[Qgis-user] Reproject from WGS84 (EPSG: 4326) to ETRS89/UTM32N (EPSG: 25832) considering the epoch

[Greg Troxel]

Jochem Kail via QGIS-User <qgis-user at lists.osgeo.org> writes:

> I would like to reproject a point layer with the CRS being WGS84 (EPSG
> 4326) to the standard German CRS being ETRS89/UTM32N (EPSG 25832) with
> high precision < 1m.

You can't do that, because WGS84 is an ensemble with an intrinsic
accuracy level of 2m.  That's the accuracy of the ensemble, before you
consider the accuracy of transformation.

> The geodatic datum ETRS89 did only correspond to
> WGS84 when it was established in 1989, but given that ETRS89 is
> plate-fixed but WGS84 is not, they presently differ by about 1m.

Yes, but surely you are referrring to a particular recent realization of
WGS84 when you say that.  Modern WGS84 and modern ITRF are close to the
same thing.

> Although some post say that it is possible in QGIS to consider the
> epoch (time the data have been mapped) when reprojecting earth-fixed
> (non-plate fixed) geographic coordinates to a plate-fixed geodatic
> datum, I found no solution to do this in the QGIS GUI.

You could also transform in gdal, if only as a test.

> I really would appreciate if someone could explain in detail where and
> how to do this transformation in the GUI of QGIS.

That's a fair question and I don't know.  Support for epoch is
relatively new in gdal and I am unclear on the state in qgis.  You
didn't say what version of qgis you are using, but if it's less than
3.44.11, you are not up to date and it would be good to fix that.
(3.44.x for some x >=4 is probably ok, but hard to tell).

I would ask that you follow up (on list) with:

  Does your data have an epoch associated with each data point?  If no,
  does it have an epoch associated with the whole dataset?

  Why do you believe that your data is in WGS84 (the ensemble)?

  Why do you believe that your data is accurate to on the order of a
  mater?

  How were the coordinates obtained?  (As in what kind of measurements,
  relative to what sort of controls, and what if any transformations.)

  Do the people who managed/prepared this data understand that 4326 is
  an ensemble and that it contains WGS84(TRANSIT)?

  Do the people who managed/prepared this data understand that there are
  no methods to obtain accurate coordinates in any realization of 4326
  (assuming those people are not in the US Air Force or NIMA, and I'm
  not going to help you with military issues!).

I realize these questions sound difficult, and I expect you won't get
good answers, but that trying will be helpful.

Greg