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<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">Thanks Javier, for the effort of explaining it so clearly to all readers. Two things are missing in your overview, though.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">(1) Each lat-lon CRS has 3 different EPSG codes:<o:p></o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2"><span lang="EN-GB" style="mso-fareast-language:EN-US">2D lat, lon (e.g. WGS84: EPSG:4326)<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2"><span lang="EN-GB" style="mso-fareast-language:EN-US">3D lat, lon, ellipsoidal height (e.g. WGS84: EPSG:4979)<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2"><span lang="EN-GB" style="mso-fareast-language:EN-US">XYZ in metres (e.g. WGS84: EPSG:4978)<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">NB: XYZ is not a projected CRS and the axis are generally not aligned with the earth surface.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">(2) There are four Global Navigation Satellite Systems (official abbreviation: GNSS, but commonly all referred to as GPS):<o:p></o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span lang="EN-GB" style="mso-fareast-language:EN-US">GPS Navstar by US military with CRS: WGS84<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span lang="EN-GB" style="mso-fareast-language:EN-US">Glonass by Russian military with CRS: PZ-90<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span lang="EN-GB" style="mso-fareast-language:EN-US">Galileo by EU (civilian) with CRS: GTRS<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><span lang="EN-GB" style="mso-fareast-language:EN-US">BeiDou by Chinese military with CRS: BDC<o:p></o:p></span></li></ul>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">The 4 GNSS are managed by organisations of different countries. Using WGS84 as the standard CRS is a political choice. But the choice for WGS84 mostly originates from ignorance about
the different world-wide datums. Therefore, the term WGS84 often means: “lat-lon coordinates in any datum, I don’t know which”.
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">There is a more precise and politically neutral scientific alternative CRS available: ITRS, with most recent realisation ITRF2014 (2D EPSG:9000, 3D EPSG:7912, XYZ EPSG7789). There is
no EPSG code for the datum ensemble ITRS, nor projected CRS for ITRF2014 in EPSG.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US">Jochem<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> PROJ <proj-bounces@lists.osgeo.org>
<b>On Behalf Of </b>Javier Jimenez Shaw<br>
<b>Sent:</b> zondag 18 april 2021 22:54<br>
<b>To:</b> proj <PROJ@lists.osgeo.org><br>
<b>Subject:</b> Re: [PROJ] World UTM in a proper datum<o:p></o:p></span></p>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Reading some answers, I realized that the term "WGS84" is very polysemic. To see the different meanings (are there more?), let's use this WKT:<o:p></o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal"><span style="font-family:"Courier New"">projinfo EPSG:32632 -o WKT1:GDAL -q</span><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><span style="font-family:"Courier New""><br>
PROJCS["WGS 84 / UTM zone 32N",<br>
GEOGCS["WGS 84",<br>
DATUM["WGS_1984",<br>
SPHEROID["WGS 84",6378137,298.257223563,<br>
AUTHORITY["EPSG","7030"]],<br>
AUTHORITY["EPSG","6326"]],<br>
PRIMEM["Greenwich",0,<br>
AUTHORITY["EPSG","8901"]],<br>
UNIT["degree",0.0174532925199433,<br>
AUTHORITY["EPSG","9122"]],<br>
AUTHORITY["EPSG","4326"]],<br>
PROJECTION["Transverse_Mercator"],<br>
PARAMETER["latitude_of_origin",0],<br>
PARAMETER["central_meridian",9],<br>
PARAMETER["scale_factor",0.9996],<br>
PARAMETER["false_easting",500000],<br>
PARAMETER["false_northing",0],<br>
UNIT["metre",1,<br>
AUTHORITY["EPSG","9001"]],<br>
AXIS["Easting",EAST],<br>
AXIS["Northing",NORTH],<br>
AUTHORITY["EPSG","32632"]]</span><o:p></o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">We see that WGS84 can be:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> - Spheroid/Ellipsoid (EPSG:7030)<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> - Datum (EPSG:6320). A datum ensemble in this case (more about that later)<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> - The prefix of all the 6 datums members of that datum ensemble EPSG:6320, like WGS84(transit) or WGS84(G1762)<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> - Geographic CRS (EPSG:4326)<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> - The prefix of many projected CRS. All those that have EPSG:4326 as the base geographic crs, like "WGS 84 / UTM zone 32N"<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">In addition to that, I met some people using it wrongly<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> - Some people use it to refer to any UTM projection over WGS84 (the one that affects them). They say "WGS84" instead of "WGS84 / UTM zone 30N"<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"> - As the "one and only" lat-long system<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">The accuracy problem I mentioned in previous emails is due to the fact that the datum "WGS_1984" is a datum ensemble, that produces that you are not sure about which one are you talking about:<o:p></o:p></p>
<div>
<p class="MsoNormal"><a href="https://epsg.org/datum_6326/World-Geodetic-System-1984-ensemble.html">https://epsg.org/datum_6326/World-Geodetic-System-1984-ensemble.html</a><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">(actually EPSG marks it with an accuracy of 2 m)<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">The same happens to the associated geographic crs, WGS84 EPSG:4326. And that "pollutes" all the projected crs that use EPSG:4326 as the geographic crs.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">The last member of that ensemble, WGS84(G1762), has a geographic crs, EPSG:9057. However, there are no projected crs on it in EPSG.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Cheers,<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Javier<br clear="all">
<o:p></o:p></p>
<div>
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt">.___ ._ ..._ .. . ._. .___ .. __ . _. . __.. ... .... ._ .__<br>
Entre dos pensamientos racionales <br>
hay infinitos pensamientos irracionales.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<div>
<p class="MsoNormal">On Sun, 18 Apr 2021 at 16:37, Lesparre, Jochem <<a href="mailto:Jochem.Lesparre@kadaster.nl">Jochem.Lesparre@kadaster.nl</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Duncan wrote:<o:p></o:p></p>
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<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB">> As an onlooker, I have a question: what is UTM/WGS84 intended to mean?
</span>Is it (A) the UTM projection using the WGS84 ellipsoid<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB">as a parameter, or (B) is it meant to imply also that the coordinates are in a datum that is one of the several labelled WGS84?
</span>A is clear and completely unambiguous; B isn't, partly because it is subject to change as data are collected (not to mention plate motions). Data-dependence is also a problem with geoid models: it is why there are so many, after all.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB">Duncan, I think WGS84-UTM is normally intended to mean both A and B so it is using a datum of WGS84 realisation and the WGS84 ellipsoid. According to ISO and
EPSG de ellipsoid is part of the datum, so in that case the answer A and B is the same as just B.
</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="EN-GB">Jochem
</span><o:p></o:p></p>
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<p class="MsoNormal"><br>
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