A geodetic datum is the agreement that makes a coordinate mean something. Measurements by themselves are relative: a GNSS receiver measures ranges to satellites, a total station measures angles and distances between points, a level measures height differences. None of that becomes a latitude, a northing or an elevation until you fix a reference surface, fix its size and shape, fix where its center sits relative to the Earth, and fix the orientation of its axes. That whole package of agreements is the datum.
Surveying uses two families of datum because horizontal position and height are answering two different physical questions. A horizontal datum answers where am I on a smooth mathematical model of the Earth. A vertical datum answers which way does water flow. Those questions have different natural reference surfaces, so they get different datums, and a complete position always cites both.
The definition, unpacked
- Geodetic datum
- A reference system that defines the size and shape of a surface approximating the Earth, together with the position and orientation of that surface relative to the Earth itself, so that coordinates computed on it are repeatable and comparable.
Two ingredients hide inside that sentence, and separating them prevents most of the confusion in this subject. The first ingredient is the reference surface — for horizontal work an ellipsoid of revolution, described by a semi-major axis and a flattening. The second ingredient is the realization: the actual set of physical monuments and continuously operating stations, with published coordinates, that gives everyone else access to the datum. The surface is a definition. The realization is how you get onto it.
This distinction matters in practice. Nobody surveys from an equation. You survey from a monument in the ground or a reference station on a rooftop, and the coordinate published for that mark is the datum as far as your project is concerned. When that published coordinate is readjusted, the definition has not changed but your coordinates have.
Horizontal datums
A horizontal datum uses an ellipsoid because an ellipsoid is smooth, computable and a good global fit to sea level. You can run a geodetic inverse across a continent on an ellipsoid and get a closed answer. You cannot do that on the real, lumpy Earth.
The historical horizontal datums of North America are NAD 27, built on the Clarke 1866 ellipsoid and oriented by holding one station in Kansas fixed, and NAD 83, built on the GRS 80 ellipsoid and intended to be geocentric — its origin at the center of mass of the Earth. Modern satellite geodesy made the geocentric approach possible and, in hindsight, made the residual errors in the older approach visible.
A horizontal datum gives you latitude and longitude directly. It gives you a State Plane or UTM northing and easting only after a projection is applied, which is a separate agreement layered on top.
Vertical datums
A vertical datum cannot use an ellipsoid, because an ellipsoid has nothing to do with the direction water flows. Height in the engineering sense has to be referenced to a level surface — a surface everywhere perpendicular to gravity. The particular level surface chosen as zero is the geoid, and heights measured from it are orthometric heights.
The historical vertical datums of the United States are NGVD 29, which held a set of tide-gauge readings fixed, and NAVD 88, which held a single benchmark at Father Point in Rimouski, Quebec. Both are realized through a national network of levelled benchmarks rather than through an equation.
Why a datum is never finished
The Earth is not static. Tectonic plates move at centimetres per year, the crust rebounds from vanished ice sheets, coastal ground subsides, and earthquakes displace whole regions instantly. A datum realized in 1986 from the measurements then available will not match a datum realized in 2011 from far better measurements on ground that has since moved.
So a datum acquires versions. NAD 83 has several realizations, each a readjustment of the network, and each carrying a reference epoch — a date the coordinates are valid for. Ignoring the realization is like ignoring the datum: it silently costs you centimetres to decimetres.
What is changing
The National Geodetic Survey is replacing both national datums as part of modernizing the National Spatial Reference System. NAD 83 is being replaced by the North American Terrestrial Reference Frame of 2022, and NAVD 88 by the North American-Pacific Geopotential Datum of 2022. The horizontal replacement is a properly geocentric, plate-fixed frame with an explicit velocity model. The vertical replacement abandons the levelled-benchmark model in favour of a gravimetric geoid, so that a GNSS observation plus a geoid model produces an orthometric height directly.
That work is in progress. What a surveyor should take from it now is procedural rather than technical: label every coordinate you publish with its datum, its realization and its epoch, because the answer to which datum is this is about to get harder, not easier.
How to state a position completely
- The horizontal datum and its realization, for example NAD 83 (2011).
- The reference epoch of the horizontal coordinates, for example 2010.00.
- The coordinate system and zone if the values are projected, for example a named State Plane zone.
- The linear unit, stated explicitly — meters, US survey feet or international feet.
- The vertical datum, for example NAVD 88.
- The geoid model used, if the orthometric height came from GNSS.
A coordinate missing any of those six items is ambiguous by somewhere between a few centimetres and a few hundred meters. Most of the datum errors that reach a courtroom or a construction claim are not computational mistakes. They are labelling failures.