Marc, I'm surely no pro on this topic but the definitions of "boundry" and "range" seem to be synonymous(sp)
Both consider that there is a finite area. And I have seen neither in actual usage.
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From:
http://www.unidata.ucar.edu/packages/netcdf/coords/0107.html
Multiple values:
I feel that the various multiply valued coordinates you have referred to are of several distinct types. I would argue that the "representative" or "midpoint" value is the principal coordinate value. This one should always exist, and it is this which must be monotonic (or at least ordered) if it is one-dimensional.
GDT distinguish three kinds of subsidiary coordinates:
(1) Boundary (section 21). We group the upper and lower boundaries into one variable for tidiness and ease of access.
(2) Component (section 18). These are for cases where the coordinate values are tuples, such as for the hybrid pressure-sigma vertical coordinate. However, an ordinary principal coordinate value must still be provided, for ordering the axis.
(3) Associate (section 19). Associate values are additional information, or extra ways of labelling the points, such as your "lev_label".
I think these are all truly different. Moreover, component and associated coordinate values can have boundaries, and associated coordinates can have components. For this reason, rather than coordinates = "lon lat (lev_upper lev_lower lev_midpoint lev_label)";
I think that it would be better to specify
:coordinates="lon lat lev";
and provide the boundary, component and associate coordinates by attaching them to lev, the representative or midpoint value. That makes for a simpler and clearer definition of the coordinate system, and it shows that the other information really is subsidiary to lev.
I think we are converging on a listing of the various "types" (meanings) we want to have for coordinates:
1) "point", "representative point", or "principal value".
2) "boundry" or "range"
3) "label", "nominal" or "associate"
Independently is the possibility that a coordinate value is specified by a tuple, eg (year,month,day,daysec): in this case we can represent such a value along a single axis………………
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Any up to date experts out there?
Al...