A History of Datums in America, Part 2: NAD 83, HARN, NAVD 88, and the GPS Era

The first half of America’s datum history was built with chains, precision baselines, theodolites, triangulation towers, spirit levels, and hand computation. The second half was transformed by electronic distance measurement, satellites, computers, and the realization that the solid Earth is not actually solid.

This is Part 2 of a two-part history. Part 1 traces the story from colonial surveys through the United States Standard Datum, NAD 27, State Plane coordinates, and NGVD 29.

Why NAD 27 eventually had to be replaced

NAD 27 was a remarkable achievement. It unified an enormous classical triangulation network around MEADES RANCH, Kansas, on the Clarke 1866 ellipsoid. For decades it served mapping, surveying, engineering, and property work across North America.

Its limitations were built into its origins. NAD 27 was fitted to the North American continent rather than centered on Earth’s center of mass. Much of the network had been observed by conventional methods and extended through long chains of triangles. Small errors and local distortions accumulated. Distant and offshore areas could not be connected as uniformly as they can be by satellite.

Electronic distance measurement improved terrestrial work after World War II, while Doppler satellite observations and later the Global Positioning System offered something revolutionary: positions could be related to a global, geocentric framework without carrying a line of triangles across the ground.

NAD 83: a new kind of North American datum

The North American Datum of 1983 adopted the Geodetic Reference System 1980 ellipsoid, usually called GRS 80. Unlike NAD 27, it was intended as a three-dimensional, Earth-centered datum. It did not depend on one surface monument serving as the origin. Instead, its definition and realization drew upon Earth-centered coordinates and a continent-wide adjustment.

The original realization is commonly labeled NAD 83(1986) because the adjustment was completed and implemented in 1986. The “1983” belongs to the datum’s name; it should not be read as the publication date of every coordinate carrying that name.

Moving from NAD 27 to NAD 83 changed latitude and longitude values, and State Plane systems were redesigned as SPCS 83. The change was not a single translation that applied everywhere. NAD 27 contained spatially varying network distortion, so NGS developed grid-based transformations such as NADCON to model geographic differences.

For a land surveyor, the lesson is simple: the monument did not leap across the ground when NAD 83 arrived. The coordinate assigned to the monument changed because the measuring framework changed.

GPS reveals the weaknesses in the first NAD 83 adjustment

NAD 83(1986) was calculated from the best national collection available at the time, but GPS quickly delivered relative positioning of much higher precision than much of the older terrestrial network. Surveyors could now occupy widely separated stations and determine their relationship without intervisibility.

That precision exposed distortions that had been tolerable—or invisible—within the classical network. It also created a practical problem. If a GPS survey was fit tightly to nearby NAD 83(1986) monuments, the GPS observations might be more internally precise than the published relationship among those monuments.

NAD 83(1990), HPGN, and HARN

States and NGS responded by establishing GPS-observed networks of high-accuracy stations. Early projects were often called High Precision Geodetic Networks, or HPGNs. The broader term High Accuracy Reference Network, or HARN, became common for these state and regional readjustments.

Labels such as NAD 83(1990) usually identify a particular state adjustment associated with this early GPS era. The year and official designation can vary by jurisdiction. It is therefore unsafe to assume that every coordinate called “83/90” represents the same nationwide realization. One must determine which state adjustment, epoch, published control, and transformation are actually meant.

NGS reports that HARN coordinates commonly differed from NAD 83(1986) by roughly 0.2 to 1.0 meter, depending on location. The new values were not corrections to a badly surveyed monument. They were a more accurate realization of NAD 83 created with GPS and improved adjustment methods.

HARNs were normally implemented state by state. This produced excellent local control, but the staggered approach also left a legacy of state-specific names, adjustment dates, epochs, and transformation grids. Survey records from the 1990s may say NAD 83, NAD 83/90, HPGN, HARN, or a state-specific equivalent. Those descriptions must be read literally and investigated.

Federal Base Networks and Cooperative Base Networks

NGS next developed Federal Base Network stations to provide a nationwide skeleton of accessible, high-accuracy passive control. Cooperative Base Network stations densified that framework through partnerships with states and other organizations.

The label NAD 83(FBN) identifies coordinates associated with Federal Base Network readjustments. As with HARN, FBN is not merely an informal quality adjective. It identifies a realization and adjustment history. A coordinate labeled NAD 83(FBN) should not be silently treated as NAD 83(1986), HARN, NSRS2007, or 2011.

From passive monuments to continuously observed control

Traditional geodetic control was passive. A disk, rod, bolt, or other mark waited for a surveyor to occupy it. Its published coordinate depended on the observations and adjustment that had been made, sometimes decades earlier.

A Continuously Operating Reference Station is different. A CORS continuously collects GNSS observations. Networks of these stations allow geodesists to monitor motion, estimate velocities, improve satellite orbits, and provide surveyors with an active connection to the reference frame.

