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 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
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