Although we've got plenty of food for thought already, here's an extra
helping:
How ESRI (arguably the largest purveyor of geospatial software, e.g. 10,000
users at their annual conference) handles vertical datums:
http://resources.arcgis.com/en/help/main/10.1/index.html#//003r00000015000000
http://edndoc.esri.com/arcsde/9.3/api/japi/docs/com/esri/sde/sdk/pe/PeVertCS.html
-Rich
On Mon, Feb 17, 2014 at 4:10 PM, John Graybeal <
john.graybeal at 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.gregory at 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 at cgd.ucar.edu
> > http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
>
> _______________________________________________
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>
--
Dr. Richard P. Signell (508) 457-2229
USGS, 384 Woods Hole Rd.
Woods Hole, MA 02543-1598
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