Updates to how special variables are identified?

[ChrisJohnNOAA]

This started as a discussion on a code review: #373 (comment)

I wanted to broaden the discussion to how best to identify special variables in datasets. Mostly we currently use destinationName, there was a question on if would be a good approach to use ioos_category (which is done for the new pressure option in that pull request) for existing special variables.

Feel free to share input/opinions on this.

[srstsavage]

https://cfconventions.org/Data/cf-conventions/cf-conventions-1.12/cf-conventions.html#coordinate-types

Because identification of a coordinate type by its units is complicated by requiring the use of an external package [UDUNITS], we provide two optional methods that yield a direct identification. The attribute axis may be attached to a coordinate variable and given one of the values X, Y, Z or T which stand for a longitude, latitude, vertical, or time axis respectively. Alternatively the standard_name attribute may be used for direct identification. But note that these optional attributes are in addition to the required COARDS metadata.

To identify generic spatial coordinates we recommend that the axis attribute be attached to these coordinates and given one of the values X, Y or Z. The values X and Y for the axis attribute should be used to identify horizontal coordinate variables. If both X- and Y-axis are identified, X-Y-up should define a right-handed coordinate system, i.e. rotation from the positive X direction to the positive Y direction is anticlockwise if viewed from above. We strongly recommend that coordinate variables be used for all coordinate types whenever they are applicable.

Would examination of an axis attribute containing X, Y, Z, or T be sufficient?

https://cfconventions.org/Data/cf-conventions/cf-conventions-1.12/cf-conventions.html#vertical-coordinate

Axis Z could also use the positive attribute (down or up).

I hesitate to suggest tightly coupling ERDDAP to CF conventions, but they are definitely less specific than IOOS conventions.