Jonathan Gregory wrote:
>Dear Rich
>
>
>
>>mean sea level should *not* be thought of as the local geopotential
>>for ocean models.
>>Consider the Gulf Stream region, where there is a jump in mean sea level
>>of order 1 meter!
>>
>>
>
>Exactly. Hence the geoid (or geopotential surface) and the "surface" (= mean
>sea level) should not be confused in defining standard names.
>
>
>
>>We could have differing standard names, or we could just say that depth
>>is defined
>>relative to "the datum" and have a "datum_offset" field that contains the
>>offset between "the datum" and the geoid.
>>
>>
>
>I tend to think it is better to make the surface/geoid distinction even more
>blatant by putting it in the standard name.
>
>
>
Ok, that's fine. I would just like to make sure that we allow the
possibility of using a
local datum for z=0 and specifying the offset between this local datum
and the geoid.
This is important, since many people use local datums that are not the
geoid, and we don't want
to break any of their software. (we don't break anything if we only
*add* variables and attributes)
Here's another possible solution:
Just as we allow different units, origin and offset for time:
(e.g. units:"days since 1968-05-23 00:00:00 EST")
we could allow different units, origin and offset for height:
units:"meters above WGS84 -10.24"
where you specify the geoid and the offset to the geoid (-10.24) in this
case.
Well, I guess that would create more problems than it would solve. But
I mention it
just in case it's a helpful way to think about this issue...
>>z=0 means the same thing whether free surface or rigid lid.
>>In free surface models, there is a variable (e.g. "zeta") that
>>represents the departure
>>of the free surface from the "datum" (z=0).
>>
>>
>
>The difference is that a rigid-lid ocean has fixed volume so z=0 is always its
>geoid in effect. However, unless zeta is constrained to have a zero global
>average, in a free-surface model the volume is not fixed so it might be that
>the geoid is a field with z = the global average of zeta, rather than z=0, if
>you define the geoid to be the surface the ocean would assume at rest. What
>do you think?
>
>
>
I agree with you that the average of the free-surface is not the datum
(z=0). I'm not
sure I understand the issue, however. This is why we want to be able
to specify the
offset between the datum and some pedigreed geoid.
>>Yes, why not list the reference ellipsoid.
>>
>>
>if relevant, yes. If OGC has a standard naming convention for them, as John
>Caron suggests, we could adopt it. Does anyone know where to find it?
>
>Best wishes
>
>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 Wed Oct 27 2004 - 14:13:46 BST
