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[CF-metadata] fixed sensors, depth, datum

From: Ethan Davis <edavis>
Date: Thu, 11 Sep 2008 11:31:16 -0600

Hi all,

Jonathan Gregory wrote:
> Dear Dale
>
> It seems none of us who has so far contributed claims to be an expert. If any
> experts are listening in, they are welcome to comment!
>

Yes, please.

>> The recommendation is that I request a few new standard names.
>> Standard names like height_above_geoid and
>> height_above_reference_ellipsoid seem appropriate
>>
> Yes.
>

As I understand it, a vertical datum can be based on sea levels,
ellipsoids, or gravitational models. I think it would be better to add
height_above_vertical_datum rather than the two more specific terms above.

>> Can one use in any variable an
>> attribute normally found in the crs variable such as
>> vertical_datum_name? For example
>> float ht_wgs84
>> ht_wgs84:long_name = "Height referenced to WGS84";
>> ht_wgs84:standard_name = height_above_reference_ellipsoid
>> ht_wgs84:vertical_datum_name = "World Geodetic System 1984"
>>
> vertical_datum_name is not a CF attribute. CF doesn't preclude other attributes
> being included, so that's fine, but software implementing CF would not use
> the attribute.
>
> In my earlier posting, I suggested that we could define the geoid by
> extending the grid_mapping variable (CF 5.6). There is currently not a standard
> CF way to name the geoid or ellipsoid, but the ellipsoid can be defined
> geometrically by the grid_mapping variable. Any solution to this would need
> an addition to the CF standard.
>

Up to this point, grid mappings have only dealt with horizontal
coordinates. I think the vertical datum information would fit better as
attributes on vertical coordinates. That way it is closer to the actual
coordinate variable and doesn't require a redirection to the grid
mapping variable. This seems to more closely follow the lead of other
CRS models (ISO and OGC, including Well-Known-Text) in keeping the
vertical and horizontal coordinates separate. Combining a 2-D horizontal
coordinate system and a 1-D vertical coordinate system to build a 3-D
CRS. [With, as discussed earlier on this list I believe, an exception
for the case where the 3-D CRS is fully based on an ellipsoid model of
the earth. And for that we would need some way for the vertical
coordinate to reference a grid mapping variable as the place the
vertical datum is defined.]


Dale you also asked:
> In addition, there are locally derived datums that I assume are
> derived by observation over a specific period of time (e.g. tide datum
> epoch from1983-2001). Once again one can propose specific standard
> names, e.g. height_above_mllw. If there is no attribute like
> vertical_datum_epoch, then the epoch info could go in the comments
> attribute, but that does not seem satisfying.

I would rather see the definition and description of the vertical datum
kept somewhere external to the data file and probably external to CF.
With CF providing a place to reference the keeper of the vertical datum
definition and an ID for the referenced vertical datum (maybe
vertical_datum_authority and vertical_datum_name, or _id). Are there
cases when a vertical datum is well defined but not used repeatedly so
not worth standardizing? If so, then maybe CF should make it possible to
capture that information.

Thanks,

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 Thu Sep 11 2008 - 11:31:16 BST

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