GeoData Tilesets: Clarify whether tilesets-* links should be included in /collections

[jerstlouis]

It is not clear from Requirement 13 whether the tilesets-* links are required to be present in the collection lists in /collections.

Requirement 13 /req/geodata-tilesets/desc-links
A) If the Web API based server has a mechanism for geospatial data resources to expose links to geospatial resource aspects (e.g., feature items, metadata…​), the API implementation SHALL include at least one of three link with the href pointing to tilesets list for geospatial data resources and with rel: http://www.opengis.net/def/rel/ogc/1.0/tilesets-vector, http://www.opengis.net/def/rel/ogc/1.0/tilesets-map and http://www.opengis.net/def/rel/ogc/1.0/tilesets-coverage.

Not requiring this means that if the tilesets- links are missing, an implementation may need to make one request for every collections to figure out whether it supports tilesets or not, which significantly slows down access to OGC API - Tiles implementations with a few collections.

We noted that ldproxy (https://demo.ldproxy.net/vineyards/collections?f=json) did not include the tilesets-* links at the /collections level (although it does includes items links there) and this was preventing the GNOSIS client from accessing the API as Tiles, potentially also a reason for issues accessing it from GDAL.

@joanma747 @cportele @ghobona