6.3 Horizontal and Vertical Datums

What a Datum Is

A datum is the reference frame that gives coordinates meaning. A horizontal (geodetic) datum ties latitude and longitude to a specific ellipsoid and a network of monumented control; a vertical datum defines the zero surface for elevations. A coordinate pair without its datum is ambiguous, because the same physical point has different numerical coordinates in different datums. The FS exam tests this repeatedly: stating that a point is at a given latitude/longitude or elevation is incomplete unless the datum (and realization/epoch) is named.

Horizontal datums in US practice:

• NAD 27 (North American Datum of 1927) - based on the Clarke 1866 ellipsoid, fixed at Meades Ranch, Kansas, with a non-geocentric origin. It is a local, distortion-prone datum.
• NAD 83 (1986 and later realizations such as NAD 83(2011)) - based on the GRS80 ellipsoid, originally intended to be geocentric. It is the current official horizontal datum but is now known to be offset from the true geocenter by about 2.2 m.

The shift from NAD 27 to NAD 83 moves coordinates by tens to over a hundred meters, varying by location, so the two can never be used together without a transformation.

Vertical Datums and Transformations

Vertical datums in US practice:

• NGVD 29 (National Geodetic Vertical Datum of 1929) - the older elevation datum, tied to 26 tide-gauge sea-level determinations.
• NAVD 88 (North American Vertical Datum of 1988) - the current official datum, adjusted to a single primary benchmark (Father Point/Rimouski, Quebec). NGVD 29 and NAVD 88 differ by variable amounts (commonly a few decimeters to over a meter across the country), so elevations must be labeled with their datum.

Transformations are not relabeling. To move coordinates between datums you must apply published parameters or interpolation grids:

Using the wrong tool, or assuming a constant shift, produces errors of meters - a classic FS distractor.

The 2022 NSRS Modernization

The National Geodetic Survey is replacing both NAD 83 and NAVD 88 in its modernized National Spatial Reference System (NSRS). The FS exam now expects awareness of this transition. Key facts:

• NAD 83 is replaced by four plate-fixed terrestrial reference frames, each rotating with its tectonic plate and defined relative to the International Terrestrial Reference Frame (ITRF2020): NATRF2022 (North America), PATRF2022 (Pacific), CATRF2022 (Caribbean), and MATRF2022 (Mariana).
• NAVD 88 is replaced by the North American-Pacific Geopotential Datum of 2022 (NAPGD2022), a gravimetric geoid-based vertical datum whose model is GEOID2022.
• Motivation: NAD 83 is non-geocentric by ~2.2 m and NAVD 88 is biased ~0.5 m and tilted ~1 m coast to coast relative to modern global geoid models.
• The new frames are time-dependent (coordinates carry an epoch) because tectonic plates move. Official transition is anticipated around 2027.

Expect coordinate shifts of roughly a meter or more when transforming from NAD 83(2011)/NAVD 88 to the modernized frames - again requiring published transformation tools, not relabeling.

Realizations, Epochs, and Why Datums Drift

A single datum name hides several realizations - successive readjustments as new data arrive. NAD 83 has been realized as NAD 83(1986), NAD 83(HARN/HPGN), NAD 83(CORS96), and NAD 83(2011), each tied to a reference epoch (NAD 83(2011) uses epoch 2010.00). Coordinates can differ by centimeters to decimeters between realizations of the same datum, so a complete coordinate statement names the datum, the realization, and the epoch - for example "NAD 83(2011) epoch 2010.00." Treating two realizations as identical is a subtle but real FS error.

The deeper reason the modernized NSRS is time-dependent is that the North American plate moves roughly 2 to 3 cm per year relative to the global frame. A fixed-coordinate datum slowly disagrees with reality; the new plate-fixed frames (NATRF2022 and its siblings) rotate with their plates so that points on stable crust keep nearly constant coordinates, while the relationship to the global ITRF2020 carries the time information through the epoch.

The practical message for the FS candidate: always carry datum, realization, epoch, and the transformation tool used; never assume coordinates with different tags can be combined directly.

A further distinction the exam draws is dynamic vs static vertical references. NAVD 88 is a static geopotential datum fixed to a single benchmark, so it does not perfectly track the present-day global geoid. NAPGD2022 is a gravimetric datum derived from a continuously improving geoid model rather than from a single leveling adjustment, which is how it removes the roughly half-meter bias and one-meter tilt that NGS attributes to NAVD 88.

Understanding that the vertical datum is becoming a model-defined surface rather than a network of leveled marks is the single most important conceptual update the modernization brings to elevation work.