A reference surface used to measure heights and elevations of points on the Earth's surface.

Vertical Datum

Overview

A vertical datum is a reference surface or level from which all vertical measurements, such as elevations and heights, are measured in surveying, mapping, and civil engineering applications. It serves as the fundamental baseline for determining how high or low any point is relative to a chosen reference level.

Definition and Purpose

The vertical datum establishes a standardized zero point for elevation measurements across a geographic region or entire country. Without a consistent vertical datum, elevation data from different surveys would be incompatible and could not be reliably compared or integrated. The primary purpose of a vertical datum is to provide a uniform, recognized reference system that allows surveyors, engineers, and planners to accurately document terrain heights and design infrastructure appropriately.

Types of Vertical Datums

Tidal Datums are based on ocean water levels and are commonly used in coastal areas. These include mean sea level (MSL), which is calculated from long-term observations of tide gauges. MSL has historically been the primary vertical datum used in many countries.

Geodetic Datums are based on the Earth's gravitational field and geoid, which is the equipotential surface closest to mean sea level. Modern vertical datums increasingly use the geoid as their reference surface.

Local Datums may be established for specific projects or regions where a temporary reference point is set to local conditions.

National Vertical Datums

Most countries maintain official vertical datums established through government surveying agencies. In the United States, the National Geodetic Vertical Datum (NGVD) was historically used, but has been replaced by the North American Vertical Datum of 1988 (NAVD 88). Canada uses the Canadian Geodetic Vertical Datum (CGVD).

These national datums are realized through networks of benchmark monuments distributed across the country. Benchmarks are permanent markers with precisely determined elevations that serve as reference points for local surveys.

Datum Establishment

Vertical datums are established through precise leveling surveys conducted over many years. Surveyors use spirit levels, electronic levels, and modern GPS/GNSS technology to create networks of control points with known elevations. Tidal gauge stations in coastal areas provide data for calculating mean sea level, which traditionally anchored the datum.

Modern Developments

Contemporary vertical datums are increasingly based on geoid models rather than tidal observations alone. This allows for greater consistency across regions and improved compatibility with GPS-derived heights. The relationship between ellipsoidal heights (from GPS) and orthometric heights (traditional elevations) is defined by geoid undulation values.

Applications

Vertical datums are essential for:

Construction and infrastructure design

Flood risk mapping and management

Hydrological studies

Land surveying and property boundaries

Navigation and coastal charting

Urban planning and development

Environmental monitoring

Datum Transformation

When surveys from different vertical datums need to be compared or integrated, transformation equations must be applied. These transformations account for differences between datums, which can vary significantly across large geographic areas.

Conclusion

The vertical datum remains a cornerstone of surveying practice, providing the essential reference framework for all elevation-based measurements and analyses. As surveying technology evolves, vertical datums continue to be refined for greater accuracy and utility in an increasingly data-integrated world.