In a legal description, a datum is the fixed reference surface that every coordinate, elevation, or distance in the document is measured from. It is the agreed starting point that turns a latitude, longitude, or height into a location anyone else can find on the ground. Asking what a datum is in a legal description is really asking why those numbers need an anchor at all: Earth is not a perfect sphere, its gravity is uneven, and its surface moves with the tectonic plates, so two surveyors measuring the same corner can produce different numbers unless they agree on the model they are measuring against.
Pick the wrong datum, or leave it out entirely, and a boundary line can shift dozens of feet or a flood elevation can be off by enough to change an insurance premium.
What a Datum Actually Does
A datum is the “zero” that everyone measures from. When a description says a corner sits at a particular latitude and longitude, those numbers only mean something once you know which mathematical model of Earth’s shape produced them. The model accounts for Earth’s bulge at the equator, its uneven gravity, and the slow drift of its crust, and it locks measurements to a defined surface so that work done by different surveyors in different years lines up.
Two kinds of models do this work. An ellipsoid is a smooth approximation of Earth’s shape used for horizontal measurements like latitude and longitude. A geoid is an irregular surface shaped by gravity that better represents actual sea level and is used for elevations. Horizontal datums ride on ellipsoids; vertical datums ride on geoids. That is why a single property can reference two different datums in the same document: one for its boundary coordinates and one for its elevations.
The Datums You’ll See Named in a Description
Three categories show up in U.S. legal descriptions: horizontal datums for position, vertical datums for elevation, and tidal datums for coastal boundaries.
Horizontal Datums (NAD 83 and NAD 27)
The standard horizontal datum across the United States is the North American Datum of 1983, usually written as NAD 83. It replaced the North American Datum of 1927 (NAD 27), which was built from fewer survey points and a less accurate ellipsoid.1National Geodetic Survey. North American Datum of 1983 (NAD 83) – Horizontal and Geometric Datums Federal filings that require coordinates generally require NAD 83; the FCC, for instance, mandates NAD 83 for every geographic coordinate submitted through its licensing system.2Federal Communications Commission. North American Datum Of 1983 Coordinates
The gap between the two is not small. Within the lower 48, the same physical point can carry coordinates that differ by 10 to 100 meters on the ground depending on which datum is used.3U.S. Geological Survey. How Large Is the North American Datum of 1927 (NAD 27) to NAD 83 Shift A description written in NAD 27 and read as NAD 83 can place a boundary line hundreds of feet from where it belongs. Older deeds recorded before the mid-1980s sometimes use NAD 27 without saying so, which is one reason surveyors treat datum identification as non-negotiable.
Vertical Datums (NAVD 88 and NGVD 29)
Vertical datums set a zero-elevation surface for measuring heights. The current vertical datum for the contiguous United States and Alaska is the North American Vertical Datum of 1988 (NAVD 88), the official standard since 1993.4National Geodetic Survey. North American Vertical Datum of 1988 (NAVD 88) – Vertical Datum It replaced the National Geodetic Vertical Datum of 1929 (NGVD 29). The difference between them can reach several feet depending on location: in parts of New York City, converting between NAVD 88 and older local datums produces shifts of roughly 1.1 to 3.2 feet.5FEMA Region II Coastal. Understanding Vertical Datums: Frequently Asked Questions
Tidal Datums and Coastal Boundaries
Along coastlines, a third category of datum defines where private land ends and publicly owned tidelands begin. Tidal datums are derived from observed water levels averaged over a 19-year cycle called the National Tidal Datum Epoch (currently 1983–2001, with an update based on 2002–2020 data in progress).6NOAA Tides & Currents. Tidal Datums The most legally significant is Mean High Water, the average height of all high tides over the epoch. In coastal states, Mean High Water typically serves as the legal boundary between private upland and state-owned foreshore. Because tidal datums shift as sea levels change and as new epochs are adopted, a coastal boundary can move over time even when no one touches the deed.
How Datums Appear on the Page
A well-drafted legal description states its datum outright. When coordinates are involved, you will see a line like “all coordinates are referenced to NAD 83” or “elevations are based on NAVD 88.” The U.S. Fish and Wildlife Service puts the rule plainly: “latitude plus longitude plus datum equals a complete coordinate,” and leaving the datum off forces users to guess, which routinely plots features in the wrong place.7U.S. Fish and Wildlife Service. Datums, Projections, and Coordinate Systems Nothing in a coordinate value itself reveals which datum it belongs to, so the datum must always be stated separately.
