[PROJ] ETRFxx to national ETRS89 realizations (e.g. ETRS89-NLD [AGRS2010], RGF93 v2/v2b, ETRS89-HUN [ETRF2000], etc.) (was Re: Time dependency in ETRFxx to ETRFyy coordinate transformations?)

[Lesparre, Jochem]

> So I'm thinking the output coordinate when applying "ETRS89-NLD [AGRS2010] to ETRF2000 (1)" is at coordinate epoch 2010.5, right?   Shouldn't the PROJ pipeline +proj=set +v_4=2010.5 ?

For the transformation from ETRS89-NLD [AGRS2010] to ETRF2000 that would make sense, but for the reverse transformation you would strictly need ETRF2000 velocities of your input points to change the epoch to 2010.50. As ETRF2000 can be considered static for most applications in the Netherlands, the epoch could be set to 2010.50 in both directions in practice. However, setting the epoch is most important if you want to transform ETRS89-NLD [AGRS2010] to ETRF2000 to ITRF2020 (or reverse). In that case you need to set the epoch to the required epoch (e.g. current epoch) for the output ITRF2020 coordinates, not 2010.50.

3 Steps between ETRS89-NLD[AGRS2010] and ITRF2020 at current:
ETRS89-NLD[AGRS2010] <-> ETRF2000 at 2010.50 (null transformation)
ETRF2000 at 2010.50 <-> ETRF2000 at current (null transformation or based on station velocity)
ETRF2000 at current <-> ITRF2020 at current (time-dependent transformation)

Jochem



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