Hi Jonathan,
Jonathan Gregory wrote:
>>>>> - height relative to the ellipsoid
>>>>> - height relative to the geoid
>>>>>
>> I would argue against them being different quantities because there
>> exist transformations between these various heights. Is that not enough
>> to indicate they are the same quantity?
>>
>
> I don't think so, because the transformations aren't a simple matter of
> convention, but involve geophysical data viz. the shape of geopotential
> surfaces. If you say that height relative to ellipsoid and geoid is the
> same quantity, aren't you saying that the geoid and ellipsoid are essentially
> the same thing? I don't think they are the same thing; they are by definition
> different from each other, and from other surfaces such as mean sea level and
> "the" surface (the bottom of the atmosphere).
>
To me, they both indicate location in a vertical dimension. And just
like latitude and longitude are dependent on the shape of the reference
ellipsoid and the prime meridian, the vertical dimension depends on the
vertical datum both to define the direction of vertical and the zero
level in the vertical. So, just like lat and lon are the same quantity
in different horizontal CRS it seems that height should be the same
quantity even when in different vertical CRS.
Which is why I think extending the horizontal CRS/grid mapping section
to include vertical CRS is the way to go. And whether the values are in
a coordinate variable or a data variable, they should reference a CRS.
Ethan
--
Ethan R. Davis Telephone: (303) 497-8155
Software Engineer Fax: (303) 497-8690
UCAR Unidata Program Center E-mail: edavis at ucar.edu
P.O. Box 3000
Boulder, CO 80307-3000 http://www.unidata.ucar.edu/
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Received on Fri Sep 26 2008 - 14:05:26 BST