State Plane Coordinates
Many modern descriptions use State Plane Coordinates instead of raw latitude and longitude. Each state defines one or more zones that project NAD 83 positions onto a flat grid, producing easting and northing values in feet or meters. A description using this system will specify both the zone and the underlying datum, such as “Delaware Coordinate System of 1983, based on the North American Datum of 1983.” The datum reference is built into the coordinate system’s definition, so changing datums changes the coordinates even for the same physical point.
Benchmarks
Datums connect to the real world through benchmarks, which are permanent markers (often brass disks set in concrete or bedrock) whose positions and elevations have been precisely determined against a known datum. A surveyor preparing an elevation certificate for your property typically starts from a nearby benchmark with a published NAVD 88 elevation and transfers that height to your building with leveling instruments. If the benchmark references an older datum, the surveyor must convert before recording any measurement on a document that calls for a current one.
What Goes Wrong When the Datum Is Missing or Wrong
Boundary disputes are the most visible consequence. A deed recorded in the 1970s using NAD 27 coordinates, read today as NAD 83, can push a boundary tens of meters off. A title company reviewing a chain of title needs each description’s coordinates to be internally consistent, and a mismatch in datums can cloud title until a surveyor sorts out which datum each document actually used.
Flood insurance is where vertical datum errors hit the hardest financially. FEMA’s elevation certificate requires that all elevations match the datum used for the base flood elevation on the applicable flood map.8Federal Emergency Management Agency. Elevation Certificate and Instructions If a certificate shows a building two feet above the flood line but the surveyor measured against the wrong datum, those two feet may not exist. The owner ends up underinsured, or pays years of premiums priced to the wrong risk category.
Construction is the third pressure point. Building codes in floodplains often require finished floors to sit at or above a specified elevation. Designing to the wrong vertical datum can produce a structure that violates local requirements even though the contractor built exactly to the plans.
The Coming Replacement of NAD 83 and NAVD 88
Both major U.S. datums are being replaced. The National Geodetic Survey is rolling out a modernized National Spatial Reference System that will swap NAD 83 for four new terrestrial reference frames, one each for the North American, Pacific, Caribbean, and Mariana tectonic plates. The frame covering the lower 48 states is the North American Terrestrial Reference Frame of 2022 (NATRF2022). NAVD 88 will give way to a gravity-based geopotential datum built from an updated geoid model.9National Geodetic Survey. New Datums
The practical improvement is that each new frame rotates with its tectonic plate, so coordinates for points on stable parts of the plate will change far less over time than they do under NAD 83. As of early 2026, the Federal Geodetic Control Subcommittee has not yet voted to approve the modernized system, though NGS expects that vote in early to mid-2026. Until then, NAD 83 and NAVD 88 remain official.10National Geodetic Survey. Release of the Modernized NSRS
Existing deeds will not need to be rewritten. NGS provides transformation tools, including the free NCAT coordinate converter and the VDatum software for vertical conversions, that let surveyors translate between legacy and new datums.11NOAA/NOS. VDatum 4.8: Vertical Datums Transformation Any legal description written after the transition that uses coordinates should still name the new datum directly to avoid ambiguity down the line.
Fixing a Datum Error in a Recorded Deed
When a recorded deed names the wrong datum, or names none at all, the fix usually depends on how serious the mistake is. A missing or misstated datum generally counts as a clerical error rather than a change to what the parties intended, which simplifies the correction.
Two common remedies cover most situations. A scrivener’s affidavit is a sworn statement by the person who drafted the deed, identifying the specific error, and it gets recorded alongside the original instrument. A correction deed is a new deed that restates the legal description with the correct datum and references the original recording information. Either document enters the public record so that anyone searching title can see what changed and why.
Harder cases, where the wrong datum caused the description to identify the wrong parcel entirely, or where the parties disagree about what was intended, can require a court action to reform the deed. Reformation requires proof of the original intent and of the mismatch between that intent and the current language. This is rarer with datum errors specifically, because physical markers on the ground usually make the intended boundary clear, but it does come up when older descriptions combine outdated datums with vague reference points.
The cheapest route is to catch the problem before closing. A surveyor doing a boundary survey will flag datum issues as part of the normal review, and fixing a draft description costs a small fraction of correcting a recorded deed later.