Negative Values on RTK Elevation

@flyinmada Have you found a solution yet? I’m in Massachusetts and often get negative ellipsoid heights then convert them using VDATUM. These ellipsoid heights are correct and when I use VDatum to convert, the results align relatively well with established local geodetic markers. Trying using WGS84 transit as your source. When prompted for epoch, put 2010.0 in both boxes. See if these results line up with a nearby record elevation.

carefull in areas with allot of movement a “known reference” may just be a out of date reference. tectonic plate movement, subsidence, earth quakes, sea level change…its all moveing

which is controlled to what? if you want your stuff to match a existing data set thats one thing but dont trust someone elses work untill you have validated thier methods.

this is what i do, it may not seem elegant but it is the right way to check your work.

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Just out of curiosity, what were the negative values like? I just did a quick test and it seems that around the Puget Sound area, the geoid undulation is around -21m, meaning that for a MSL elevation of 0m (at the coast), the GPS should return -21m.

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Agreed! For sites that I work on, the “known reference” is often a rebar stake that my team has driven into a beach or other surface. Using that reference as a basis for other work (construction, engineering design, etc.) at the same site works well enough for my purposes within a timeframe of a few years at sites that are usually less than an acre in size. Different methods are suitable for different applications.

Same goes for our use of LiDAR. In Washington, the state Dept. of Natural Resources is continually updating their LiDAR surfaces in coastal regions every few years. For a site where highest accuracy isn’t a concern, pulling the LiDAR elevation off of a paved surface is usually close enough to get by. I agree it’s not the most accurate solution, but it’s good to get a quick and dirty survey in the ballpark of where it ought to be. For really accurate stuff, we still use a total station setup.