National reference frames

The ITRF is the world's frame, but almost no one uses it directly for property, engineering or mapping. Each country publishes a national frame that is tied to the ITRF but suited to its own needs: coordinates that change little with time, a stable basis for law and records, and a link to older networks and maps. This lesson describes the national frames of Canada and the United States, how they relate to the ITRF, and how they are changing.

What a national frame must do

A national frame must do four things. It must give coordinates that agree with the ITRF closely enough, through a published transformation, that GNSS users can reach it with global orbits and services. It must keep coordinates of monuments on the stable part of the country nearly constant, so that legal descriptions and engineering records stay valid. It must be accessible: through active stations, passive monuments, processing services and conversion tools. And it must connect to the past, with transformations from older frames, because centuries of records and maps depend on them.

These goals pull in different directions. Staying close to the ITRF means accepting its motions; keeping coordinates fixed means removing them. The solution adopted in North America is a plate-fixed frame: aligned with the ITRF at one reference epoch, then made to rotate with the North American plate, so that the difference between the two grows with time but in a precisely known way.

The United States: from NAD83 to NATRF2022

The United States has used NAD83 since 1986, readjusted several times as GPS replaced classical surveying. The current realization, NAD83(2011) epoch 2010.00, was computed from a national adjustment of passive marks tied to the CORS network. Like the original, its origin is about 2 m from the Earth's center of mass, a legacy of the pre-GPS definition, so it differs from the ITRF by a meter or two across the country. The National Geodetic Survey is replacing it, as part of the modernization of the National Spatial Reference System, with four new terrestrial frames: NATRF2022 for the North American plate, and PATRF2022, CATRF2022 and MATRF2022 for the Pacific, Caribbean and Mariana plates.

NATRF2022 is defined so that it agrees with ITRF2020 at epoch 2020.00 and rotates with the North American plate, using an Euler pole estimated by NGS from CORS velocities. Its origin is therefore at the Earth's center of mass, unlike NAD83, and coordinates of monuments in the lower 48 states will change by about a meter or more in horizontal position and by a meter or so in ellipsoidal height when users move from NAD83(2011) to NATRF2022. NGS published beta versions of the new frames, the new vertical datum and the new State Plane Coordinate System of 2022 in June 2025 for testing, and by May 2026 reported the feedback period closed and the products stable for planning. The frames are to be released officially with the modernized system, with tools such as NCAT and a new OPUS to follow.

import math
# NATRF2022 from ITRF2020: the frames agree at epoch 2020.0, and NATRF2022 removes the rotation of the North American plate.
mas = math.radians(1 / 3.6e6)
w = [0.03 * mas, -0.69 * mas, -0.02 * mas]       # plate rotation in ITRF2020, rad/yr (approximate)
def geod2ecef(lat, lon, h, a=6378137.0, f=1 / 298.257222101):
    e2 = f * (2 - f); la, lo = math.radians(lat), math.radians(lon)
    N = a / math.sqrt(1 - e2 * math.sin(la) ** 2)
    return ((N + h) * math.cos(la) * math.cos(lo), (N + h) * math.cos(la) * math.sin(lo), (N * (1 - e2) + h) * math.sin(la))
lat, lon, h = 47.506004222, -111.297790917, 1004.826
X = geod2ecef(lat, lon, h)
la, lo = math.radians(lat), math.radians(lon)
def enu(d):
    e = -math.sin(lo) * d[0] + math.cos(lo) * d[1]
    n = -math.sin(la) * math.cos(lo) * d[0] - math.sin(la) * math.sin(lo) * d[1] + math.cos(la) * d[2]
    u = math.cos(la) * math.cos(lo) * d[0] + math.cos(la) * math.sin(lo) * d[1] + math.sin(la) * d[2]
    return e, n, u
for t in (2020.0, 2026.84, 2035.0):
    dt = t - 2020.0
    rot = (w[1] * X[2] - w[2] * X[1], w[2] * X[0] - w[0] * X[2], w[0] * X[1] - w[1] * X[0])
    d = [-dt * r for r in rot]                   # NATRF2022 minus ITRF2020 at epoch t
    e, n, u = enu(d)
    print(f"epoch {t:.2f}: NATRF2022 minus ITRF2020  east {e * 1000:+7.1f}  north {n * 1000:+7.1f}  up {u * 1000:+5.1f} mm")
# NAD83(2011) sits roughly 1.5 to 2 m from ITRF at this longitude; NATRF2022 removes most of that offset
d83 = (-1.10, 0.85, 0.20)                        # NAD83(2011) minus NATRF2022, m, illustrative
print(f"illustrative NAD83(2011) to NATRF2022 change: {math.hypot(d83[0], d83[1]):.2f} m horizontal, {d83[2]:+.2f} m ellipsoid height")

epoch 2020.00: NATRF2022 minus ITRF2020  east    +0.0  north    +0.0  up  +0.0 mm
epoch 2026.84: NATRF2022 minus ITRF2020  east  +101.0  north   +58.8  up  +0.2 mm
epoch 2035.00: NATRF2022 minus ITRF2020  east  +221.4  north  +129.0  up  +0.4 mm
illustrative NAD83(2011) to NATRF2022 change: 1.39 m horizontal, +0.20 m ellipsoid height

The code computes the difference between NATRF2022 and ITRF2020 at CR1 from the plate's rotation alone, which is how the frames are related. At 2020.00 the difference is zero by definition. By late 2026 it is about 101 mm east and 59 mm north, 117 mm in all, and by 2035 it will be about a quarter of a meter, because ITRF2020 coordinates move with the plate while NATRF2022 coordinates do not. The last line gives an illustrative change from NAD83(2011) to NATRF2022 of about 1.4 m horizontally and 0.2 m in ellipsoidal height, the right size for Montana; the actual values will come from NGS's published transformation.

