John,
I like your idea of using distance to avoid automatic overloading of concepts!
The problem with defining altitude as being an orthometric distance above the
geoid is that neither satellite nor GPS vertical coordinates match this
definition. Similarly, height as “height above the surface” has an inherent
problem with regard to CRSs, because none of them define the location of the
bottom of the atmosphere. This definition of height forces it to be a relative
measurement that cannot be connected to a location on the Earth without a
corresponding measurement of the vertical distance of the surface above/below a
vertical datum. Both of these terms are doomed by history to be ambiguous. We
can define them to be a particular thing, but existing uses might be in
conflict with the new definitions.
Back to your suggested name format, I was wondering about the "above | below”
part. They look fine to me, but is anyone not OK with the idea that a negative
distance above is below and vice versa?
Jonathan, in answer to your question about geoids, WKT has a clause “VERT_CS”
where a vertical datum is defined. A geoid is, in most cases, realized as a
lat/lon grid in one or more files (often tiled), and is only identified by name.
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: jbi...@cicsnc.org
o: +1 828 271 4900
On Feb 17, 2014, at 4:10 PM, John Graybeal <john.grayb...@marinexplore.com>
wrote:
> Simple terms like height, depth, and altitude are great for onboarding --
> though complicated usage ('geoid must always be defined in the
> grid_mapping'), lessens the onboarding benefit. And if they are ambiguous,
> the long-term usability is affected. (See: sea_surface_temperature.)
>
> I want a consistent approach that starts simple -- e.g., 'altitude' is an
> alias for geodetic distance above geoid, and if no particular geoid is
> specified, a default is assumed, perhaps carrying along explicit assumptions
> about the possible error bounds.
>
> The basic concepts discussed so far seem to break down as:
> distance_[above | below]_[surface | geoid | ellipsoid | center], #
> 'distance' avoids loaded terms altitude, depth, etc.
> with the possibility of a prefix like
> orthometric | geodetic | geocentric | geometric
> and the need or possibility to specify additional parameters for at least
> some of these choices (ex: surface may default to the bottom of the
> atmosphere, but could be defined using any of the Sample Dimensions in the
> MetOcean graphic [1]).
>
>> It would be really useful if anyone could explain how the geoid is
>> identified in CRS WKT.
>
>
> Do you mean 'identified' or 'specified'? From Dru Smith's 1998 paper [2] --
> it didn't look like an 'identifier' would be sufficient any time soon, or do
> we already have controlled terms for the various 'geoid candidates' that are
> out there? (Note for non-experts like me: I found that Wikipedia's simple
> and specific definitions [3] bypass the problem of defining where 'the geoid'
> actually is.) It's hard to imagine that CF users will be in a position to
> provide those geoidal identification or specification details, though....
>
> John
>
> [1]
> http://external.opengeospatial.org/twiki_public/MetOceanDWG/MetOceanWMSBP20120206
> [2] http://www.ngs.noaa.gov/PUBS_LIB/EGM96_GEOID_PAPER/egm96_geoid_paper.html
> [3] http://en.wikipedia.org/wiki/Geoid,
> http://en.wikipedia.org/wiki/Physical_geodesy
>
> On Feb 17, 2014, at 09:50, Jonathan Gregory <j.m.greg...@reading.ac.uk> wrote:
>
>> Dear all
>>
>> Thank you for clarifications and further information.
>>
>> We used "altitude" for "height above geoid" because that's what it most
>> commonly means, I think. However, it's unclear. To avoid confusion, we could
>> rename altitude as height_above_geoid, using aliases. There are 14 standard
>> names which use the word altitude. Would that be worth doing?
>>
>> Similarly, we could rename plain "height" as height_above_surface. There are
>> about 5 standard names which would be affected. Likewise (and relating also
>> to
>> another thread), we could rename plain "depth" as depth_below_surface. There
>> are about 14 standard names using this word in that sense. Is this
>> worthwhile,
>> or shall we continue with short words and rely on the definitions? Opinions
>> would be welcome.
>>
>> It would be really useful if anyone could explain how the geoid is identified
>> in CRS WKT.
>>
>> Best wishes
>>
>> Jonathan
>> _______________________________________________
>> CF-metadata mailing list
>> CF-metadata@cgd.ucar.edu
>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
>
> _______________________________________________
> CF-metadata mailing list
> CF-metadata@cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
_______________________________________________
CF-metadata mailing list
CF-metadata@cgd.ucar.edu
http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