NAD 83(CORS96) emerged as an important active realization in the 1990s. The “96” refers to the realization, not simply the date a rover was used. CORS96 helped shift practical access to NAD 83 away from a purely monument-to-monument world and toward a continuously monitored network.

The Earth refuses to hold still

GPS made another fact impossible to ignore: coordinates change with time. Tectonic plates move. Earthquakes displace the crust. Land subsides when groundwater or hydrocarbons are withdrawn. Postglacial rebound lifts formerly ice-covered regions. Even a stable-looking monument can have a measurable velocity.

This is why an epoch matters. A reference frame tells us the system in which the coordinate is expressed. An epoch tells us the date to which the coordinate refers. A highly precise coordinate without an epoch can be incomplete, especially in areas of crustal motion.

The conterminous United States is relatively stable compared with parts of Alaska, California, Hawaii, and other tectonically active regions, but “relatively stable” is not the same as motionless. Modern geodesy treats time as part of position.

NAD 83(NSRS2007): a nationwide readjustment

By the early 2000s, the passive network contained coordinates from different state HARNs, FBN projects, epochs, and adjustment histories. NGS performed a national readjustment of GPS-derived passive control to improve consistency with the CORS framework.

The result was NAD 83(NSRS2007), with an epoch of 2007.00. The name signaled its role within the National Spatial Reference System. Rather than preserving a patchwork of state-by-state GPS adjustments, NSRS2007 supplied a more uniform national treatment of suitable GPS-observed passive stations.

NSRS2007 did not mean that every old station suddenly acquired GPS-quality coordinates. The quality of a published value still depended on the observations included, the stability of the mark, and the relationship to the adjusted network.

NAD 83(2011), epoch 2010.00

NGS continued to improve the CORS coordinates and velocities through a multi-year solution. A new national adjustment followed, producing NAD 83(2011), epoch 2010.00 for the North American plate. Related realizations—PA11 and MA11—serve the Pacific and Mariana plates.

NAD 83(2011) is the current official geometric datum realization for much federal work while the modernized NSRS remains in transition. Its coordinates are tied more consistently to the modern CORS network than earlier passive realizations.

The parenthetical year and the epoch serve different purposes. “2011” identifies the realization. “Epoch 2010.00” identifies the reference date of its coordinates. Dropping either part can create ambiguity.

Realization	Plain-language description	Common caution
NAD 83(1986)	Original continent-wide NAD 83 adjustment implemented in 1986	Often mislabeled simply “NAD 83,” concealing later differences.
NAD 83(1990) and similar state labels	Early state GPS readjustments, often associated with HPGN or HARN	The year and exact meaning vary by jurisdiction.
NAD 83(HARN)	High Accuracy Reference Network realization based on state and regional GPS surveys	HARN coordinates are not identical to NAD 83(1986).
NAD 83(FBN)	Federal Base Network readjustment and densification	Do not collapse the label into generic “NAD 83.”
NAD 83(CORS96)	Realization associated with the continuously operating GNSS network	Active control and epoch information become increasingly important.
NAD 83(NSRS2007)	National readjustment of suitable GPS-derived passive control	Published values depend on the observations and mark stability.
NAD 83(2011), epoch 2010.00	Current official North American realization based on an improved multi-year CORS solution and national adjustment	The realization year and coordinate epoch are not the same thing.

The vertical side also needed a replacement

While horizontal geodesy moved toward satellites, the vertical network still relied on NGVD 29. That datum had held 26 tide-gauge values fixed and distributed resulting inconsistencies through a continental leveling network. Decades of new leveling, crustal movement, subsidence, and knowledge about gravity revealed significant problems.

A new adjustment was required. The result was the North American Vertical Datum of 1988, or NAVD 88. The adjustment used a large body of leveling across the United States, Canada, and Mexico. Rather than holding many tide gauges fixed, it constrained the network at one primary bench mark at Father Point/Rimouski, Quebec, Canada.

NAVD 88 was adopted in the early 1990s and replaced NGVD 29 as the principal vertical datum for the conterminous United States and Alaska. The difference between NGVD 29 and NAVD 88 is not one nationwide constant. It varies by location because the two adjustments were built differently and contain different distortions.

In Florida and other low-lying regions, a difference of several tenths of a foot can matter greatly to drainage, floodplain, roadway, and coastal work. A plan that says only “elevations are national datum” is therefore dangerously incomplete. The exact datum, adjustment, bench marks, and any conversion procedure must be stated.

NAVD 88 is not a tidal datum

Surveyors frequently work near water, which creates confusion between geodetic and tidal datums. NAVD 88 is a geodetic vertical datum. Mean Sea Level, Mean Lower Low Water, Mean High Water, and other tidal datums are derived from water-level observations over a defined tidal epoch and apply to tidal applications.

A zero on a tide staff is not automatically NAVD 88. A chart datum is not automatically a property-boundary elevation. Conversions among tidal, orthometric, and ellipsoid heights require the correct models and local information. NOAA’s VDatum tool exists because these vertical reference surfaces are different and their relationships vary geographically.

