6.1 Control Datums, Realizations, and Epochs

The January 2025 California Professional Land Surveyor test plan Domain II Activity 1 asks you to determine proper control datums, realizations, and epochs. Independent OpenExamPrep teaching in this section is written for that activity and for the knowledge items on projections, datums, epochs, transformations, geoid/ellipsoid/orthometric heights, and earth movement. A candidate who can recite "NAD83" but cannot attach a realization tag and a two-decimal epoch has not finished the statutory job.

Datum, realization, and epoch are three different labels

A datum is the named reference system. For horizontal positions and ellipsoid heights, Public Resources Code (PRC) section 8852 names the official geodetic datums in California as the North American Datum of 1983 (NAD83), the North American Terrestrial Reference Frame of 2022 (NATRF2022), or the Pacific Terrestrial Reference Frame of 2022 (PATRF2022). For orthometric heights, PRC 8853 names the North American Vertical Datum of 1988 (NAVD88) or the North American-Pacific Geopotential Datum of 2022 (NAPGD2022).

A realization is a specific published solution of that datum. NAD83(2011) is not the same coordinate set as NAD83(NSRS2007) or NAD83(CORS96). PRC 8815.1 requires the realization tag in parentheses when California Coordinate System of 1983 (CCS83) coordinates are shown.

An epoch is the decimal-year date those coordinates refer to, shown to two decimal places. PRC 8815.2 says the epoch for a CCS83 survey is the published National Geodetic Survey (NGS) or California Spatial Reference Center (CSRC) datum realization and epoch of a published coordinate for a controlling station. Surveys after December 31, 1999 must be based on NAD83 (1992) epoch 1991.35 or a subsequent published NGS or CSRC realization and epoch. The statute's own example is CCS83 (2011) epoch 2010.00.

California sits on the Pacific–North America plate boundary. Marks move. A NAD83(2011) northing at epoch 2010.00 and a NAD83(2011) northing at epoch 2025.00 are not interchangeable even though both strings contain "NAD83" and "2011". That is why the exam tests realization and epoch together, and why PRC 8815.3 requires you to bring controlling stations to one consistent epoch using procedures and values published by NGS or CSRC when their published epochs differ.

What the PRC names versus what NGS currently serves as official NSRS

Senate Bill 566 (Stats. 2023, Ch. 111), effective January 1, 2024, wrote NATRF2022, PATRF2022, CCS2022, and NAPGD2022 into Division 8. That is current California statute. It is not the same event as NGS replacing the official National Spatial Reference System (NSRS).

As of September 2026, NGS still describes the official NSRS on geodesy.noaa.gov as NAD 83 and NAVD 88. Foundational modernized products, including NATRF2022 and State Plane Coordinate System of 2022 (SPCS2022), have been posted for public testing. NGS posts them as preliminary releases for testing and evaluation that are not final products and do not contain authoritative NGS data or tools. Do not treat an unpublished Federal Register cutover as California law, and do not invent a Board-required switch date. Teach what the PRC currently names: NAD83 remains an official California geodetic datum; CCS83 remains the plane system that PRC 8817 has required for new State Plane work since January 1, 1995; after January 1, 2025 new surveys may be based on CCS2022. "May" is permission. It is not a finding that NATRF2022 has already replaced NAD83 in production work.

PRC 8860 makes use of NAD83, NATRF2022, PATRF2022, NAVD88, NAPGD2022, and the California Spatial Reference Network (CSRN) optional. Optional use does not waive labeling when you do use them (PRC 8854).

NAD83, NAVD88, and H = h − N

These quantities answer different questions. Mixing them is a classic Domain II miss.

• NAD83 (with a realization and epoch) is a geometric reference for latitude, longitude, CCS83 plane coordinates, and ellipsoid height.
• NAVD88 is a gravity-related vertical datum. Orthometric heights on NAVD88 are what field crews still call elevations when they mean heights above a geoid-based surface, not heights above the ellipsoid.
• Ellipsoid height h is the height of a point above the Geodetic Reference System of 1980 (GRS 1980) ellipsoid used with NAD83.
• Geoid height N is the separation between the geoid and that ellipsoid, taken from a published model.
• Orthometric height H is the height above the geoid. The working relationship is H = h − N.

