California updates its spatial reference network

The California Spatial Reference Center (CSRC) modernized the California Spatial Reference Network (CSRN) on July 31, 2025. The new California Spatial Reference Network is denoted as CSRN Epoch 2025.00. 

These coordinates changes affect California geospatial users, but the transition process to the new epoch is something that others should understand to prepare for the new, modernized National Spatial Reference System (NSRS), which is expected to be adopted in 2026. As I mentioned in my August 2025 newsletter, NSRS users should proactively assess their geospatial data dependencies and evaluate how adoption of the new datum will affect workflows, datasets and operational decision‑making. 

The California Spatial Reference System (CSRS) is the official geodetic datum in California, as published by the California Spatial Reference Center (CSRC) according to Public Resources Code (PRC) §§8850–8861. The image below depicts the CSRN. It is rigorously aligned to the current definition of the National Spatial Reference System (NSRS) through a set of coordinate transformations from ITRF2020 to NAD83(2011) as published by the NOAA/NOS National Geodetic Survey (NGS). The California Spatial Reference System (CSRS) is realized by the geodetic coordinates and uncertainties of the CSRN on the date of 2025.00 (January 1, 2025; GPS week 2347, day 3) of 1068 GNSS stations (881 active and 187 defunct stations) in California and at the borders of Arizona, Nevada, Oregon and Baja California. CSRN Epoch 2025.00 NAD83(2011) replaces the previous CSRS Epoch 2017.50 NAD83(2011).

The latest hybrid geoid model GEOID18 published by NGS was used to compute Global Navigation Satellite System (GNSS)-derived orthometric heights (DCOH) on the North American Vertical Datum of 1988 (NAVD 88) datum in accordance with the California PRC §§8890-8902 (California Orthometric Heights).

As previously mentioned, the new CSRC Epoch 2025.00 (NAD83 (2011) replaces the previously published CSRC Epoch 2017.5 NAD83 (2011). Readers can obtain the project report that provides technical information about the new realization at the following link: https://sopac-csrc.ucsd.edu/index.php/csrn-epoch-2025-00/ . The website provides web-links to the project report and a table of stations that includes information about the coordinates.  See the image captioned “Excerpt from CSRC Epoch 2025.00 Web Page” for the links to the reports and tables.  The CSRC Epoch 2025.00 realization is aligned with NAD83 2011, Epoch 2010.0.  See the image captioned “Excerpt from Project Report V2” for the summary from the report. I have highlighted some sections of the summary that I thought others would find of interest.

Excerpt from Project Report V2

Summary

This report, prepared under California Department of Transportation (Caltrans) Contract No. 52A0157, Task Order 1, documents the modernization of the California Spatial Reference Network (CSRN) by the California Spatial Reference Center (CSRC). This updated realization aligns the CSRN with the North American Datum of 1983 (NAD83 2011, epoch 2010.00).

The new reference frame, effective on January 1, 2025 (GPS Week 2347, Day 3), is called CSRN Epoch 2025.00 NAD83(2011), referred to for short as CSRN Epoch 2025.00. It replaces the previous adjustment at Epoch 2017.50 and remains a core component of the California Spatial Reference System (CSRS).

The CSRN is defined by the geodetic coordinates and uncertainties (Table 1) of 1,068 continuous GNSS stations—881 active and 187 inactive or decommissioned—located throughout California and bordering regions in Arizona, Nevada, Oregon, and Baja California, Mexico. As California’s official geodetic reference network under Public Resources Code (PRC) §§8850–8861, all Caltrans surveys using the California Coordinate System of 1983 (CCS83) must reference CSRN control stations or comply with CSRN specifications. The definition and use of CCS83 are governed by PRC §§8801–8819. This new realization is fundamentally tied to the International Terrestrial Reference Frame 2020 (ITRF2020) through the IGb20 coordinates adopted by International GNSS Service (IGS) Analysis Centers. All multi-year processing for this project was performed within this state-of-the-art global reference frame. Furthermore, the CSRN Epoch 2025.00 is rigorously aligned with the National Spatial Reference System (NSRS) maintained by the National Geodetic Survey (NGS). Epoch 2025.00 geodetic coordinates are transformed from ITRF2020 to NAD83(2011) using the NGS Horizontal Time-Dependent (HTDP) utility (Figure 1). The ITRF2020 coordinates (X,Y,Z) of the 1068 CSRN stations are transformed into geodetic coordinates (latitude, longitude and ellipsoidal height), using the GRS80 ellipsoidal parameters (semi-major axis, a = 6378137 m and inverse flattening, 1/f = 298.257 222 101).

