Is using SI prefix with non SI units possible ?

[matdeslandes]

Question

Hello,

To represent of a volcanic ash cloud, we are producing ash concentrations within layers defined by Flight Levels. These Flight levels are expressed as hundreds of feet.
At the moment, we are using "hft" as units, but from the section about units and scale factor, a question arised whether that formulation was CF compliant because foot is not a SI unit.

So, is using "hft" as units is CF compliant or is there a better way to express the unit of Flight Level ?

Kind regards,

Mathieu

[larsbarring]

From a CF compliance perspective hectofeet (hft) is acceptable as this is understood by UDUNITS:

$ udunits2 -H "hft" -W ""
    30.48 m

$ udunits2 -H "30.48 m" -W "ft"
    30.48 m = 100 ft
    x/ft = 3.28084*(x/m)

[matdeslandes]

@larsbarring many thanks for answering me

Maybe overthinking... In fact the question comes from the sentence in 3.1.3. section :

This UDUNITS syntax for arbitrary transformation of units is not supported by the CF standard

Does that restriction applies only to the previous sentence

UDUNITS offers a syntax for indicating arbitrary scale factors and offsets to be applied to a unit.

or also to fact that UDUNITS allows as well SI prefix to be applied to non-SI units as mentionned ealier in the same paragraph ?

[JonathanGregory]

Dear Mathieu @matdeslandes

Yes, the restriction applies only the special syntax that UDUNITS offers for transformation of units, which is not connected with SI prefixes. It's things like "33 feet water" and "degF @ 32" given as examples in the udunits(3) man page. CF doesn't allow this syntax for arbitrary linear transformation of units.

Sorry for the confusion. Would it be better if we exchanged the order of the sentences "(Note that this is different ..." and "This UDUNITS syntax for arbitrary ..."?

Cheers

Jonathan

[matdeslandes]

I think that simply adding a linebreak after "...allows them to be applied to non-SI units as well" is enough and distincts the two UDNITS syntax in two differents paragraphs. It can make clearer that the restriction only apply to the second syntax... and rereading it "syntax for arbitrary transformation" was already quite helpful.

Many thanks @JonathanGregory !

[JonathanGregory]

I see, yes. You're suggesting that the first sentence could be a paragraph of its own. I have proposed this change just now in conventions issue 620. Thanks for raising the point.