Hi all,
for temperature, the units imply a zero point, but for height they
don't. For time, we specifically include the zero point in the units
(e.g., "days since 20010101") and we also distinguish among various
calendars with the "calendar" attribute. For height I wouldn't advocate
that approach, but rather the already proposed hybrid approach: the
standard name (roughly) specifies the reference surface, which can then
be more precisely defined in another place (e.g., within the grid_mapping).
best regards,
Karl
On 2/11/14, 10:05 AM, Jim Biard wrote:
Hi.
It seems to me that tacking on a description of the datum in the
standard name isn't a good plan. It creates a linkage between
standard names and grid mappings / WKT blocks. The nature of the
height of the sea surface is
not altered by the choice of datum. The values will be different,
depending on what sort of height, but you can (most of the time!)
translate heights from one CRS to another. It is definitely more
complicated, but tacking on a datum description appears to me to be
akin to having different standard names for "temperature_in_C" and
"temperature_in_K". If you have properly specified your CRS, the
question of where the zero in your height scale is located is
completely unambiguous.
Grace and peace,
Jim
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On Feb 11, 2014, at 11:43 AM, Jonathan Gregory
<j.m.greg...@reading.ac.uk <mailto:j.m.greg...@reading.ac.uk>> wrote:
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
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