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12-2. Key concepts

STAC (SpatioTemporal Asset Catalog) is a standard way of describing collections of geospatial data so that software can discover and use them. Many Earth Observation projects and data portals publish their results as STAC catalogues.

A STAC catalogue is organised in levels:

  • A catalogue groups related collections.
  • A collection is a dataset family, for example land surface temperature over Austria or a Sentinel 2 archive.
  • An item is one entry in a collection, usually one scene or one time step, with a date, a footprint and some properties (such as cloud cover).
  • An asset is an actual file belonging to an item — the COG, GeoJSON or FlatGeoBuf the Explorer will display.

Two ways of using STAC

STAC as a discovery service. In Tutorial 3 you browsed the Projects Results Repository (PRR) catalogue to find the Biomass layers. The catalogue helped you uncover data of interest, and the address of the file you chose was then added to your configuration directly. The catalogue's job ended at discovery.

STAC as a data source. In this tutorial the STAC catalogue is the data source. The configuration stores a STAC address — a collection, an item, or a query — and it is the Geospatial Explorer, not the Configuration Builder, that then goes to the catalogue and fetches the data. Because the catalogue is consulted at view time, a STAC layer can track a growing collection: new items that match the query appear automatically.

What the configuration stores

When you add STAC to a layer, the configuration stores only:

  • the URL of the collection, item, item list or search query, and
  • the asset name and its format (for example data (flatgeobuf)).

Any filters — area of interest, date range, result limit or property filters such as cloud_cover < 10 — are part of that URL. Temporary or signed file links are never saved.

Items and search endpoints

A STAC service offers two ways to ask for items:

Endpoint Shape Notes
Items .../collections/{id}/items?... The default. Serves a single collection.
Search .../search?collections={id}&... Can span several collections. Some services only evaluate property filters here.

The query editor lets you switch between the two while keeping your filters, and its Test button shows how many items each endpoint returns, so you can see whether a server honours your filters.

For more detail, see STAC browser.