Jonathan Gregory wrote:
>Deat Rich
>
>These are tricky issues, which we've worried about a bit wrt sea level
>quantities, but not sorted out thoroughly.
>
>
>
>>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.
>>
>>
>
>Yes, I suppose so. I had only thought of the connected global ocean, for
>which "geoid" is appropriate.
>
>This problem is broader than dimensionless coordinates. The basic problem is
>what "depth" means as a vertical coordinate in the ocean or the ocean
>model. At present we have a standard_name of depth, defined relative to the
>surface, which is further defined as mean sea level. But
>
>- it might be relative to the geoid or some other geopotential surface, instead
>of mean sea level, so perhaps we should have separate standard names for
>depth_below_geoid and depth_below_surface.
>
>- what does it mean for an ocean model? So far (e.g. for the IPCC data
>collection) we have been assuming that in a rigid-lid model, "geoid" means
>the rigid lid z=0. What does z=0 mean in an ocean model with a free surface?
>
>
>
>>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?
>>
>>
>
>If the local datum is a geopotential, a single number rather than a field ought
>to be enough to specify the offset, shouldn't it?
>
>
>
>>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..
>>
>>
>
>Yes, we are being vague. In some cases e.g. ocean models for climate, this
>doesn't matter. But sometimes it clearly would matter. So we may need some
>further way to specify the geoid, as one of the reference ellipsoids.
>
>Comments on the above?
>
>Cheers
>
>Jonathan
>_______________________________________________
>CF-metadata mailing list
>CF-metadata at cgd.ucar.edu
>http://www.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
>
Im sure you all know that the GIS community has been wrestling with this
level of detail for a while, although likely a lot of concepts from
modelers will be new. At Unidata we have been closely following the OGC
specs, with an eye to using them and extending when needed. I think the
BADC folks know about this in more detail than I. Can we start from OGC
definitions and see what needs to be added, then create CF conventions
for them ?
Received on Tue Oct 26 2004 - 09:50:16 BST
