[CF-metadata] Vertical datums (again)

From: Jonathan Gregory <j.m.gregory>
Date: Tue, 11 Feb 2014 16:43:33 +0000

Dear Rich

> Thanks for the detailed explanation (and analogy) of why it's useful
> to tack on the "_above_geoid" or "_above_ellipsoid" or
> "_above_tidal_datum" to the standard-name.
>
> So we do that and then specify the geoid, ellipsoid or tidal datum
> elsewhere in the grid_mapping variable, right?

Yes, that is the way we've been proceeding up to now. In fact we do not have
any stdnames yet referring to tidal datum, but you're welcome to propose them
if they're needed now, of course.

> geoid: NAVD88, GEOID93, GEOID96, USGG2009, etc
> ellipsoid: WGS84, Airy 1830, Airy Modified 1849, etc
> tidal_datum: MLLW, MLW, MTL, MHW, MHHW, etc

Thanks for these useful lists! I would tend to think that we should
give different standard names for the various different tidal datums, since
I would regard those as different geophysical quantities - would you agree?
If there was data which referred to a tidal datum but didn't actually know
which one it was, I suppose it might still be useful (if imprecise) and it
could have a standard name that referred to "tidal datum" generically. But
if you know it's mean_high_water (for instance), I would spell that out in
the standard name.

However I think the various geoids are all different estimates of the same
geophysical quantity, so they should *not* have different standard names.
Likewise the ref ellipsoid is the "best" ellipsoid approximating the geoid -
again, that is a single geophysical concept, with many alternative versions.

So we need a place to name the geoid, if that is the vertical datum. It would
be good to have a similar treatment to CRS WKT for this, but I don't see a
place in WKT where the geoid can be identified. Can anyone help? Is the geoid
implied by, or identical to, the vertical datum name, perhaps? How does one
know, in WKT, whether the vertical datum is a geoid or a ref ellipsoid?

Best wishes

Jonathan
Received on Tue Feb 11 2014 - 09:43:33 GMT