This question certainly pops up on a regular basis.
After a discussion on the MMI 'Ask' metadata discussion list, OceanSITES
decided to add 2 attributes to the Z axis variable, one is 'reference',
which
is intended for human readers, and the other is
'coordinate_reference_frame',
which contains a URN that holds the definition of the 3D coordinate
reference
system, which is, at least according to the EPSG, more useful than a simple
datum.
We are dealing only with data relative to instantaneous sea surface; we
don't have
a mechanism for describing height above the sea floor, which we may need
soon.
DEPTH:reference=<R>;
where currently vetted values for R are "mean_sea_level",
"mean_lower_low_water",
"wgs84_geoid" and the default, "sea_level".
DEPTH:coordinate_reference_frame="urn:ogc:crs:EPSG::5831"; or
HEIGHT:coordinate_reference_frame="urn:ogc:crs:EPSG::5829";
5831 and 5829 can be resolved at http://www.epsg-registry.org/ but are not
meant for human information. They're defined as 'depth (or height)
relative to instantaneous
water level, uncorrected for tide.'
We'd be happy to implement this differently, if CF comes up with a scheme.
Thanks - Nan
On 2/4/14 10:08 AM, Jonathan Gregory wrote:
Dear Rich
Nothing further has been done in CF about this, but I'd be glad if it could be,
since it is often asked. I think there is no reason not to do something, since
there is an evident need, if we could only agree what should be done, and the
main problem is getting a sufficient understanding of these complexities. So
I can only repeat my last para in the email you point to:
The discussion stalled because we didn't really understand what "vertical
datum" means! ... [If this concerns the definition of the geoid], we should
extend grid_mapping so it can identify the geoid by name, perhaps also giving
a URN as you say. Unlike the ref ellipsoid, the geoid cannot be specified
by metadata, as it's too complicated!
Is that the answer to your question of yesterday? i.e. would it suffice to
specify that by "geoid" in this data, "NAVD88" was meant? Of course, in other
datasets, geoid might mean a different definition. Specifying which geoid is
meant is not always possible (for instance in the case of model data), but we
could make it possible if that's useful. Is NAVD88 not a geoid name in your
example?
Best wishes
Jonathan
----- Forwarded message from "Signell, Richard" <rsign...@usgs.gov> -----
On a telecon yesterday with a coastal inundation modeling group, one
of the PIs asked me how to handle vertical datums in NetCDF --
specifically where to specify that the model bathymetry and water
levels were were relative to NAVD88. I wasn't sure how to reply.
Was there any resolution to the 2nd half of this question asked back in 2011?
http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2011/054483.html
I looked at the draft 1.7 spec, and the only vertical datum reference
info I found was the ability to specify VERT_DATUM in the new CRS
well-known-text (WKT) section:
http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.7-draft1/ch05s06.html#idp5644304
Is this how we should specify the vertical datum in CF, using VERT_DATUM in WKT?
Thanks,
Rich
--
Dr. Richard P. Signell (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
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