Hi.
Heights are surprisingly complicated. The vertical datum can be an ellipsoid (with a sphere being a degenerate case), a geoid, or even a plane. Every vertical datum has a reference ellipsoid. The heights above the datum can be orthometric (normal to the datum surface at the point) or geodetic (normal to the ellipsoid). If your vertical datum is an ellipsoid, then the two types of heights are the same. If you are using a non-ellipsoidal vertical datum, you are probably going to report orthometric heights, but it is possible, if you happened to get your heights and X/Y positions from two different sources.
Grace and peace,
Jim
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On Feb 7, 2014, at 1:21 PM, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
> Dear all
>
> I expect people remember the very lengthy debate we had about including WKT
> in CF, in tickets 69 and 80, which should both be included in the next version
> of CF, although in the current draft it seems that only ticket 69 is there so
> far. In 5.6.1 of the new draft, it reads
>
> "The crs_wkt attribute is intended to act as a supplement to other
> single-property CF grid mapping attributes (as described in Appendix F); it is
> not intended to replace those attributes. If data producers omit the
> single-property grid mapping attributes in favour of the compound crs_wkt
> attribute, software which cannot interpret crs_wkt will be unable to use the
> grid_mapping information. Therefore the CRS should be described as thoroughly
> as possible with the single-property attributes as well as by crs_wkt."
>
> Ticket 80 adds a number of new attributes for grid_mapping, which will be
> in Table F1 of CF. See
> https://cf-pcmdi.llnl.gov/trac/attachment/ticket/80/issue80.2.txt
>
> One of these attributes is reference_ellipsoid_name. That corresponds to the
> SPHEROID name in WKT. As far as I can see in
> the definition of WKT,
> http://www.geoapi.org/3.0/javadoc/org/opengis/referencing/doc-files/WKT.html
> there isn't a way to name a geoid. I guess that's because it's only used for
> vertical reference, so it's probably implied by the VERT_DATUM name. Is
> that right?
>
> We could add a vertical_datum_name attribute in Table F1, just as ticket 80
> adds a horizontal_datum_name attribute. That could then be exactly what Rich
> requested. However, I have the same concern as before about the purpose of it.
> Is the vertical datum always a geoid? If so, that's not a problem; we could
> just say that the vertical_datum_name identifies the geoid, and therefore
> affects quantities whose standard_names indicate they refer to the geoid, such
> as altitude and sea_surface_height_above_geoid. If the vertical datum is not
> necessarily a geoid, what else could it be? I think we have to be careful about
> what quantities might be affected by the vertical datum. Insight about this
> would be most welcome!
>
> Best wishes
>
> Jonathan
>
>
> ----- Forwarded message from Jim Biard <jbiard at cicsnc.org> -----
>
>> From: Jim Biard <jbiard at cicsnc.org>
>> Date: Fri, 7 Feb 2014 10:38:23 -0500
>> To: CF metadata <cf-metadata at cgd.ucar.edu>
>> X-Mailer: Apple Mail (2.1827)
>> Subject: Re: [CF-metadata] Vertical datums (again)
>>
>> Hi.
>>
>> I think there is still value in adding an attribute to the grid_mapping set where the name of the vertical datum can be supplied. The datums (data?) have ?standard? names (not CF standard names). If you aren?t using a named vertical datum, you could specify either ?none? (or just don?t specify the attribute) or ?custom? if you are using a datum that hasn?t been given a name. I agree that the name alone is not necessarily sufficient, but it does provide a human-readable marker, whereas the WKT and URN approaches require further digging.
>>
>> Just to be clear, I?m not advocating either/or with regards to the more rigorous approaches. A ?vertical_datum? attribute that contains a human-readable name should be present in addition to one (or more) of the other approaches.
>>
>> Grace and peace,
>>
>> Jim
>>
>> Visit us on
>> Facebook Jim Biard
>> Research Scholar
>> Cooperative Institute for Climate and Satellites NC
>> North Carolina State University
>> NOAA's National Climatic Data Center
>> 151 Patton Ave, Asheville, NC 28801
>> e: jbiard at cicsnc.org
>> o: +1 828 271 4900
>>
>>
>>
>>
>> On Feb 7, 2014, at 10:25 AM, Hedley, Mark <mark.hedley at metoffice.gov.uk> wrote:
>>
>>> Hello Rich
>>>
>>> I think that using the WKT representation for vertical datum definitions is a good approach
>>>
>>> As you have indicated, it is to be supported in CF 1.7 and provides a controlled terminology set for this purpose.
>>>
>>> There is an example using the OS Newlyn datum in the draft spec which fits quite nicely.
>>>
>>> I'd rather see us adopting WKT for complex issues like this than creating a syntax for encoding CF grid_mapping attributes, there's a lot of prior art we can benefit from.
>>>
>>> For example WKT enables me to specify more than just the EPSG code, which is useful as not all datum instances are provided by EPSG
>>>
>>> mark
>>> ________________________________________
>>> From: CF-metadata [cf-metadata-bounces at cgd.ucar.edu] on behalf of Signell, Richard [rsignell at usgs.gov]
>>> Sent: 04 February 2014 11:47
>>> To: CF metadata
>>> Subject: [CF-metadata] Vertical datums (again)
>>>
>>> CF folks,
>>>
>>> On a telecon yesterday with a coastal inundation modeling group, one
>>> of the PIs asked me how to handle vertical datums in NetCDF --
>>> specifically where to specify that the model bathymetry and water
>>> levels were were relative to NAVD88. I wasn't sure how to reply.
>>>
>>> Was there any resolution to the 2nd half of this question asked back in 2011?
>>> http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2011/054483.html
>>>
>>> I looked at the draft 1.7 spec, and the only vertical datum reference
>>> info I found was the ability to specify VERT_DATUM in the new CRS
>>> well-known-text (WKT) section:
>>> http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.7-draft1/ch05s06.html#idp5644304
>>>
>>> Is this how we should specify the vertical datum in CF, using VERT_DATUM in WKT?
>>>
>>> Thanks,
>>> Rich
>>> --
>>> Dr. Richard P. Signell (508) 457-2229
>>> USGS, 384 Woods Hole Rd.
>>> Woods Hole, MA 02543-1598
>>> _______________________________________________
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>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>
>
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>
> ----- End forwarded message -----
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