Hi.
Regarding how ESRI represents vertical CRSs:
When ESRI specifies a particular vertical datum and doesn?t specify the ellipsoid, it is because the specified vertical datum specifies the ellipsoid. The fully realized definition still contains a specification of the ellipsoid that the geoid heights were measured against. I don?t think we should overload the add_offset attribute to do anything besides its meaning for rescaling.
I?ll write more on this later, but there?s my quick $0.02 worth.
Grace and peace,
Jim
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On Mar 11, 2014, at 8:59 AM, Jonathan Gregory <j.m.gregory at reading.ac.uk> wrote:
> Dear Rich
>
> Thanks for keeping this going.
>
>> In CF, we have some of these parameters of the vertical coordinate
>> system specified with the "units" and "positive" attributes. And we
>> already have "add_offset" that could be used for the vertical shift.
>>
>> So that just leaves the geoid-based "Vertical Datum" or spheroid-based
>> "Datum", where the spheroid can be specified with parameters.
>
> In the agreed trac ticket 80
> https://cf-pcmdi.llnl.gov/trac/ticket/80
> we added several more attributes to grid mapping to describe reference
> surfaces in ways analogous to CRS WKT. We already have attributes to name
> and specify the reference ellipsoid (spheroid). As far as I can see, we need
> only to add an attribute of geoid_name to Appendix F to identify the geoid.
> Would that meet your requirement? Earlier discussions implied that CRS WKT
> does not have a way to name the geoid specifically.
>
> Best wishes
>
> Jonathan
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