[PROJ] World UTM in a proper datum

Lesparre, Jochem Jochem.Lesparre at kadaster.nl
Mon Apr 19 01:09:04 PDT 2021 

Thanks Javier, for the effort of explaining it so clearly to all readers. Two things are missing in your overview, though.

(1) Each lat-lon CRS has 3 different EPSG codes:

  *   2D lat, lon (e.g. WGS84: EPSG:4326)
  *   3D lat, lon, ellipsoidal height (e.g. WGS84: EPSG:4979)
  *   XYZ in metres (e.g. WGS84: EPSG:4978)

NB: XYZ is not a projected CRS and the axis are generally not aligned with the earth surface.


(2) There are four Global Navigation Satellite Systems (official abbreviation: GNSS, but commonly all referred to as GPS):

  *   GPS Navstar by US military with CRS: WGS84
  *   Glonass by Russian military with CRS: PZ-90
  *   Galileo by EU (civilian) with CRS: GTRS
  *   BeiDou by Chinese military with CRS: BDC

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”.

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.

Jochem


From: PROJ <proj-bounces at lists.osgeo.org> On Behalf Of Javier Jimenez Shaw
Sent: zondag 18 april 2021 22:54
To: proj <PROJ at lists.osgeo.org>
Subject: Re: [PROJ] World UTM in a proper datum

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<mailto: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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