5/1/2023 0 Comments Datum conversion calculator![]() ![]() It is true the shift varies over the CONtinental US (CONUS), but that relationship between the two vertical datums is a snapshot in time dating back to 1990, the original date of the publication of NAVD88. ![]() While VertCon is indeed imbedded in CorpsCon, its use is of dubious value. Until then, current values are accessible at All data is open to the public and free for downloading. NGS continues to update its geoid models each year for the time being, and is currently flying gravimetric surveys over the entire US, territories and possessions in the "GRAV-D" project in preparation for the planned publication of the new "3D" datum in 2022. When "updated" by NGS or by private surveyors using approved NGS Standards & Practices, fewer significant digits are displayed in the published values. By observing ellipsoid heights according to procedures established and published by the National Geodetic Survey, we "update" static, passive benchmarks to their current elevations by "hanging off the current geoid." In summary, NAVD88 is still valid, some BMs are still "good" according to published values, some have to be updated from time-to-time, and their present elevations are indeed published by NGS. As of the original adjustment in the 1990s, selected groups of benchmarks have been periodically updated through observations of GPS campaigns termed "GPS Leveling." The concept presently being utilized is that the NAVD88 still exists, but some benchmarks are moving away from their initial positions, some subsiding, some rising. Since then, crustal motion has caused the static, passive benchmarks to become "stale" or obsolete with respect to their initial vertical positions as of the national adjustment. ![]() Crustal motion was recognized to exist at the time in several areas of CONUS and was initially defined by the static, passive benchmark monuments as of 1990 in most of the continent, published in following years for the various areas experiencing crustal motion. The current datum, the "North American Vertical Datum of 1988" was defined by the datum adjustment performed by least squares and indexed to "Father's Point" in Rimulski, Canada along the St. That datum was essentially a "snapshot" in time, and crustal motion was not a consideration when published. renamed the "National Geodetic Vertical Datum of 1929" was defined by the datum adjustment performed by least squares and indexed to the primary benchmark of the U.S. ![]()
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