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

[Joseph Boardman]

OK, I'll take my licks and ask again.

How would I use Proj to do a precision transform from a particular NAD83 
(GRS80) realization and a particular WGS-84 realization?

On 1/19/2022 4:06 PM, Greg Troxel wrote:
> 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.