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<p class="MsoNormal"><span lang="DA" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Hi Bo,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="DA" style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">You are on the right path. It is true that ETRS89 is plate-fixed and that WGS84 is not and that the two slowly drift apart. There are two facts that makes this
more complicated than most of us would like:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">There exists 6 realizations, or versions, of WGS84 that differ up to a meter between the two versions that are farthest apart. Since in general in GIS
we never specific *<b>which</b>* of the six WGS84’s the uncertainty in the transformation is larger than the 30 years of continental plate movement.
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">To do an accurate transform between a global reference like WGS84 and a plate-fixed frame like ETRS89 you need to know the observation time of your
coordinate. What if you WGS84 coordinate was measured in 1995? Then you only need to account for a few years of plate movement compared to a coordinate from today where 30 years is needed.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">Because of the two above facts the default transformation between WGS84 and ETRS89 is the identity transformation. Improving the situation required both time-stamped
coordinates and knowledge of the specific WGS84 version in use.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">For the Danish case, if you really need high accuracy transformations you can take a look at
<a href="https://github.com/NordicGeodesy/NordicTransformations">https://github.com/NordicGeodesy/NordicTransformations</a> where transformations with cm-accuracy between ITRS2014 (basically WGS84 today) and the Danish realization of ETRS89 is available in
the form of PROJ parameter and grid files.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D">/Kristian<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"> QGIS-Developer <qgis-developer-bounces@lists.osgeo.org>
<b>On Behalf Of </b>Bo Victor Thomsen<br>
<b>Sent:</b> 25. september 2019 08:24<br>
<b>To:</b> QGIS Developer Mailing List <qgis-developer@lists.osgeo.org><br>
<b>Subject:</b> [QGIS-Developer] CRS problems with WGS84 and ETRS89<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p>Hi list members -<o:p></o:p></p>
<p>I have question regarding coordinate reference systems.<o:p></o:p></p>
<p>As I understand the difference between the datums WGS84 and ETRS89:<o:p></o:p></p>
<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
They are based on the same ellipsoid. And in 1989 there were - for all practical purposes -
<i>no</i> difference between the the 2 datums coordinate wise<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
However, the datums are "drifting" apart because the ETRS89 is coordinate bound to the Eurasian continental plate and WGS84 is not.
<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
The continental drift is ca 2.5 cm / year. And around 70 cm in all.<o:p></o:p></li></ul>
<p>So a specific position in Denmark (or Europe) in the same mathematical projection i.e UTM32N should have 2
<i>different</i> x,y values using the WGS84 datum and ETRS89 datum. And the present "distance" should be ca. 70 cm.
<o:p></o:p></p>
<p>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.<o:p></o:p></p>
<p>(Doing the same experiment using ED50 instead of WGS84 show the expected difference around 200 m between ED50 and ETRS89)
<o:p></o:p></p>
<p>How come ? Or is the error-source sitting ca. 70 cm in front of the screen ?? (I have a 34" large flatscreen :-)
<o:p></o:p></p>
<p> <o:p></o:p></p>
<pre>-- <o:p></o:p></pre>
<pre>Med venlig hilsen / Kind regards<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>Bo Victor Thomsen<o:p></o:p></pre>
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