[PROJ] Fwd: some help with cm-level GRS80 to WGS-84 transformation?

[Greg Troxel]

Kirk Waters - NOAA Federal via PROJ <proj at lists.osgeo.org> writes:

> I'm sure others with more experience will jump in, but I think the main
> issue here is that WGS84 is a family of reference frames that are all based
> on the GRS80 ellipsoid. So, the problem doesn't exactly make sense and the
> various realizations of WGS84 differ at the cm level. Are you perhaps
> trying to go from a NAD83 realization (such as NAD83(2011)) to a WGS84
> realization (maybe ITRF14
> or something)?

I think Kirk has it exactly right.

Also, GRS80 is not a datum, but only a name for an ellipsoid.  So
"converting from GRS80" does not make sense.


Further into the weeds, WGS84 realizations are dynamic datums and if you
are talking about coordinates of crust-fixed objects you need to pay
attention to epoch.  If you care about a few cm, you really need to pay
attention.
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