units requirement for dimensionless vertical coordinate

[ilaflott]

Topic for discussion

from v1.13, chapter 4.3, Vertical (Height or Depth) Coordinate:

Variables representing dimensional height or depth axes must always explicitly include the units attribute; there is no default value.

this seems contradicted shortly after, in chapter 4.3.2, Dimensionless Vertical Coordinate:

The units attribute is not required for dimensionless coordinates.

i understand "dimensionless" means "no units", but perhaps what is meant in 4.3.2 is that the (required) units attribute may be an empty string? or something equally indicative of no-units, e.g. 1 ? otherwise, it appears that a units attribute is in fact, not always required, contradicting 4.3's starting language.

[taylor13]

I would say the information in 4.3 is poorly organized into sections and subsections, which may be causing some confusion, but I don't think the actual sentences quoted above are contradictory. They simply say that an coordinate variable expressing vertical position as a dimensional quantity must have a units attribute, but a coordinate variable expressing the position by layer number or some other dimensionless measure needs no units attribute.

[ilaflott]

oh, "... dimensional ... must always ... include...". Now i see your point about the poor organization.

[JonathanGregory]

I agree that the text of 4.3 is not well-organised and therefore could be confusing; also, it's rather repetitive. I have therefore proposed rewording it in conventions issue 635. Is that better? Please comment on that issue, rather than here.