I was reviewing some information regarding datum pairing and understand the article refers to NAD83 and Geoid pairing.?ÿ But what about the NAVD88 vertical datum, when is this considered over a geoid model as a vertical datum?
As I think I understand it, NAVD88 is pinned to the orthometric height of physical bench marks that have been leveled to in the past, and the current geoid model is derived from GPS measurements of ellipsoidal heights at those marks (and other data). If you want to interpolate between marks you use that geoid model and the ellipsoid used by NAD83.
The new datum in the works will appear maybe in 2025 and reverse that, with the official elevation being obtained from the GPS ellipsoidal height combined with a new geoid model.
Mike can correct me if needed.
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Bill's got it right, the current/recent geoids have been hybrid models - a gravimetric model that is constrained to known/observed geoid heights at published stations.
You can check the estimated geoid uncertainty with the NGS online computation tool.
As far as when you want to hold to an NAVD88 levelled elevation versus a geoid-derived elevation, that's entirely a function of project location, size, goals and potential pitfalls.
If you're using GNSS and looking to be on NAVD88, the easiest way to get there is with a geoid. Whether you hold a published orthometric elevation or a published ellipsoid height is up to you, and can have implications for the rest of the project area.
In my experience it's the larger projects, and corridor projects, that can get tricky. Precise levelling over long-ish distances may reveal geoid separations that do not quite jive with current model, or at least the rate of change does not match up well enough to meet project specs when using GNSS.?ÿThis can lead to a situation where, if you want to hold levelled elevations, you're either trying to create/modify a custom geoid (only heard of it being done, never done it myself), doing a site calibration (danger Will Robinson!) or you're going to have to use terrestrial methods. Again, it's largely project-dependent.
Ultimately, best practices is to publish both ellipsoid and orthometric on control sheets, noting which orthos were levelled and which were geoid-derived.
As can be seen in the graphic, there is a significant cross-CONUS tilt between NAVD88 and the best information available.
Implicit in the various NGS guideline documents 58 and 59 is recognition that transferring heights to areas without benchmarks can be done efficiently via GNSS and a geoid model. Over local scales, differential leveling is preferable.
The ??Leveling Projects Page? https://beta.ngs.noaa.gov/datasheets/leveling-projects/index.html can be used to assess leveling projects published by the NGS.
Lots of decades-old leveling, lots of lost monuments, lots of disturbed monuments.?ÿ
Providing a National vertical datum by means other than GNSS + a geoid model is not going to happen.?ÿ
Of course in the future we will probability see our height system determined by relativistic geodesy using atomic clocks and gravity meters.?ÿ
Some tools useful for analysis of NGS-leveling data.
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1.?ÿ https://geodesy.noaa.gov/datasheets/ ?ÿ - web page to retrieve information for points in the NGS data base.
2.?ÿhttps://beta.ngs.noaa.gov/datasheets/leveling-projects/index.html /a>?ÿ- beta site page to display information about a level project using it's unique identifier.?ÿ
3.?ÿ https://geodesy.noaa.gov/cgi-bin/phase1.prl ?ÿ - input page for the retrieval of field adjusted heights for a level project. It allows selection of corrections to be applied. Requires an input file.
4.?ÿ https://beta.ngs.noaa.gov/GEOID/xGEOID20/computation.shtml ?ÿ- interactive computation tool prompting for latitude, longitude, ellipsoid height, output epoch (optional) and name.?ÿ
They have you covered six ways to Sunday. ?ÿ