CSRC submitted to the European Petroleum Survey Group (EPSG) definitions for datums, transformations, and coordinate reference systems for Epoch 2025.00 to facilitate unique terminology with associated metadata.

GPS data (phases and pseudoranges contained in RINEX data files) collected at the CSRN stations from June 10, 1992 to May 17, 2025, and about 300 global tracking stations of the IGS network were re-analyzed in the ITRF2020 reference frame. The complete set of RINEX data and metadata are accessible from the Scripps Orbit and Permanent Array Center data archive.

The latest hybrid geoid model GEOID18 published by NGS is used to interpolate geoid heights for each of the CSRN stations as the basis of Global Navigation Satellite System (GNSS) derived California Orthometric Heights (DCOH) on the NAVD 88 datum in accordance with the California PRC §§8890-8902 (California Orthometric Heights).

As provided in the summary of the report, a diagram noted that the ITRF 2020 cartesian (XYZ) coordinates were transformed into NAD83 (2011) cartesian (XYZ) coordinates, and then into local topocentric coordinates (NEU) to obtain the CSRC Epoch 2025.00 NAD83 (2011) coordinates. 

I downloaded the table of stations with their various coordinates and plotted the differences between the new CSRC Epoch 2025.00 NAD83 (2011) and the previous CSRC Epoch 2017.50 (NAD83 (2011) for stations that were designed as operational stations in 2025.  The following plots depict the difference in coordinates between Epoch 2025.00 and Epoch 2017.50.  One can see that there’s a reason that California needs to periodically update the coordinates of the California Spatial Reference Network.  Some of the horizontal coordinates have changed over 300 mm or around one foot.  The vertical coordinate changes are not as large, but some do shift more than 4 cm.

Note: The plots do not include newer stations with less than 6 months of solutions (no velocities estimated) and defunct stations (stations in Epoch 2017.50 but no data before January 1, 2025.

The image below provides some statistics about the differences in coordinates between Epoch 2025.00 and Epoch 2017.50.

This newsletter highlighted that the CSRC has adopted a new Public Resources Code–compliant geodetic datum (reference frame) for California: CSRN Epoch 2025.00 NAD83(2011), which replaces CSRN Epoch 2017.50 NAD83(2011). The updated datum incorporates secular (linear) tectonic motions across the North America–Pacific plate boundary, transient motions (such as coseismic and postseismic deformation and fault creep), vertical land motion (subsidence and uplift), and data from new stations established since Epoch 2017.50. Additionally, the new vertical datum provides a comprehensive set of California Orthometric Heights on the NAVD88 datum for all CSRN stations.

In essence, the CSRC has released three new datums. The first is tied to ITRF2020, the second to NAD83(2011), and the third to NAVD88. Transformation parameters are available between the first two datums. The NAD83(2011)-based datum satisfies California’s Public Resources Code requirements and is the recommended standard for geodetic control in the state. The NAVD88-based datum provides GNSS-derived California Orthometric Heights of 1988 (COH88).

These new datums will be added to the European Petroleum Survey Group (EPSG) database, the worldwide standard for coordinate reference systems (CRSs) and transformations. Each will receive a unique EPSG code, making it easy to reference and use. This will ensure that CSRN Epoch 2025.00 NAD83(2011), CSRN Epoch 2025.00 (ITRF2020), and COH88 Epoch 2025.00 (NAVD88) can be seamlessly integrated into industry software.

The CSRC report also noted that NGS has released a beta version of the modernized horizontal and vertical datums for the NSRS: NGS New Datums.

Once the modernized NSRS is fully published, and in response to the needs of California’s user community, CSRC will continue working to secure resources that support its partnership with NGS and ensure ongoing compatibility with national programs.