Using UCDs as tags

[Roy Williams]

> I'd like to have your opinion on how to solve the sort of sql statement I
put at the
> end of my previous email:
>
> Select
>    service_name, service_description, service_rowcount
> from Registry
> where
>    service_category = "CATALOG BROSWER"
>    and data_class = "OBJECT CATALOG"
>    and subject = "GALAXY"
>    and querable_parameter = "GALAXY DISTANCE"
>    and output_parameter = "GALAXY NAME"
>
> How would you make it work without a dictionary ?

Option (1) is the very successful and simple approach that Google takes -- 
matching words in the query with words in the descriptions of the resources.
Let us not forget this option!

Option (2) we can query on the UCDs. If I am looking for, say catalogs with
optical photometry, I would choose a suitable UCD, perhaps
"phot.flux.optical", and look for strong matches with UCDs in the output of
the services, or the queryable parameters, etc. The primary method of UCD is
a matching function: "how much are these UCDs alike?".

The data model people want a vocabulary that provides exact matches for all
astronomers. I believe this to be impossible. In contrast, the UCD approach
concentrates on finding "strong semantic matching".