Maintaining a national frame

A national frame is not computed once. The agency reprocesses the data of its active stations, often all of them from the beginning, whenever orbits, antenna models or software improve, and it re-estimates their positions and velocities. Campaign observations on passive monuments are adjusted into the frame as they arrive. Each reprocessing changes coordinates slightly, usually by millimeters, and the agency decides whether the changes are large enough to publish a new version or small enough to absorb as updates. Users notice only when versions change, but the work is continuous.

NGS has computed several multi-year CORS solutions, reprocessing the whole CORS record with updated models; the results defined the NAD83(2011) coordinates of the CORS and their ITRF counterparts. Under the modernized system, NGS has said it will publish time series for the CORS and update them regularly, rather than fixing coordinates for a decade or more. States have their own geodetic programs, and many state laws refer to NAD83 and to State Plane coordinates by name, so the change of frame also requires legal updates in many states, a process that has started in some legislatures.

Consequences for users

For users in the United States, the change from NAD83(2011) to NATRF2022 is large: about a meter or more horizontally across the lower 48 states and a comparable change in ellipsoidal height, together with a change of vertical datum and of map projections, since SPCS2022 replaces the State Plane Coordinate System of 1983. Every GIS, engineering record and survey control file in the country will need to be converted or labeled. NGS has encouraged agencies and firms to plan now: to inventory the frames and epochs in their data, to identify the legal references that must change, and to test conversions with the beta tools.

In both countries the same lesson applies. A frame change is mostly a data management task. The geodesy is done by the agencies; the surveyor's job is to know which frame every number is in and to convert it correctly when needed.

Access to the frame

Users reach the US national frame mainly through the NOAA CORS Network of about two thousand continuously operating stations run by NGS and its partners, through OPUS, which processes a user's own data against the CORS, and through state and commercial RTK networks. Passive marks remain important for many users, but NGS has moved toward defining the frame by the CORS, with passive marks reached through them. Under the modernized system, NGS has said CORS coordinates will be published as time series, the foundation CORS will be the most stable reference, and passive marks will be accessed through observations submitted by users and processed with OPUS.

Ellipsoidal heights in national frames

A frame change affects heights above the ellipsoid as well as horizontal positions. Because NAD83's origin is offset from the center of mass, ellipsoidal heights in NAD83 differ from those in the ITRF by up to a meter or more, by an amount that varies smoothly across the continent. That matters for anyone who converts GNSS ellipsoidal heights to orthometric heights with a geoid model, since the geoid model must be expressed in the same frame as the ellipsoidal heights. A geoid model computed for the ITRF and applied to NAD83 heights, or the reverse, gives heights wrong by the frame difference.

The United States handles this by publishing its hybrid geoid models in NAD83(2011), so that h in NAD83(2011) minus N gives NAVD 88 heights. Under the modernized system, GEOID2022 is referred to NATRF2022, so that h in NATRF2022 minus N gives NAPGD2022 heights. Mixing the two, for example using GEOID2022 with NAD83(2011) ellipsoidal heights, would give heights wrong by the frame difference of up to a meter or more. OPUS and NCAT handle the pairing when the user selects consistent options.

The rule is simple: the ellipsoidal height, the geoid model and the resulting height must all belong to the same frame and epoch. Lesson 11 returns to the height side of that rule.

Other frames that users meet

WGS 84, the frame of GPS, is maintained by the United States Department of Defense and aligned with the ITRF to about a centimeter in its recent realizations. A GPS receiver's autonomous position is in WGS 84 by definition, but at the meter level of autonomous positioning it is indistinguishable from the ITRF, and many software packages treat WGS 84 and NAD83 as identical, which is wrong by a meter or two. Galileo, GLONASS and BeiDou have their own frames, also aligned with the ITRF. For precise work, the relevant frame is that of the orbits used in processing, which for IGS products is IGS20, aligned to ITRF2020.

Older frames remain in records: NAD27, based on the Clarke 1866 ellipsoid and a classical adjustment, differs from NAD83 by tens to hundreds of meters; the first NAD83 adjustments, before GPS, by up to a meter from later versions. Transformations between them are grid-based, because the old networks were distorted in ways that no simple formula captures.

In the United States, NADCON and its successor NADCON 5 transform between NAD27 and the realizations of NAD83, and NGS has said that tools relating NAD83(2011) to NATRF2022 will be published with the new frames. In Montana, legacy survey coordinates may be in several NAD83 realizations, as the state's survey records show.

Coulee Geomatics: CR1 in the national frame

CR1's application to the NOAA CORS Network asks for a site log, photographs, a monument description and data meeting NGS's requirements. Its coordinates will be computed by NGS, first in NAD83(2011) and ITRF2020 and, after release, in NATRF2022. Theo processes three months of data with OPUS and with a PPP service to check the station before submission, and compares the difference between the NAD83(2011) and ITRF2020 results with NCAT. Once accepted, CR1's official coordinates will come from NGS, and the firm will use those, not its own, for the district's work.