[QGIS-Developer] CRS problems with WGS84 and ETRS89

[Bo Victor Thomsen]

Hi list members -

I have question regarding coordinate reference systems.

As I understand the difference between the datums WGS84 and ETRS89:

  * They are based on the same ellipsoid. And in 1989 there were - for
    all practical purposes - /no/ difference between the the 2 datums
    coordinate wise
  * However, the datums are "drifting" apart because the ETRS89 is
    coordinate bound  to the Eurasian continental plate and WGS84 is not.
  * The continental drift is ca 2.5 cm / year. And around 70 cm in all.

So a specific position in Denmark (or Europe) in  the same mathematical 
projection i.e UTM32N should have 2 /different/ x,y values using the 
WGS84 datum  and ETRS89 datum. And the present "distance" should be ca. 
70 cm.

However, that is not the case in QGIS (v. 3.8):  A point digitized with 
position (550000, 6080000) in ETRS89/UTM32N has the (almost) exact same 
x/y values in WGS84/UTM32N when you change the projection in the map 
window from ETRS89/UTM32N to WGS84/UTM32N. The x/y values differs around 
.1 mm. Certainly no "drift" around 70 cm.

(Doing the same experiment using ED50 instead of WGS84 show the expected 
difference around 200 m between ED50 and ETRS89)

How come ? Or is the error-source sitting ca. 70 cm in front of the 
screen ?? (I have a 34" large flatscreen :-)


-- 
Med venlig hilsen / Kind regards

Bo Victor Thomsen

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