Jonathon,
Good point. I didn't think about datums. Most ocean modelers don't
make explicit what
vertical datum they are working with, but I guess if you are thinking
about using tools
like IDV, they need to know! Usually ocean modelere work with water
depths relative
to some local geopotential, which for places like the Great Lakes could
be hundreds of
meters above the geoid.
What do you think about adding to the CF convention the specification of
a 2D variable
(function of lat, lon) called "geoid_offset" (or equivalent) that would
specify the
offset between the "local datum" (z=0) and the geoid? This would allow
people to take
advantage of new software features in tools that support CF conventions
without breaking
the tools they already have.
And while we are on the subject, are we being intentionally ambiguous by
referring
to "the geoid"? I'm thinking about NAD27, GRS80, WGS84, etc..
-Rich
nathan Gregory wrote:
>Dear Rich
>
>I agree with you that it would be better to replace the existing definition
>of the ocean_s_coordinate with your new, less prescriptive, one. Contacting
>people who might be using it would be a good idea if they aren't subscribed
>to this group!
>
>Although you're right that sigma is a special case of s, I think we can keep
>sigma, just as we have both atmosphere sigma and hybrid-sigma-pressure.
>
>In all the definitions of sigma and s for ocean we have "relative to ocean
>datum (e.g. mean sea level)". Could we be more precise about these? We should
>be clear what quantity the calculated z will be, and that depends on the
>definition of its inputs. Would it be correct to say the heights and depths
>are all wrt geoid, for instance?
>
>Cheers
>
>Jonathan
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>
--
Richard P. Signell rsignell at usgs.gov
U.S. Geological Survey Phone: (508) 457-2229
384 Woods Hole Road Fax: (508) 457-2310
Woods Hole, MA 02543-1598
Received on Tue Oct 26 2004 - 08:21:24 BST
