[CF-metadata] meaning of depth in the ocean

[Rich Signell]

Jonathan,

To summarize:   We will have a standard name
which defines the datum, and a parameter attribute that gives the offset.
Right?  I am happy with this.

-Rich

Gregory wrote:

>Dear Rich
>
>  
>
>>>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"
>>    
>>
>
>I see what you mean. I don't think we can do exactly what you propose because
>that units string doesn't comply with udunits so would break other software.
>I agree that this is effectively an offset in the units, like the difference
>between degC and K. udunits provides a syntax for specifying an offset but we
>have disallowed it in CF.
>
>In another thread we are discussing the introduction of a parameters attribute
>to supply numbers which are used to define quantities when the standard_name
>alone is not sufficient. We could have a standard_name of height_above_
>geopotential_surface (more general than geoid) with parameters to specify the
>the height of the geopotential surface above the geoid and to identify the
>geoid/ellipsoid.
>
>  
>
>>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.
>>    
>>
>
>The issue is that when comparing a free-surface model simulation with altimetry
>we would probably like heights wrt geoid i.e. subtract the area-average of zeta
>uniformly from zeta. In other applications the model might like to output zeta
>itself. Again, these are two different quantities (though only by a constant
>offset, as you say). I think we agree we need to be able to distinguish them.
>
>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