A vertical datum is a reference surface that defines zero elevation, the baseline from which all heights and depths are measured. Every time you see an elevation number on a topographic map, a flood zone designation, or a trail marker telling you you’re 5,000 feet above sea level, that number is measured relative to a vertical datum. Without one, elevation would be meaningless, since there’s no natural “zero” built into the landscape.
How a Vertical Datum Works
Think of a vertical datum as an agreed-upon floor for measuring height. If you say a mountain peak sits at 4,392 feet, the natural follow-up is: 4,392 feet above what? The vertical datum answers that question. It provides the zero surface, and every elevation value is simply a distance above (or below) that surface.
Traditionally, surveyors established vertical datums using a technique called geodetic leveling. Teams moved across the country with precision instruments, measuring tiny height differences between benchmarks, metal disks set into concrete or bedrock. These benchmarks formed a network, and by tying that network to a known water-level observation (typically mean sea level at a coastal tide gauge), every inland point could be assigned an elevation. The result was a consistent, nationwide system of heights.
Tidal Datums vs. Geodetic Datums
Not all vertical datums work the same way. The two main categories are tidal datums and geodetic datums, and they serve different purposes.
Tidal datums are based on actual water-level observations at specific coastal locations. NOAA collects tide data over a 19-year cycle (called an epoch) and averages it to create reference points like Mean Sea Level (MSL), Mean High Water (MHW), and Mean Lower Low Water (MLLW). MLLW, the average of the lowest daily low tide over that 19-year period, is the standard reference for nautical charts and tide predictions. When a chart says a channel is 12 feet deep, that depth is measured from MLLW. MHW, the average of all high tides, often defines legal shoreline boundaries.
Geodetic datums, by contrast, measure the height of land rather than water. They’re built from survey networks and gravity measurements, and they’re designed to work consistently across an entire continent, not just at the coast. The current official geodetic vertical datum in the United States is the North American Vertical Datum of 1988 (NAVD 88).
One important distinction: Mean Sea Level and a geodetic datum are not the same thing, even though people often treat them interchangeably. Because ocean currents, temperature, salinity, and atmospheric pressure all affect local water levels, actual mean sea level varies from place to place. A geodetic datum represents a best-fit surface over a broad area, so its relationship to local mean sea level differs depending on where you are.
The Role of Gravity and the Geoid
Gravity is what makes vertical datums tricky. The Earth isn’t a perfect sphere. It’s lumpy, with denser rock in some places and lighter material in others. These density variations cause gravity to pull slightly harder in some spots, which means a level water surface (one where water would naturally rest if it could flow freely) isn’t a smooth shape. That bumpy, gravity-defined surface is called the geoid, and it’s the most physically meaningful reference for “sea level” because water naturally follows gravity.
Modern satellite positioning systems like GPS measure height above a smooth mathematical shape called an ellipsoid, which is a simplified model of Earth’s surface. The ellipsoid ignores gravity variations, so GPS-derived heights don’t match the elevations you’d see on a topographic map. To convert between the two, you need to know the separation between the ellipsoid and the geoid at your location. That conversion follows a simple relationship: subtract the geoid-ellipsoid separation from the GPS height, and you get an orthometric height, the kind of elevation people actually use.
The most widely used global model of the geoid is EGM2008 (Earth Gravitational Model 2008). In areas with good gravity data, it matches independent ground-truth measurements to within about 5 to 10 centimeters. Using gravity to define a reference surface also solves a practical problem: it lets you determine where sea level “would be” even hundreds of miles from any coast.
NGVD 29 and NAVD 88
The United States has used two major geodetic vertical datums over the past century. The first, the National Geodetic Vertical Datum of 1929 (NGVD 29), was based on a network of leveling surveys tied to 26 tide gauges along the Atlantic, Pacific, and Gulf coasts. It served the country for decades, but problems accumulated over time.
By the 1980s, roughly 625,000 kilometers of new leveling data had been added to the national network since 1929. Thousands of original benchmarks had been destroyed, mostly by highway construction after World War II. Others had shifted due to earthquakes, the slow rebound of land that was compressed under glaciers during the last ice age, and ground subsidence from pumping underground water and oil. Forcing all the new survey data to fit the old 1929 values introduced serious distortions. The largest relative discrepancy, between St. Augustine, Florida, and Fort Stevens, Oregon, reached 86 centimeters (nearly three feet).
NAVD 88 fixed these problems through a complete readjustment of the leveling network. Instead of tying the datum to multiple tide gauges (each with its own local biases), the adjustment held fixed a single tidal benchmark at Father Point/Rimouski in Quebec, Canada. This minimized distortions across the network and produced a more internally consistent set of heights.
Why the Datum You Use Matters
Choosing the wrong vertical datum, or mixing up two different ones, can have real consequences. The clearest example is flood mapping. FEMA’s Flood Insurance Rate Maps (FIRMs) show base flood elevations, the water height expected during a 100-year flood. Builders compare their proposed structure elevations to these base flood elevations to make sure new construction will be reasonably safe from flooding. The margin above the base flood elevation is sometimes kept to a minimum for cost reasons, so even a small error in the reference datum can put a building in the flood zone.
The challenge is that many flood maps were originally created using NGVD 29, while newer elevation data uses NAVD 88. The difference between the two datums varies by location, sometimes by more than a foot. If a builder reads a base flood elevation referenced to NGVD 29 but measures their site elevation using NAVD 88 data, the numbers won’t match correctly. FEMA generally considers a change significant enough to require republishing a flood map when it shifts the flood elevation by more than half a foot, and differences of a foot or more trigger a full restudy of the affected area.
This same issue affects any field that relies on precise elevations: stormwater engineering, road grading, pipeline design, and coastal management all depend on everyone using the same datum.
The Coming Shift to NAPGD2022
NAVD 88 is itself being replaced. The National Geodetic Survey plans to roll out a modernized National Spatial Reference System in 2025 or 2026, which will include a new vertical reference called the North American-Pacific Geopotential Datum of 2022 (NAPGD2022).
The key change is how heights will be determined. Rather than relying on physical benchmarks connected by leveling surveys, NAPGD2022 will use a high-resolution geoid model built from satellite data and airborne gravity measurements (collected through NOAA’s GRAV-D project). This means accurate elevations will be available anywhere a GPS receiver can get a signal, not just near a physical benchmark. After release, the system will go through at least six months of public testing before the Federal Geodetic Control Subcommittee votes on final approval, likely in 2026. Full tools and services are expected within five years of the initial rollout.
For anyone working with elevation data, the transition will eventually require updating reference values, just as the shift from NGVD 29 to NAVD 88 did. But the long-term payoff is a more accurate, more accessible, and more consistent system for measuring height across North America.