17 Professional Land Surveyors of Oregon | www.plso.org Featured Article CONFUSION IS COMING By Andrew Plett, PLS As most surveyors are aware, the National Geodetic Survey is working hard on the next set of datums for the United States. This work encompasses a new Horizontal Datum, a new Vertical Datum, and new map projections for each state. They are: • North American Terrestrial Reference Frame of 2022 (NATRF2022) will replace NAD83. • North American-Pacific Geopotential Datum of 2022 (NAPGD2022) will replace NAVD88. • GEOID2022 will replace Geoid18. • State Plane Coordinate System of 2022 (SPCS2022) will replace the current State Plane zones, and in many states, add new low-distortion zones similar to Oregon DOT’s OCRS zones. Due to many factors, the release of these new systems is delayed, with the current FAQ page (https://geodesy.noaa.gov/ datums/newdatums/FAQNewDatums. shtml) stating, “It is not out of the question to consider a complete roll-out of the modernized NSRS to be somewhere in the 2024–2025 timeframe.” NAPGD2022’s Elevation Shift This article focuses on changes that NAPGD2022 will bring and the unique impact that it will have on the Pacific Northwest. I have worked in the Pacific Northwest for my entire 18-year career in land surveying. A rule of thumb I was taught was if you are checking into elevations of existing control from an unknown datum and your measurements are around three feet different, the most likely cause is a difference in vertical datum. NAVD88 and NGVD29 are about 3.5 feet apart on average in Oregon. When NAVD88 was introduced in 1991, many cities and counties in Oregon did not update due to cost and stayed on NGVD29. This created a mismatch between many urban and rural communities, but due to the significant difference between the two datums, it was easy to detect. Preliminary models available from NGS on its xGeoid page (https://beta.ngs.noaa. gov/GEOID/xGEOID/) reveal that in the Pacific Northwest, the new NAPGD2022 vertical datum will drop by about one meter in many parts of the state. Because the shift is similar to a gradient, some communities in Oregon will see the new elevations come within 0.10 feet or less of the old NGVD29 elevations, while other areas will have differences of a few tenths up to about one foot compared to NGVD29. According to the current FAQ page, this shift back to seemingly NGVD29 elevations is coincidental. As a result, Oregon surveyors will have to be more careful in the future to determine the datum of existing control, and to adequately define the vertical datum on current projects. In the future, we will Approximate Orthometric Height Change Figure 1: NGS diagram showing the Approximate Orthometric Height Change nationwide, in meters no longer be able to rely on a 3.x-foot difference to warn us of a vertical datum difference. A Statewide Review of Upcoming Elevation Differences To provide a preview of the expected magnitude of the NAVD88 to NAPGD2022 difference and the relationship to NGVD29, I sampled existing NGS vertical control benchmarks (of First Class or better, where available) around the state that had existing NAVD88 elevations and superseded NGVD29 elevations. From there, I used the xGeoid20 Interactive Computation page (https:// beta.ngs.noaa.gov/GEOID/xGEOID20/ computation.shtml) to calculate approximate NAPGD2022 elevations at each location. The results are shown in the table on page 18. To help visualize the shift at a larger scale than Figure 1, I used Trimble Business Center to create a surface model of the difference between NAPGD2022 and NGVD29, which is shown in Figure 2. continues
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