Hi.
The contents of the grid_mapping variable are doing as much mapping when specifying a projected coordinate system as they are when specifying a geographic (i.e., lon/lat) coordinate system. It is telling you how to understand the spatial coordinate information, particularly in relation to any other choice of coordinate system. It has an unfortunate name, in that it leads us to think about it as ?the thing that tells me how I got from my X & Y coordinate variables to my lon & lat grids?. When you have both X/Y coordinates and lon/lat auxiliary coordinates, the grid_mapping variable is actually telling you how to understand both. There is really no such thing as a ?null mapping?.
Grace and peace,
Jim
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On Mar 14, 2014, at 8:25 AM, David Hassell <d.c.hassell at reading.ac.uk> wrote:
> Dear Jonathan,
>
> I like the idea of name for a grid mapping that doesn't actually map,
> but suggest a name of "null_mapping" (instead of "null") to better
> indicate to humans that whilst the variable is not mapping, it is
> doing something.
>
> All the best,
>
> David
>
> ---- Original message from Jonathan Gregory (02PM 11 Mar 14)
>
>> Date: Tue, 11 Mar 2014 14:29:25 +0000
>> From: Jonathan Gregory <j.m.gregory at reading.ac.uk>
>> To: cf-metadata at cgd.ucar.edu
>> User-Agent: Mutt/1.5.21 (2010-09-15)
>> Subject: Re: [CF-metadata] Vertical datums (again)
>>
>> Dear David
>>
>>> I'm a bit confused as to the use of these grid_mapping parameters with
>>> vertical coordinates - do we need new grid_mapping_names? I'm
>>> thinking, for example, of linking a geoid specification to a vertical
>>> "altitude" coordinate.
>>
>> That's a good point, thanks. I was thinking of the grid_mapping as a place to
>> record the reference surface in the same way as it does for latitude_longitude,
>> which was added as a "null" horizontal mapping. No "mapping" is required for
>> the vertical coordinate; it just needs its reference surface (vertical datum)
>> to be more precisely specified than its geophysical description of "geoid" or
>> "reference_ellipsoid" alone, for some applications.
>>
>> I'd like to suggest we add a new grid_mapping_name of "null" to Appendix F,
>> thus: "This grid_mapping does not imply any mapping of horizontal or vertical
>> coordinates. Its purpose is to describe the reference ellipsoid or the geoid."
>> No map parameters, no map coordinates.
>>
>> I would further suggest that we make latitude_longitude an alias for null.
>>
>> It might happen that a different reference ellipsoid is required for vertical
>> and horizontal coordinates. Thanks to Mark's accepted ticket 70, this will be
>> no problem; it allows there to be more than one grid_mapping, and the
>> grid_mapping attribute indicates which coordinate variable each is relevant to.
>>
>> Best wishes
>>
>> Jonathan
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>
>
> --
> David Hassell
> National Centre for Atmospheric Science (NCAS)
> Department of Meteorology, University of Reading,
> Earley Gate, PO Box 243,
> Reading RG6 6BB, U.K.
>
> Tel : +44 118 3785613
> E-mail: d.c.hassell at reading.ac.uk
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