[PROJ] Transforming from epsg:4326 to epsg:31468 produces a 2 meter offset

[Greg Troxel]

"Lesparre, Jochem via PROJ" <proj at lists.osgeo.org> writes:

> I don’t understand why you are using EPSG:4326 (WGS84) when a 2 metre
> offset is not acceptable for you. EPSG:4326 is a general code for a
> collection (datum ensemble) of more precise defined datums (WGS84
> realisations) with their own codes. The differences within the datums
> of the ensemble prevent a transformation to EPSG:4326 that is more
> precise than 2 metres. So if you what a higher accuracy, you will have
> to use a CRS based on another datum, e.g. EPSG:9000 (ITRF2014).

OSM is defined to use WGS84.  From a viewpoint with an understanding of
geodesy in 2021, that's a bug.  In 2007 when people had available
single-frequency GPS receivers that only did navigation solutions, it
made sense.  But even the WGS84 that was in use in 2007 was very close;
if the pre-2007 realizations are excluded, the fuzz wouldn't be a
problem for most.

TMS is also defined to use WGS84, and that has the exact same problem.
I am not aware of any momentum to fix that.  Plus, with 15cm or better
imagery and velocities on the order of 5 cm, one needs to carry epoch
and velocity models, and that is well beyond what it seems webmapping is
up to dealing with.

Someone needs to rant less and actually figure out how to make proj
choose to transform to WGS84(G1762) when asked to transform to or from
WGS84 and submit a patch :-)   It's fair that the accuracy should be
degraded but there's no reason to drop the best-guess transform.


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