NGS hybrid model GEOID18 converts NAD 83 (2011) epoch 2010.0 ellipsoid heights to NAVD 88 orthometric heights in the conterminous United States. NGS states that ellipsoid heights on other NAD83 realizations, WGS 84, or ITRF are not compatible with GEOID18. PRC 8894 splits California Orthometric Heights of 1988 (COH88) into leveled COH88 from differential leveling and derived COH88 from GPS plus a geoid model. PRC 8895 requires the latest geoid model published by NGS when you derive COH88. PRC 8896 allows a documented local orthometric height correction. Interpolate N from the published model at the point. Do not invent a site-specific N for a county or a project.

Worked height example (you supply N from the model, not from memory): you occupy a control mark and obtain h = 32.418 m ellipsoid height on NAD83(2011) epoch 2010.00. You interpolate GEOID18 at that latitude and longitude and obtain N. Then H = 32.418 − N, in meters. Over much of CONUS, N is negative, so H is larger than h. If the ellipsoid height is not on NAD83(2011) epoch 2010.00, GEOID18 is the wrong pairing until you transform using published NGS tools—NCAT between NAD83 realizations, HTDP between frames or epochs—and you accept the additional transformation uncertainty NGS warns about.

CSRC published CSRN Epoch 2025.00 NAD83(2011) on July 31, 2025: coordinates of 1,068 GNSS stations (881 active and 187 defunct) at epoch date 2025.00 (January 1, 2025), replacing CSRS Epoch 2017.50 NAD83(2011). CSRC obtained NAD83(2011) values from ITRF2020 using NGS HTDP and interpolated GEOID18 for derived California Orthometric Heights under PRC 8890–8902. The California Real Time Network (CRTN) began transmitting those epoch-dated coordinates in RTCM 3.3 streams on August 11, 2025. If you mix a 2010.00 NGS datasheet with a 2025.00 CRTN vector without a PRC 8815.3 epoch reduction, you have not determined a proper epoch.

HTDP is the epoch-shift tool — not a source of invented centimeters

The NGS Horizontal Time-Dependent Positioning (HTDP) utility (User Guide version 3.6.0, April 7, 2025) lets you:

• Estimate horizontal crustal velocities.
• Estimate crustal displacements between two dates, including coseismic and postseismic models.
• Update (or backdate) positional coordinates from one date to another.
• Transform coordinates, certain observations, and velocities between reference frames and dates.

HTDP includes velocity grids covering Southern California, Northern California, and western CONUS, plus rigid-plate models outside those grids. Use HTDP or CSRC published station coordinates and velocities to perform the epoch shift. Do not invent a county-wide displacement and write it on a map. PRC 8815.3 points you to procedures and values published by NGS or CSRC. HTDP does not replace the realization-and-epoch note required by PRC 8815.1.

CSRN accuracy floors that belong in a datum decision

PRC 8855 names CSRN as the official geodetic reference network for California. Stations enter CSRN only if they meet Chapter 3 tests, including publication by NGS or CSRC. Horizontal CSRN stations must be referenced to NAD83 or NATRF2022, determined by GPS methods, published at 2 cm network accuracy or better (FGDC) or first order or better (FGCS), and have a published or determinable horizontal velocity (PRC 8856). Ellipsoid heights need 5 cm network accuracy or FGCS fourth order, class II or better (PRC 8857). GPS-derived orthometric heights need NAVD88 on NAD83 (or NATRF2022 and NAPGD2022) at 5 cm network accuracy or better (PRC 8858). Leveled orthometric heights need NAVD88 at FGCS third order, class II or better (PRC 8859).

PRC 8854 requires any document that shows horizontal positions, ellipsoid heights, or orthometric heights to show the geodetic datum, including realization tag and epoch date if appropriate.