Vertical Datum – Reference for Elevation Measurements in Surveying

A vertical datum is the foundational reference surface from which elevations—heights above or below a defined zero level—are measured. This concept is central to surveying, mapping, engineering, aviation, hydrography, and all geospatial sciences. A common vertical datum ensures that elevation data from different sources or projects are directly comparable, enabling accurate data integration for flood risk analysis, construction, topographic mapping, coastal management, and more.

The role of a vertical datum becomes critical when integrating datasets, as discrepancies of even a few centimeters can have significant consequences in engineering or environmental planning. For example, the United States uses the North American Vertical Datum of 1988 (NAVD 88) as the standard elevation reference for most federal and state projects. Other countries use their own national or regional datums, often based on mean sea level observed at specific tide gauges.

A vertical datum can be realized in two principal ways:

• Physical realization, using a network of benchmarks connected by precise leveling.
• Mathematical modeling, using reference surfaces such as the geoid or ellipsoid, often in conjunction with satellite-based positioning.

Types of Vertical Datums

Tidal Datums

Tidal datums are derived from long-term observations of sea level at specific tide gauges, typically averaged over a 19-year tidal epoch. Common tidal datums include:

• Mean Sea Level (MSL): The average sea level over the tidal epoch.
• Mean High Water (MHW): The average of all high tides.
• Mean Lower Low Water (MLLW): The average of the lowest daily tides.

Tidal datums are inherently local, as sea level varies due to currents, atmospheric pressure, and geography. In the United States, NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS) defines and maintains tidal datums. International standards, such as the IHO S-44, require explicit documentation of the tidal datum and epoch used in hydrographic products.

Example: Nautical charts often use MLLW as the reference for sounding depths, providing mariners with the minimum expected water depth.

Geodetic (Orthometric) Datums

Geodetic or orthometric datums are defined by nationwide leveling networks and gravity measurements, referenced to a physically meaningful surface like the geoid. The geoid is a model of mean sea level extended beneath the continents and is the best approximation of Earth’s “zero” elevation when accounting for gravitational differences.

• NAVD 88: The primary vertical datum for the U.S., established by continent-wide leveling.
• NGVD 29: The earlier U.S. datum, now replaced due to gravity-related inconsistencies.

Geodetic datums are used in large-scale mapping, engineering, and scientific research, and are supported by organizations like the International Association of Geodesy (IAG).

Local Datums

Local datums are established for limited geographic areas, often for site-specific projects or historical convenience. These may be based on a single benchmark and may not correspond to mean sea level or the geoid. Local datums should be rigorously documented and related to national datums to avoid inconsistencies.

Key Concepts and Terms

Geoid

The geoid is an equipotential surface representing mean sea level globally, undisturbed by tides or currents. It is irregular and undulating due to Earth’s variable gravity, and serves as the reference for orthometric heights.

Modern geoid models, such as GEOID18 in the United States, allow conversion from GNSS-derived ellipsoidal heights to orthometric heights compatible with NAVD 88 and other datums.

Reference Ellipsoid

A reference ellipsoid is a mathematically defined, smooth surface approximating Earth’s shape, with specific parameters for equatorial radius and flattening. It is used in satellite navigation (GPS/GNSS) and mapping.

• WGS84: The global ellipsoid used for GPS.
• GRS80: Used for NAD 83 in North America.

Ellipsoidal heights must be corrected using geoid models to obtain physically meaningful elevations.

Orthometric Height

Orthometric height (H) is the elevation above the geoid (mean sea level), measured along the direction of gravity. It is the standard for most mapping, construction, and floodplain management.

Formula:
H = h – N
where h is ellipsoidal height and N is geoid height.

Ellipsoidal Height

Ellipsoidal height (h) is the vertical distance from the reference ellipsoid to a point on Earth’s surface, as measured by GPS/GNSS. It is not physically meaningful without geoid correction.

Geoid Height (Separation)

Geoid height (N) is the difference between the geoid and reference ellipsoid at a specific location. It is used to convert ellipsoidal heights to orthometric heights.

Benchmark

A benchmark is a permanently installed marker with a precisely known elevation, referenced to a vertical datum. Benchmarks are used for local surveying, mapping, and construction control.

National Spatial Reference System (NSRS)

The NSRS is the United States’ official coordinate system, maintained by NGS. It includes both horizontal and vertical datums (e.g., NAD 83, NAVD 88), a network of benchmarks, and a system of GNSS reference stations (CORS).

North American Vertical Datum of 1988 (NAVD 88)

NAVD 88 is the official vertical datum for North America, established by continent-wide adjustment of leveling networks and used for all federal mapping, floodplain management, and engineering projects.

National Geodetic Vertical Datum of 1929 (NGVD 29)

NGVD 29 is the predecessor to NAVD 88, based on sea level observations at multiple tide gauges but lacking corrections for gravity variations. It is now obsolete but still found in legacy data.

GEOID18

GEOID18 is the most recent U.S. geoid model, used to convert GNSS heights to NAVD 88 orthometric heights with improved accuracy. It supersedes earlier models and is distributed by NGS.

Tidal Epoch

A tidal epoch is a standardized 19-year period over which tidal datums are defined—currently 1983–2001 in the U.S. It averages out short-term variations due to astronomical cycles and climate.

Applications and Importance

• Surveying and Mapping: Provides consistent elevation references for topographic maps, cadastral surveys, and GIS.
• Engineering and Construction: Ensures correct elevations for infrastructure, floodplain boundaries, and grading.
• Aviation: Used for runway elevation, instrument approach procedures, and obstacle clearance.
• Hydrography: Reference for charting depths, tidal predictions, and coastal management.
• Disaster Management: Essential for flood risk assessment, evacuation planning, and climate resilience.

Best Practices

• Always document the vertical datum, including geoid model and epoch, for all elevation data.
• Use national or international standards (e.g., NAVD 88, WGS84) rather than local or arbitrary datums.
• Apply transformations and geoid corrections when integrating datasets from different sources.
• Regularly update elevation data as new datums and geoid models are released.

References

• NOAA National Geodetic Survey – Vertical Datums
• International Association of Geodesy (IAG)
• International Hydrographic Organization (IHO) S-44 Standards
• USGS – Benchmarks and Control

A vertical datum is the backbone of reliable elevation data. By standardizing elevation references across disciplines and geographies, it underpins safe navigation, resilient infrastructure, and effective environmental management.