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