Asset specific metadata

[matthewhanson]

A few of us had a quick discussion today relating to asset specific metadata: the ability to specify fields that are Item properties on a per asset basis.

It's discussed here in the context of the proposed "grids" extension:
#688 (comment)
but could apply to other fields as well.

We concluded this was a good feature to have, with the following notes:

• properties defined at the asset level should always be first defined at the Item level. The asset field, if present, overrides the Item field for that asset
• make clear that fields defined per asset will not be searchable....users will search on the Item properties so that should be some sort of "nominal" value
• recommendation should include a short list of fields this may make sense for, e.g., grids extension, gsd
• otherwise, providers should think carefully on whether or not they should be doing this, or if what they really need is separate Items for this data

did I miss anything @m-mohr @cholmes @anayeaye ?

[m-mohr]

This seems mostly complete, although it doesn't clearly state how the relation between asset-level metadata and common metadata + extension fields is. There's a potential conflict between the "title" fields, should be mentioned at least. Also, what's the take on things like eo:bands that differs between properties and asset level? I think that needs some thoughts and explanation.
We probably need to clarify that the asset extension can also inherit the "new" fields.

[m-mohr]

We just had a use case where we have to specify the "datetime" field on the asset level (i.e. export of a single band time series), which IMHO is reasonable to do although STAC Items are more modeled for data with a single timestamp. And that makes me think of #613 again. ;-)

[matthewhanson]

closed with #788