Link relation type for an individual tileset

[tschaub]

As far as I can tell, the spec doesn't recommend a rel for a link that points to an individual tileset. There are rel values for tileset lists. And within a tileset list, items have a link with a rel value of self to point to the individual tileset.

I think people are using tiles as a link relation when pointing to an individual tileset from another resource. Is this already mentioned in the spec? If not, should it be?

My use case is a STAC API / OGC API – Features service where each item represents an image that has a single associated tileset. I am providing a link from the item to the tileset, and am wanting to use a rel value that might be understood by others.

[jerstlouis]

As far as I can tell, the spec doesn't recommend a rel for a link that points to an individual tileset.

That is correct.

I think people are using tiles as a link relation when pointing to an individual tileset from another resource. Is this already mentioned in the spec?

No, it is not.

If not, should it be?

We could debate what the correct link relation to an individual tileset should be (e.g., tileset-vector, tileset-map singular...).
There is a clear distinction between a "tile" and a "tile set" in 2DTMS and AS Topic 22, but of course a "tile set" is a set of tiles.
However, a tileset has more characteristics, such as using the same 2DTMS, all tiles representing the same data, not containing multiple tiles for the same level/row/column, etc. We could also discuss whether the link relation should include the extra -vector, -coverage, -map to qualify it.

There is currently no planned upcoming revision to OGC API - Tiles.
It might be possible to register whichever link relation is agreed upon with the OGC Naming Authority, but normally it is done soon before publication of a standard that references it.

In addition to already having standardized link relations, there are other advantages to link to a OGC API collection (http://www.opengis.net/def/rel/ogc/1.0/geodata) or to the list of tilesets (http://www.opengis.net/def/rel/ogc/1.0/tilesets-*) instead if they can be made available as well, such as the ability to support multiple 2D TileMatrixSets, or retrieving the data using other access mechanisms such as OGC API - Maps for example.

[m-mohr]

I have the same use case (for https://github.com/stac-extensions/web-map-links) and it would be great if OGC could recommend a relation type for linking to individual tilesets. Doesn't necessarily need to be in the spec, maybe just in a best practice document or so?

[jerstlouis]

@m-mohr Perhaps we could make an exception and register a link relation type with the definition server before a new standard or update is released, and include it in any future revision / corrigendum? @ghobona

I think my own preference would be a singular version of the existing tilesets list link relations:

• http://www.opengis.net/def/rel/ogc/1.0/tileset-vector
• http://www.opengis.net/def/rel/ogc/1.0/tileset-map
• http://www.opengis.net/def/rel/ogc/1.0/tileset-coverage

These are not needed in the Standard because the self relation is used from within the tilesets array.

@joanma747 thoughts?

@m-mohr Note that I'd still recommend linking to an OGC API collection with http://www.opengis.net/def/rel/ogc/1.0/geodata instead if you have those extra metadata end-points defined, which offers a client the ability to choose between different available Tile Matrix Sets (e.g., GDAL already supports it as an input for the OGC API driver) and improves interoperability.

[m-mohr]

Thanks @jerstlouis

I'd need to know upfront whether the API collection is a API Tiles collection or any other OGC collection. As such using http://www.opengis.net/def/rel/ogc/1.0/geodata is not possible.

[jerstlouis]

@m-mohr That's not possible. A client would need to fetch the collection and check whether the links contain tilesets-*.

A collection can support multiple access mechanisms, so I don't think there is any plan to define multiple link relation types for it.
There was already some pushback to define separate tilesets-* link relation types.

If you really want to only links to datasets that only contain tiles, I would suggest using the tilesets-map (plural, that is already defined) that can support multiple TileMatrixSets, and would allow the client to choose their preferred 2DTMS from those offered. The tileset can also link back to the collection with rel: geodata