receivers.

EGM2008

Earth Gravitational Model 2008, released October 2008 by NGA (National Geospatial-Intelligence Agency). Spherical-harmonic expansion to degree and order 2160 (corresponding to ~10 km resolution). Major improvement over EGM96.

Data sources:

• GRACE satellite gravity measurements (2002–2017, twin satellites measuring inter-satellite distance).
• Surface gravity measurements from many countries.
• Satellite altimetry over ocean areas.

EGM2008 is the dominant global geoid model for practical applications. It's embedded in most GPS-based surveying software, most engineering applications, and most online geoid calculators.

EGM2020

Earth Gravitational Model 2020, released 2020 by NGA. Spherical-harmonic expansion to degree 2190 with refined coefficients incorporating:

• GRACE-FO (Gravity Recovery and Climate Experiment Follow-On) data (2018–present).
• Swarm satellite gravity measurements.
• GRAV-D airborne survey results.
• Updated surface gravity measurements.

EGM2020 is slightly more accurate than EGM2008 but the differences are small for most applications (sub-meter geoid undulation differences). Many engineering applications haven't migrated and continue using EGM2008. The migration is gradual.

Regional geoid models

For higher precision, regional or national geoid models exist:

• USGG2012 / GEOID18 (US National Geodetic Survey): fit specifically for the continental US, accurate to ~1–2 cm.
• DTU17 / DTU21: Danish Technical University global models.
• GRAV-D results will feed into a future high-resolution US model.

Regional models combine global EGM data with denser local measurements, achieving better accuracy in the covered area.

The International Height Reference System

The International Height Reference System (IHRS) is a long-running effort by the International Association of Geodesy (IAG) and the International Union of Geodesy and Geophysics (IUGG) to define a single global vertical datum.

Definition

IHRS was formally defined in 2015 by IAG Resolution

1. The defining elements:

• Reference potential: W₀ = 62,636,853.4 m²/s² (the gravity potential at the IHRS reference surface).
• No tide-gauge anchor: the datum is defined by the potential value alone, independent of any tide gauge.
• Realized through: globally distributed geodetic measurements (GNSS, satellite gravity, terrestrial gravity).

Realization

IHRS implementation is slow. Countries are expected to align their national vertical datums to IHRS over time. As of 2026:

• No country has fully migrated yet.
• Pilot studies in Germany, Australia, and others are underway.
• The first global IHRS realization (IHRF, the International Height Reference Frame) is being computed by IAG's Joint Working Groups.

The goal: heights in different countries should agree to within a few centimeters, supporting cross-border projects (water management in Europe, trans-continental infrastructure, sea-level monitoring).

Why is it so slow?

Vertical datums are deeply embedded in national infrastructure:

• Engineering drawings, GIS databases, surveying records, regulatory frameworks — all reference the national datum.
• Changing the datum requires updating millions of records.
• The cost-benefit is unclear for non-cross-border uses; most national applications are self-consistent in the current datum.

IHRS implementation will likely span decades, with major migrations coinciding with other datum modernizations (NAPGD2022 in North America, EVRF updates in Europe).

Modern surveying workflow

A typical modern surveying workflow combining the above:

1. GPS/GNSS receiver provides positions in WGS 84 ellipsoidal frame: latitude, longitude, and ellipsoidal height h.
2. Geoid model lookup: at the survey location, look up the geoid undulation N from EGM2020 or a regional model.
3. Orthometric height computation: H = h - N.
4. Datum conversion: if the project requires NAVD88 specifically (not just orthometric height), apply the NAVD88-specific geoid model (GEOID18) instead of EGM2020 for the conversion.
5. Cross-check: tie the GPS-derived height to nearby NAVD88 benchmarks via short-baseline leveling. Reduces N-model error.

For high-precision work (cm-level), the entire workflow may be done with Real-Time Kinematic (RTK) GPS (see /learn/rtk-gps) plus a regional geoid model, achieving 1–3 cm vertical accuracy.

Common misconceptions

“Vertical datum and horizontal datum are the same thing.” They're separate. A position on Earth requires both a horizontal datum (for lat/lon — typically WGS 84 or NAD 83) and a vertical datum (for height — typically NAVD88, EVRF, AHD, etc.). The two can be combined into a 3D datum, but they have separate realizations and uses. See /learn/horizontal-vs-vertical-datum.

“GPS gives orthometric heights.” GPS gives ellipsoidal heights. Conversion to orthometric height requires a geoid model. Many GPS receivers display orthometric heights by default, applying an internal geoid model (often EGM96 or EGM2008) automatically — but the underlying GPS measurement is ellipsoidal.

“Orthometric height is the same as elevation above sea level.” Approximately, but distinctly:

• Orthometric height: height above the geoid.
• Mean sea level (MSL): height of a 19-year tide-gauge average; differs from the geoid by sea surface topography (±1 m).
• For most everyday purposes the two are interchangeable; for precision applications they must be distinguished.

“NAVD88 is universal in North America.” The US, Canada, and parts of Mexico use NAVD88, but Mexico has variants and isolated regional datums exist in Alaska, US territories, etc. NAPGD2022 will unify these when adopted.

“EGM2008 is obsolete.” It's still widely used and accurate to ~10 cm globally — fine for most engineering applications. EGM2020 is the successor but the gradual migration means EGM2008 will remain in service for years.

“Vertical datums never change.” They do — NAVD88 is being replaced by NAPGD2022; NAD 83 is being replaced by NSRS 2022; EVRF is on its third realization. Vertical datum maintenance is an ongoing geodetic activity.

“Heights from different datums are interchangeable with a constant offset.” The offset between datums varies by location. Two points might have a +30 cm NAVD88-vs-NGVD29 offset; two other points might have a -45 cm offset. The full conversion requires either a transformation grid or re-leveling.

“The geoid is a smooth surface.” It's smooth over short distances but has substantial variation globally (±100 m). The variation is the geoid undulation, captured by EGM2020.

“The IHRS is in production use.” It's formally defined but not yet operationally adopted by any country. Pilot studies are underway; operational adoption is expected in the late 2020s or 2030s.

“Tide gauges define vertical datums forever.” Traditional datums (NAVD88, ODN, NAP) are tide-gauge-anchored at the realization epoch. As sea levels rise and land moves, the tide-gauge value drifts from current MSL. Gravimetric datums (NAPGD2022) avoid this by being gravity-defined rather than tide-gauge-defined.

“Engineers always need orthometric height.” Often, but not always. Aviation altimetry uses barometric altitude (referenced to atmospheric pressure standards). Maritime applications use chart datums (MLLW, LAT — see /learn/mean-sea-level-explained). GPS positioning uses ellipsoidal heights directly. The choice depends on the application.