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Commons Extension Specification

Extension Maturity Classification: Proposal

A group of STAC Item objects from a single source can share a lot of common metadata. This is especially true with satellite imagery that uses the STAC EO Extension. Rather than including these common metadata fields on every Item, they can be provided into the STAC Collection that the STAC Items belong to.

  • Examples:
    • Landsat 8: An Item that uses the Commons extension to place shared data in a Collection.
  • JSON Schema (missing, PRs welcome)

WARNING

This is still an early version of the STAC Commons Spec, expect that there may be some changes before everything is finalized.

Implementations are encouraged, however, as good effort will be made to not change anything too drastically. Using the specification now will ensure that needed changes can be made before everything is locked in. So now is an ideal time to implement, as your feedback will be directly incorporated.

Item fields

Unlike other extensions the Commons extension does not add any fields to a STAC Item, instead it allows one to move fields out of Item and into the parent STAC Collection, from which any member Item will inherit. Any field under an Items properties can be removed and added to the Collection properties. Since a Collection contains no properties itself, anything under properties are metadata fields that are common across all member Items.

This provides maximum flexibility to data providers, as the set of common metadata fields can vary between different types of data. For instance, Landsat and Sentinel data always has a eo:off_nadir value of 0, because those satellites are always pointed downward (i.e., nadir), while satellite that can be pointed will have varying eo:off_nadir values. The Commons extension allow the data provider to define the set of metadata that defines the colleciton. While some metadata fields are more likely to be part of the common set, such as or eo:instrument rather than eo:cloud_cover, it depends on how the data provider chooses to organize their data.

If a metadata field is specified in the Collection properties, it will be ignored in any Item that links to that Collection. This is important because a Collection is the metadata that is common across all Item objects. If a field is variable at all, it should not be part of the Commons.

A Collection may not link to any Items, it may just be a definition of a Collection, in which case the Commons extension could still be used by defining any properties that would be shared by any member Item.

Linking to a STAC Collection

All Items link to a Collection (this is part of the core STAC spec) and is specified in two places. One is a field under properties called collection which is the id of a STAC Collection record. The collection field provides an easy way for a user to search for any Items that belong in a specified Collection.

A STAC Item must also provide a link to the STAC Collection using the collection rel type:

"links": [
  ...
  { "rel": "collection", "href": "link/to/collection/record.json" }
]

Using a STAC Collection

To get the complete record of an Item (both individual and commons properties), the properties from the Collection can be merged with the Item.

An incomplete Collection:

{
  "id": "landsat-8-l1",
  "title": "Landsat 8 L1",
  "description": "Landat 8 imagery radiometrically calibrated and orthorectified using gound points and Digital Elevation Model (DEM) data to correct relief displacement.",
  "version": "0.1.0",
  "extent": {...},
  "license": "PDDL-1.0",
  "properties": {
    "eo:gsd": 15,
    "eo:platform": "landsat-8",
    "eo:instrument": "OLI_TIRS",
    "eo:off_nadir": 0,
    "eo:bands": [
      {
        "id": "B1",
        "common_name": "coastal",
        "gsd": 30,
        "center_wavelength": 0.44,
        "full_width_half_max": 0.02
      },
      ...
    ]
  },
  "links": [...]
}

An incomplete item:

{
  "type": "Feature",
  "id": "LC08_L1TP_107018_20181001_20181001_01_RT",
  "bbox": [...],
  "geometry": {...},
  "properties": {
    "collection": "landsat-8-l1",
    "datetime": "2018-10-01T01:08:32.033Z",
    "eo:cloud_cover": 78,
    "eo:sun_azimuth": 168.8989761,
    "eo:sun_elevation": 26.32596431,
    "landsat:path": 107,
    "landsat:row": 18
  },
  "assets": {...},
  "links": [...]
}

The merged Item then looks like this:

{
  "type": "Feature",
  "id": "LC08_L1TP_107018_20181001_20181001_01_RT",
  "bbox": [],
  "geometry": {},
  "properties": {
    "collection": "landsat-8-l1",
    "datetime": "2018-10-01T01:08:32.033Z",
    "eo:cloud_cover": 78,
    "eo:sun_azimuth": 168.8989761,
    "eo:sun_elevation": 26.32596431,
    "landsat:path": 107,
    "landsat:row": 18,
    "eo:gsd": 15,
    "eo:platform": "landsat-8",
    "eo:constellation": "landsat-8",
    "eo:instrument": "OLI_TIRS",
    "eo:off_nadir": 0,
    "eo:bands": [
      {
        "id": "B1",
        "common_name": "coastal",
        "gsd": 30,
        "center_wavelength": 0.44,
        "full_width_half_max": 0.02
      },
      ...
    ]
  },
  "assets": {...},
  "links": [...]
}

Implementations

sat-api has pioneered this functionality. You can see it in action at https://sat-api.developmentseed.org/stac.