![]() It is important to note that the errors seen in Photo 4 don’t correlate with the distance from Father Point. They range from about 16 cm in Florida to -112 cm in the Pacific Northwest. Dru Smith, called “Improving the National Spatial Reference System”Ī figure of the error accumulation of NAVD 88. Image excerpted from the white paper by Dr. Because of the high accuracy of the data from both the GPS and GRACE (usually found in the 1 to 3 centimeters range) any significant inconsistencies could be identified as an error in NAVD 88. Spatial filtering of the leveling and GPS data to the wavelengths offered by GRACE removed any of the localized issues and allowed for a continent-wide view of the discrepancies. This estimate used NAVD 88 heights, co-located with GPS-derived ellipsoid heights, and a long-wavelength geoid model, completes the Gravity Recovery and Climate Experiment (GRACE). Such errors (a combination of leveling, geoid modeling, and VLBI errors) marked discrepancies in the -105 to +76 centimeter range at various locations around the country.Īnother estimate of error accumulation from leveling was performed in 2004 (Wang, et al, 2004). Observational errors account for some of these issues, but theoretical errors, such as the formulae used to convert leveled height differences into geopotential numbers in mountainous terrain, are also to blame. While this removed the warping found in NGVD 29, it introduced the potential for an accumulation of systematic errors. These numbers were computed through an adjustment of geodetic leveling, holding the single fixed point of Father Point/Rimouski. This is a prime example of why NGS is replacing both the NAD 83 datum and the NAVD 88 datum, because they are so closely intertwined.Īs previously discussed, NAVD 88 is realized through geopotential numbers and orthometric heights at hundreds of thousands of passive geodetic control monuments across North America. With that modification, the equation reads: = – N*. To “close” the earlier equation, NGS has produced a “hybrid geoid” model, (N*). This error ranges from 1.5 meters in Florida to 0.3 meters in the Pacific Northwest. That equation is: H = h – N.Ĭurrently, in NAVD 88, this equation is incorrect by a few meters, due to errors in the orthometric heights and a tilted set of systematic errors in ellipsoid heights. Smith points out that it is important to consider one of the most fundamental height equations of geodesy, which converts the ellipsoid heights (h) as derived from GPS into orthometric heights (H), as used on topographic maps, through a model of geoid undulations (N). While the error in horizontal coordinates is concerning, a vertical error is just as problematic. Subsidence, uplift, and other crustal motions of the Earth Īs a result of those problems, coupled with issues surrounding the nature of a passive-monument-based system of vertical geodetic control in general, NGS created and released its 10-year plan.Bias in the NAVD 88 H=0 reference surface compared to the geoid.Fragility and location of passive monuments.Cross-country accumulation of errors from geodetic leveling.Smith goes on to mention six major issues with NAVD 88, which warrant its replacement in 2022. Without an active maintenance plan or any consideration for readjustment, regional distortions impact its value. NAVD 88 holds a single point (Father Point, in Rimouski, Canada) as its fixed point. According to Smith’s white paper, some localized leveling allowed original heights to be superseded, while in some cases, like the state of Louisiana, a number of questionable heights were removed in favor of updated leveling and GPS-based heights. However, it hasn’t been readjusted since its implementation. NAVD 88 was an improvement over the 1929 datum, with 100,000 benchmarks growing to 450,000 and leveling numbers going from 102,724 kilometers to 1,001,500 kilometers. Those scientific developments, as well as the 625,000 kilometers of leveling that were performed post-NGVD 29, were used to create NAVD 88. ![]() ![]() Changes included a widespread multi-agency effort to collect terrestrial gravity measurements, development of new corrections to leveling, and a deeper understanding of the differences between local mean sea level (LMSL) at disparate tide gages all contributed to the questioning of the National Geodetic Vertical Datum of 1929 (NGVD 29). ![]() ![]() The mid-20th century experienced various changes to the science of geodetic leveling, or the process of determining elevations above mean sea-level. ![]()
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