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

[Nicolas Cadieux]

Hi,

I think he is trying to go from the GRS80 Ellipsoid to the WGS84 
Ellipsoid...  If this is the case, the difference is already 
sub-millimeter.   Both have the Semi-major axis of 6378137 but the 
Inverse of flattening is 298.257223563 for wgs84 and 298.257222101 for 
GRS80.

Nicolas


On 2022-01-19 6:06 p.m., 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.
>
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-- 
Nicolas Cadieux
https://gitlab.com/njacadieux