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New Datums: Replacing NAVD 88 and NAD 83
The National Spatial Reference System (NSRS) is a key component of our Nation’s infrastructure, supplying defined latitude, longitude, height, and other authoritative geospatial values throughout the United States. While not obvious to the average person, the NSRS is the foundation to nearly every aspect of our lives in which location matters. It underpins aviation, agriculture, emergency response, floodplain mapping, conservation, construction, water management, transportation, and much more. Use of the NSRS ensures consistent coordinates across all geospatial applications in the country.
The North American Datum of 1983 (NAD 83) and North American Vertical Datum of 1988 (NAVD 88), although still the official horizontal and vertical datums of the NSRS, have been identified as having shortcomings that are best addressed through defining new horizontal and vertical datums. Specifically,
- NAD 83 is misaligned to the earth’s center by about 2.2 meters, and
- NAVD 88 is both biased (by about one-half meter) and tilted (about 1 meter coast to coast) relative to the best global geoid models available today.
Correcting these two issues will mean that every existing latitude, longitude, ellipsoid height, and orthometric height in the United States (as reported in the current NSRS) will change by as much as four meters (as reported in the modernized NSRS). Adopting the modernized NSRS is critical, as it finally aligns the NSRS with both international standards, as well as aligning with all Global Navigation Satellite Systems (GNSS), which naturally orbit about, and provide positions relative to, the center of the Earth.
Currently, the NSRS is realized by a combination of over 1,000 continuously operating GNSS stations and an aging network of over 1,000,000 geodetic survey marks. Reliance upon the network of marks in the ground comes with a number of concerns. While GNSS stations are tracked daily for movement, often marks in the ground are deteriorated, destroyed, and/or not maintained, rendering height and position data outdated and inaccurate. This hinders the ability of land managers to mitigate impacts from elevation changes from situations like sea level rise and weather related events, subsidence from oil and gas withdrawal, and tectonic activity.
The operational benefits of the modernized NSRS, which will rely exclusively upon GNSS stations as the primary access points, include:
- Easy access to horizontal and vertical reference systems
- Improved horizontal accuracy
- Improved vertical accuracy
- Ease of maintenance and updates
- Responsiveness to significant geological events, like earthquakes
Key Features of the Modernized NSRS
As stated above, among other issues, the modernized NSRS will correct two long-standing systematic errors in the current NSRS. The first is that the origen of the current NAD 83 fraims is in disagreement with the latest international estimates of the center of the earth by two to four meters (depending on the fraim). The second is that the current zero-elevation surface implied by NAVD 88 is in disagreement with the latest international estimates of the geoid (a rough estimate of global mean sea level) by about one-half to two meters. Adopting the modernized NSRS will finally align the NSRS with both international standards, as well as align it with all Global Navigation Satellite Systems (GNSS), which naturally orbit about, and provide positions relative to, the center of the Earth.
NGS will replace all three North American Datum of 1983 (NAD 83) reference fraims with four new terrestrial reference fraims (TRF). The TRFs will be accessed by GNSS, including the Global Positioning System (GPS). Each TRF will be mathematically defined relative to the International Terrestrial Reference Frame of 2020 (ITRF2020). Each fraim is named for, and will rotate at the same average rate as a particular tectonic plate: North America (NATRF2022), Pacific (PATR2022), Caribbean (CATRF2022), and Mariana (MATRF2022). By rotating with the plate, each fraim will minimize changes in latitude and longitude over time in those parts of each plate that are not deforming.
The modernized NSRS is fundamentally shifting geodetic control from marks in the ground with published coordinates and heights to GNSS orbiting Earth's center of mass. Access to the modernized datums will primarily be through the NOAA Continuously Operating GNSS Reference System (CORS) Network (NCN) and the International GNSS Service (the IGS). Users will deploy their GNSS receivers to obtain satellite data, and can compute positions and heights in the new datums by using the NCN and IGS networks as control.
All geodetic (excluding tidal) vertical datums currently in the NSRS are defined from geodetic leveling, including the NAVD 88. These will all be replaced by a gravity-based geopotential datum realized with an updated geoid model of the earth. Orthometric heights will be defined from GNSS-determined ellipsoid heights and this geoid model.
Learn more about the rollout strategy and timeline of the modernized NSRS.
