Describing ParameterGroup objects

[jonblower]

In the current spec, the ParameterGroup object itself has an observedProperty but does not give any information about what kind of grouping it is. It seems that the client must infer this from the information that's attached to the "children" of the group. However I think it would be useful to have extra information at the ParameterGroup level to indicate what kind of grouping it is. (It may not be easy or possible to find this from inspection of the children alone.)

I think the two main use cases are:

• Grouping the components of a velocity field
• Grouping the components of an uncertain variable

So having "tags" of some kind for each of these cases could be useful. I can see two options: firstly, giving information about the observedProperty:

{
  "type": "ParameterGroup",
  "observedProperty": {
    "label": {
      "en": "Wind velocity"
    }
    "narrowerThan" : "http://some.uri.for/velocity"
  },
  "members": ["WIND_SPEED", "WIND_DIR"]
}

But I don't think this works for uncertain variables. (The concept of sea surface temperature is not narrower than the concept of a probability density function.) I think that something like a "groupType" property would work better, attached directly to the ParameterGroup object. This could have have values as in the table below:

groupType	Meaning	Notes
vector	Children are components of a vector field	Metadata attached to the children describes what kind of components we have (magnitude and direction, x and y components, etc)
statistics	Children are different statistics of the same quantity	E.g. mean and standard deviation, using URIs from UncertML
pdf	Children hold the values of the components of the probability density function	May need another property (from UncertML) to describe which PDF we are using
samples	Children hold the values from individual realisations of a sampling	E.g. an ensemble

(See http://www.uncertml.org for more details about encoding uncertain data.)

Or, instead of these general groupTypes we could use a much more specific URI to denote what the grouping represents (e.g. http://www.uncertml.org/distributions/normal for a normally-distributed variable). The children of the group would need to be similarly described by URIs to work out which child holds which information.