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STAC Catalog Specification

This document explains the structure and content of a STAC Catalog. A STAC Catalog is a collection of STAC Items. These Items can be linked to directly from a Catalog, or the Catalog can link to other Catalogs (often called sub-Catalogs) that contain links to Items. The division of sub-catalogs is up to the implementor, but is generally done to aid the ease of online browsing by people.

Catalogs are not intended to be queried. Their purpose is discovery: to be browsed by people and crawled by machines to build a search index. A Catalog can be represented in JSON format. Any JSON object that contains all the required fields is a valid STAC Catalog.

This Catalog specification primarily defines a structure for information to be discoverable. Any use that is publishing a set of related spatiotemporal assets is strongly recommended to also use the STAC Collection specification to provide additional information about a set of Items contained in a catalog, to give contextual information to aid in discovery. Every STAC Collection is also a valid STAC Catalog.

WARNING

This is still an early version of the STAC 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.

Catalog Definitions

There are two required element types of a Catalog: Catalog and Item. A STAC Catalog points to STAC Items, or to other STAC catalogs. It provides a simple linking structure that can be used recursively so that many Items can be included in a single Catalog, organized however the implementor desires.

STAC makes no formal distinction between a "root" catalog and the "child" catalogs. A root catalog is simply a top-most catalog (which has no parent). A nested catalog structure is useful (and recommended) for breaking up massive numbers of catalog items into logical groupings. For example, it might make sense to organize a catalog by date (year, month, day), or geography (continent, country, state/prov). Any scheme may be used, but it's considered a best practice to keep the size of each catalog under a megabyte.

A simple Catalog structure might look like this:

catalog (root)
  - catalog
    - catalog
      - item
        - asset
      - item
        - asset
    - item
      - asset
      - asset

This example might be considered a somewhat "typical" structure. However, catalogs and items can describe a number of different relationships. The following shows various relationships between catalogs and items:

  • Catalog -> Item (this is a common structure for a catalog to list links to items)
  • Catalog -> Catalog (this is a common tree structure to group sets of items. Each catalog in this relationship may also include item links as well as catalog links)
  • Item -> Catalog (example: an item may point to a catalog to describe a set of derived assets, where it may be desirable to have the origin asset as a "parent", such as NDVI generated from RGB/IR)
  • Item -> Item (example: this relationship may be used to describe a 1-1 parent-child relationship, such as a single derived item from one parent item)

As all STAC Collections are also valid STAC Catalogs, all Catalogs described here could also be Collections.

The relationships are all described by a common links object structure, making use of the rel attribute to further describe the relationship.

There are a few types of catalogs that implementors occasionally refer to. These get defined by the links structure.

  • A sub-catalog is a Catalog that is linked to from another Catalog that is used to better organize data. For example a Landsat collection might have sub-catalogs for each Path and Row, so as to create a nice tree structure for users to follow.
  • A root catalog is a Catalog that only links to sub-catalogs. These are typically entry points for browsing data. Often they will contain the STAC Collection definition, but in implementations that publish diverse information it may contain sub-catalogs that provide a variety of collections.
  • A parent catalog is the Catalog that sits directly above a sub-catalog. Following parent catalog links continuously will naturally end up at a root catalog definition.

It should be noted that a Catalog does not have to link back to all the other Catalogs that point to it. Thus a published root catalog might be a sub-catalog of someone else's structure. The goal is for data providers to publish all the information and links they want to, while also encouraging a natural web of information to arise as Catalogs and Items are linked to across the web.

There are a number of emerging 'best practices' for how to organize and implement good catalogs. These can be found in the best practices document, and include things like catalog layout, use of self links, publishing catalogs, and more. This specification is designed for maximum flexbility, but the best practices provide guidance for good recommendations when implementing.

