[PROJ] World UTM in a proper datum

[Javier Jimenez Shaw]

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:

projinfo EPSG:32632 -o WKT1:GDAL -q

PROJCS["WGS 84 / UTM zone 32N",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.257223563,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0,
            AUTHORITY["EPSG","8901"]],
        UNIT["degree",0.0174532925199433,
            AUTHORITY["EPSG","9122"]],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",9],
    PARAMETER["scale_factor",0.9996],
    PARAMETER["false_easting",500000],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AXIS["Easting",EAST],
    AXIS["Northing",NORTH],
    AUTHORITY["EPSG","32632"]]

We see that WGS84 can be:
 - Spheroid/Ellipsoid (EPSG:7030)
 - Datum (EPSG:6320). A datum ensemble in this case (more about that later)
 - The prefix of all the 6 datums members of that datum ensemble EPSG:6320,
like WGS84(transit) or WGS84(G1762)
 - Geographic CRS (EPSG:4326)
 - The prefix of many projected CRS. All those that have EPSG:4326 as the
base geographic crs, like "WGS 84 / UTM zone 32N"

In addition to that, I met some people using it wrongly
 - 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"
 - As the "one and only" lat-long system

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:
https://epsg.org/datum_6326/World-Geodetic-System-1984-ensemble.html
(actually EPSG marks it with an accuracy of 2 m)
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.

The last member of that ensemble, WGS84(G1762), has a geographic crs,
EPSG:9057. However, there are no projected crs on it in EPSG.

Cheers,
Javier
.___ ._ ..._ .. . ._.  .___ .. __ . _. . __..  ... .... ._ .__
Entre dos pensamientos racionales
hay infinitos pensamientos irracionales.



On Sun, 18 Apr 2021 at 16:37, Lesparre, Jochem <Jochem.Lesparre at kadaster.nl>
wrote:

> Duncan wrote:
>
> > As an onlooker, I have a question: what is UTM/WGS84 intended to mean? Is
> it (A) the UTM projection using the WGS84 ellipsoid
>
> 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? 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.
>
>
>
> 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.
>
>
>
> Jochem
>
>
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