[PROJ] Static/Dynamic Webmapping Problem version 2.0

[Greg Troxel]

Martin Desruisseaux <martin.desruisseaux at geomatys.com> writes:

> I did not yet had the time to follow this thread closely (will try to do
> later), but I would like to react on some points:
>
>   * In my reading of ISO 19111 §3.1.16 [1], the difference between WGS84
>     realizations can be close to one meter. Note huge, but not
>     irrelevant neither.
>   * ISO 19111 §3.1.62 seems to define a static reference frame as a
>     frame without time evolution, regardless if fixed to the plate or not.
>   * In past emails I have seen WGS84 cited as a "dynamic datum". In my
>     understanding, it is rather a "datum ensemble", which is not the
>     same thing.

I think the differing usages stem from there being three interpretations
of an unqualified "WGS84":

   The original, WGS84(TRANSIT)
   The one currently used by GPS, WGS84(G1762)
   A set of all of them, and the notion that one doesn't know which one

It's not useful to argue about which concept the bare word should
properly be bound to, but I think it is a leap to assume that any time
anyone uses the bare word that they are intentionally referring to the
set/ensemble meaning and intending to include WGS84(TRANSIT), superceded
in 1994.

You have succeeded in convincing me that I am not sure what static and
dynamic really mean!

> Useful quote from ISO 19111:
>
>     “WGS 84” as an undifferentiated group of realizations including WGS
>     84 (TRANSIT), WGS 84 (G730), WGS 84 (G873), WGS 84 (G1150), WGS 84
>     (G1674) and WGS 84 (G1762). At the surface of the Earth these have
>     changed on average by 0.7m between the TRANSIT and G730
>     realizations, a further 0.2m between G730 and G873, 0.06m between
>     G873 and G1150, 0.2m between G1150 and G1674 and 0.02m between G1674
>     and G1762).

Thanks for digging that out.  There's a useful chart at

 https://confluence.qps.nl/qinsy/latest/en/world-geodetic-system-1984-wgs84-29855173.html

While it seems pedantically true about WGS84 being a group of datums
with a difference close to a meter, note that WGS 84 (G1150) was
introduced in 1997, and the variations from G1150 on are about 0.22m by
what you quote, and 0.02m from the page I linked.  So if one qualify
WGS84 to "WGS 84 from the last 20 years", the scary large variance is
not so present.  (I agree that without some metadata you can't assume
that, but non-differential positions from pre-1997 GPS are much worse
than 1m anyway, SA and all.)

> [1] http://docs.opengeospatial.org/as/18-005r4/18-005r4.html