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At the OGC Tech Conference in September 2014 held in Calgary, Canada, an Ad Hoc Working Group on Vertical CRS was called. The group agreed to collect information on different examples of vertical CRSs prior to then analysing them and forming a conceptual model as a basis for extending or changing existing OGC standards.

Meteorology and Oceanography uses a number of vertical coordinates:
  • Altitude above Mean Seal Level or other datum
  • Elevation above the land surface (actual, or sometimes a coarse Digital Terrain Model used by a specific Numerical Weather Prediction model)
  • Pressure levels, measured in hectoPascals (which the same numerical values as milliBars)
  • Potential Temperature levels (the potential temperature of a parcel of air is the temperature it would be if brought down to the surface or a standard pressure level, using the adiabatic gas laws)
  • Isopycnic levels below the sea surface, where the coordinate is density. This is useful because both temperature and salinity affect density.
  • Sigma levels, which are the pressure level values normalised by the surface pressure, in the range (0,1), with 1 at the surface and 0 at the top of the atmosphere.
  • Oceanographic sigma levels, which may be normalised density values.
  • Hybrid sigma levels (sometimes called eta levels), which are linear combinations of sigma and scaled altitude.
  • etc
The key feature of many of these CRS is that the datum is itself a 2 dimensional near-horizontal field or coverage, which is also time varying

-- ChrisLittle - 20 Nov 2014
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Topic revision: r1 - 20 Nov 2014, ChrisLittle

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