Geoid models connect satellite heights to elevations

GNSS naturally produces an ellipsoid height: the point’s height relative to the reference ellipsoid. Most engineering and surveying work needs an orthometric height, the quantity commonly treated as elevation.

A geoid model supplies the modeled separation between the ellipsoid and an equipotential surface related to gravity. In simplified form, the relationship is:

H = h − N

Here, H is orthometric height, h is ellipsoid height, and N is geoid undulation. Models such as GEOID96, GEOID99, GEOID03, GEOID09, GEOID12, and GEOID18 represent continuing efforts to connect GNSS-derived ellipsoid heights with NAVD 88 bench-mark heights.

The model name is part of the result. An elevation calculated with one geoid model should not be documented as though the model were irrelevant. NGS’s GPS on Bench Marks program has been especially important because observations on marks with reliable leveling provide data to build and test these hybrid geoid models.

Why later realizations do not erase earlier records

A surveyor may be tempted to treat the newest coordinate as the only “correct” coordinate. History is not that simple. An old plat, highway plan, right-of-way map, subdivision, or control sheet must be interpreted in the reference system actually used when it was created.

NAD 83(1986), HARN, FBN, NSRS2007, and NAD 83(2011) can assign different coordinates to the same undisturbed monument. Those differences do not automatically prove that an earlier survey was defective. They may record a new realization, a new adjustment, improved observations, crustal motion, or a different epoch.

The professional task is to preserve the chain of evidence. Record the mark, datum, realization, epoch, projection, zone, units, geoid model, transformation, and source. If a legacy coordinate must be brought into a newer system, use an appropriate transformation or readjust the original observations. Never change the label and leave the numbers untouched.

The path toward the modernized NSRS

NAD 83 and NAVD 88 were major advances, but they retain fundamental compromises. NAD 83 is offset from the best modern estimate of Earth’s center of mass. NAVD 88 contains bias and tilt relative to modern gravity-based geoid models. Passive marks can move between surveys, while published values may not reveal the motion promptly.

NGS is therefore preparing a modernized National Spatial Reference System based on four plate-fixed terrestrial reference frames—NATRF2022, PATRF2022, CATRF2022, and MATRF2022—and the North American-Pacific Geopotential Datum of 2022, or NAPGD2022. State Plane Coordinate System of 2022 will provide projected coordinates tied to the applicable new frame.

The modernized system carries the history of every earlier change forward. It favors active GNSS control, gravity-based height access, velocity models, and explicit epochs. In other words, the latest chapter is not a rejection of two centuries of surveying. It is the logical result of everything surveyors learned while building, adjusting, checking, and rebuilding the national networks.

For a practical explanation of the coming system, see The New NSRS Is Coming: A Field-to-Office Guide for Surveyors.

A field record should preserve the whole coordinate identity

• The datum or reference frame, including the realization in parentheses.
• The coordinate epoch, when applicable.
• The map projection and State Plane zone.
• The linear and angular units.
• The vertical datum and geoid model.
• The control stations, CORS, RTN mount point, or published marks used.
• The observation dates, raw files, antenna model, antenna height method, software, and adjustment report.
• The transformation method and version if coordinates were converted from another realization.

The long view

American surveying began with local descriptions because local descriptions were what the tools, government, and economy required. Triangulation created regional and then national horizontal networks. Spirit leveling created a national language of elevation. NAD 27 and NGVD 29 supported much of the twentieth century. Satellites then revealed distortions and motion that optical networks could not fully resolve.

NAD 83 was not the final answer because geodesy does not produce final answers in a moving world. Its sequence of realizations—1986, 1990-era state adjustments, HARN, FBN, CORS96, NSRS2007, and 2011—records the profession’s improving ability to measure the same Earth.

The names can seem like alphabet soup, but the history behind them is straightforward. Every realization answered a practical question: How can we make positions more accurate, more consistent, easier to access, and more honest about time?

That question connects Hassler’s triangulation, Bache’s network adjustments, a level crew crossing the continent, a 1990s GPS HARN survey, today’s CORS network, and the modernized NSRS. The equipment changed. The duty did not: measure carefully, document completely, and leave the next surveyor enough evidence to understand what was done.

Sources and further reading

• NOAA NGS: Datums and Reference Frames
• NOAA NGS: North American Datum of 1983
• NOAA NGS: NAD 83 High Accuracy Reference Network
• NOAA NGS: Coordinate Conversion and Transformation Tool
• NOAA Special Publication: The New Horizontal Control Datum for North America—NAD 83
• NOAA NGS: National Geodetic Vertical Datum of 1929
• NOAA NGS: North American Vertical Datum of 1988
• NOAA VDatum
• NOAA NGS: Replacing NAD 83 and NAVD 88

This article is a historical and educational overview. It is not a substitute for the controlling survey record, agency specification, statute, transformation documentation, or a project-specific geodetic analysis.