As most readers already know, the National Geodetic Survey (NGS) is modernizing the National Spatial Reference System (NSRS) through the introduction of a new horizontal reference frame and vertical datum to replace NAD 83 and NAVD 88. One important aspect of the modernized NSRS is that it will be dynamic to account for the earth’s crustal motion. To ensure coordinates are comparable, users will need to convert and transform coordinates collected at a given time — called an “epoch” — to other epochs, including NGS’s standard “reference epochs.”
NGS has released a new Technical Memorandum that defines the mathematical relationship between coordinates across different reference frames and epochs. The two most important frames for North American users are NATRF2022 (the North American Terrestrial Reference Frame of 2022) and ITRF2020 (the International Terrestrial Reference Frame of 2020). While ITRF coordinates are allowed to drift with crustal motion, NATRF2022 is an Earth-centered Earth-fixed (ECEF) frame from which rigid North American plate motion has been removed.
To transform NATRF coordinates, users will need to apply NGS Euler Pole Parameters (EPPs) that describe the rigid motion of the North American Plate. These parameters will be made available in a model called EPP2022, to be published by NGS.
However, rigid plate motion is not the only cause of coordinate shift. Another source is intraplate deformation due to faulting and glacial isostatic adjustment. Thus NGS is also developing an intra-frame velocity model (IFVM) that will model motion for every survey mark in the NSRS. The model will be called IFVM2022. One complicating factor is that NGS is designing IFVM2022 to include, not just intraplate deformation, but also motion due to rigid plate rotation. This means it will be impossible for users to apply IFVM2022 directly to NATRF2022 coordinates without first transforming their coordinates to the ITRF.
For example, a surveyor interested in comparing coordinates in NATRF2022 at epoch “A” to NATRF2022 coordinates at epoch “B” would need to: a) apply EPP2022 to transform the NATRF coordinates at epoch A to ITRF2020 coordinates at epoch A, b) apply IFVM2022 to transform the ITRF2020 coordinates at epoch A to ITRF2020 coordinates at epoch B, and c) apply EPP2022 to transform the ITRF2020 coordinates at epoch B to NATRF2022 coordinates at epoch B. One model (IFVM2022) changes the epoch but not the frame, while the other model (EPP2022) changes the frame but not the epoch.
The NGS Technical Memorandum addresses the inefficiency of this approach by proposing a “virtual” version of IFVM2022 — labelled IFVM2022(N) — that would operate within NATRF2022 rather than ITRF2020. The paper gives the mathematical foundation to show that a one-step path through IFVM2022(N) is identical to the multi-step approach using IFVM2022 and EPP2022. The paper does not indicate whether NGS will develop such a virtual model, but rather proves that it is mathematically possible.
The same logic applies to the three other new NSRS terrestrial reference frames: the Pacific Terrestrial Reference Frame of 2022 (PATRF2022), Mariana Terrestrial Reference Frame of 2022 (MATRF2022) and Caribbean Terrestrial Reference Frame of 2022 (CATRF2022).
To add one more (small) wrinkle, NGS — in collaboration with the Canadian Geodetic Survey — has changed the name of the Intra-Frame Velocity Model (IFVM2022) to Intra-Frame Deformation Model (IFDM2022).
For additional SCO postings on these topics, as well as more information on Wisconsin’s response to the NGS NSRS modernization effort, see the WSRS2022 (Wisconsin Spatial Reference System 2022 Task Force) page here.