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.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 > -- Dr. Richard P. Signell (508) 457-2229 USGS, 384 Woods Hole Rd. Woods Hole, MA 02543-1598
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