Catalog Types

Though it is technically an implementation detail outside the scope of the core specification, it is worth mentioning that implementations generally fall into two different 'types':

  • Static Catalogs can be implemented as simply files online, often stored in an cloud storage service like Amazon S3. or Google Cloud Storage. The core JSON documents and link structures are encoded in the file, and work as long as things are structured properly.
  • Dynamic Catalogs are implemented in software, returning the JSON documents and links dynamically. This is mostly used when data holdings are already exposed through a dynamic interface, and STAC can be an alternate facade on the same core database or search cluster.

The two catalog types both implement the same fields and links, and can be treated as the same by clients. For more details on the two types and how you might use them see the Static and Dynamic Catalogs section of the best practices document.

Catalog fields

Element Type Description
stac_version string REQUIRED. The STAC version the catalog implements.
id string REQUIRED. Identifier for the catalog.
title string A short descriptive one-line title for the catalog.
description string REQUIRED. Detailed multi-line description to fully explain the catalog. CommonMark 0.28 syntax MAY be used for rich text representation.
links [Link Object] REQUIRED. A list of references to other documents.

stac_version: It is not allowed to mix STAC versions. The root catalog or the root collection respectively MUST specify the implemented STAC version. Child Catalogs and child Collections MUST NOT specify a different STAC version.

Examples:

A catalog of NAIP imagery might look something like this:

{
  "stac_version": "0.7.0",
  "id": "NAIP",
  "description": "Catalog of NAIP Imagery",
  "links": [
    { "rel": "self", "href": "https://www.fsa.usda.gov/naip/catalog.json" },
    { "rel": "child", "href": "https://www.fsa.usda.gov/naip/30087/catalog.json" },
    { "rel": "root", "href": "https://www.fsa.usda.gov/catalog.json" }
  ]
}

In addition, the catalog shown above is strongly recommended to also follow the STAC Collection specification to add more information about the NAIP imagery such as the spatial and temporal extents, a license and more.

A typical 'child' sub-catalog could look similar:

{
  "stac_version": "0.7.0",
  "id": "NAIP",
  "description": "Catalog of NAIP Imagery - 30087",
  "links": [
    { "rel": "self", "href": "https://www.fsa.usda.gov/naip/30087/catalog.json" },
    { "rel": "parent", "href": "../catalog.json" },
    { "rel": "root", "href": "https://www.fsa.usda.gov/catalog.json" },
    { "rel": "item", "href": "https://www.fsa.usda.gov/naip/30087/m_3008718_sw_16_1_20130805.json" },
    { "rel": "item", "href": "https://www.fsa.usda.gov/naip/30087/m_3008718_sw_16_1_20130806.json" }
  ]
}

The root catalog in this example could hold a set of sub-catalogs with different STAC collections, e.g. data from other satellites or processed variants of the NAIP imagery.

Link Object

This object describes a relationship with another entity. Data providers are advised to be liberal with links.

Field Name Type Description
href string REQUIRED. The actual link in the format of an URL. Relative and absolute links are both allowed.
rel string REQUIRED. Relationship between the current document and the linked document. See chapter "Relation types" for more information.
type string Media type of the referenced entity.
title string A human readable title to be used in rendered displays of the link.

A more complete list of possible 'rel' types can be seen at the IANA page of Link Relation Types.

Please see the chapter 'relative vs absolute links' in the Item spec for a discussion on that topic, as well as the use of links section of the catalog best practices document.

Relation types

The following types are commonly used as rel types in the Link Object of a STAC Collection:

Type Description
self STRONGLY RECOMMENDED. Absolute URL to the location that the catalog file can be found online, if available. This is particularly useful when in a download package that includes metadata, so that the downstream user can know where the data has come from.
root STRONGLY RECOMMENDED. URL to the root STAC Catalog or Collection. Catalogs should include a link to their root, even if it's the root and points to itself.
parent URL to the parent STAC Catalog or Collection. Non-root catalogs should include a link to their parent.
child URL to a child STAC Catalog or Collection.
item URL to a STAC Item.

Note: A link to at least one item or child catalog is REQUIRED.

Extensions

There are emerging best practices, which in time will evolve in to specification extensions for particular domains or uses.

The extensions page gives an overview about relevant extensions for STAC Catalogs.

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