US latitude, longitude and elevation are about to change by up to 4 metres: Why America is replacing its 40-year-old GPS reference system

The ground in the United States is not moving, but millions of official coordinates soon could. Latitude, longitude and elevation readings used by surveyors, engineers, builders and government agencies are set for a major overhaul in the US, reports Space Daily. The National Geodetic Survey (NGS) is replacing the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88), two systems that have been used for decades to describe where places are and how high they sit. Under the new system, some coordinates could differ from today's published figures by as much as four metres. That does not mean homes, roads or mountains are suddenly shifting. Instead, the reference system used to measure them is changing.

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Why are US coordinates changing?

A latitude, longitude or elevation is measured against a reference system. NAD 83 and NAVD 88 were developed before today's satellite technology could accurately measure the Earth's centre and its changing shape. NGS says the origin of NAD 83 is misaligned with the modern understanding of the Earth's centre by about 2.2 metres. The agency also notes that differences between existing NAD 83 frames and international estimates can range from roughly two to four metres.


The vertical system has its own problems. NGS says NAVD 88 is biased by about half a metre and tilted by roughly one metre from coast to coast when compared with modern global geoid models. That means two locations can have published elevations that do not perfectly reflect the way Earth's gravity actually defines "level."

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The headline figure of four metres is not a uniform shift across America. The actual change will depend on where a point is located, which existing datum is being used and the date, or "epoch", associated with the new coordinate. NGS has published maps showing expected changes in horizontal position and both ellipsoid and orthometric heights. So, a survey point in Ohio could receive a different coordinate than it has today, while the physical point itself remains exactly where it was.

A new gravity-based map will redefine elevation

Changing the country's vertical reference system requires understanding Earth's gravity field. NGS's Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project collected airborne gravity measurements across the United States. Those measurements helped create GEOID2022, which will support the new vertical system. The new North American-Pacific Geopotential Datum of 2022 (NAPGD2022) will replace existing geodetic vertical datums, including NAVD 88. In simple terms, the new system aims to provide a much more accurate definition of where "zero elevation" actually is.

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The horizontal system is also being modernised.

Instead of relying on one framework for the entire country, NGS is introducing four terrestrial reference frames linked to major tectonic plates: North America, Pacific, Caribbean and Mariana. These frames will move with their respective plates and will be tied to the International Terrestrial Reference Frame of 2020.

This matters because Earth's surface is constantly moving, even if people cannot feel it. Attaching coordinates to tectonic plates can make future positioning more accurate.

The current system relies heavily on physical survey marks, including more than 1 million survey monuments and marks across the country. Many have deteriorated, been destroyed or are difficult to access.

The modernised system will instead rely much more heavily on satellite positioning, including the NOAA reference station network and the International GNSS Service. Coordinates will also include an epoch, essentially a date attached to the measurement. This becomes important when technology is precise enough to detect the slow movement of Earth's surface. The change is expected to be a major shift in how the US defines and maintains its official geodetic reference system.

Why this matters to ordinary Americans

Most people will not suddenly need to change their home address or move a property boundary. But the new reference system could matter behind the scenes for construction, surveying, flood maps, infrastructure projects, aviation, GPS-based positioning and scientific measurements. For professionals working at centimetre-level accuracy, however, the change could be significant.

FAQs

1. Is the US physically moving by four metres?

No. The land is not moving four metres. The reference system used to describe its position is changing.

2. What is NAD 83?

NAD 83 is the North American Datum of 1983, a reference system used to define latitude and longitude across North America.

3. What is NAVD 88?

NAVD 88 is the North American Vertical Datum of 1988, a reference system used to define elevations.

4. Will my home's address change?

No. A home's street address does not change because the geodetic reference system is updated.

5. Why can coordinates change if the land does not move?

Coordinates are measurements made against a reference framework. Change that framework and the numerical coordinates can change even when the physical location stays put.

6. What is GEOID2022?

GEOID2022 is a geoid model developed from gravity measurements that will help define elevations under the modernised US vertical reference system.

7. Why are tectonic plates important?

Earth's tectonic plates are constantly moving. Plate-based reference frames allow coordinates to better account for that movement over time.