SpatioTemporal Asset Catalog (STAC)

Overview

The SpatioTemporal Asset Catalog (STAC) family of specifications aim to standardize the way geospatial asset metadata is structured and queried. A “spatiotemporal asset” is any file that represents information about the Earth at a certain place and time. The original focus was on scenes of satellite imagery, but the specifications now cover a broad variety of uses, including sources such as aircraft and drone and data such as hyperspectral optical, synthetic aperture radar (SAR), video, point clouds, lidar, digital elevation models (DEM), vector, machine learning labels, and composites like NDVI and mosaics. STAC is intentionally designed with a minimal core and flexible extension mechanism to support a broad set of use cases. This specification has matured over the past several years, and is used in numerous production deployments.

This is advantageous to providers of geospatial data, as they can simply use a well-designed, standard format and API without needing to design their own proprietary one. This is advantageous to consumers of geospatial data, as they can use existing libraries and tools to access metadata, instead of needing to write new code to interact with each data provider’s proprietary formats and APIs.

The STAC specifications define related JSON object types connected by link relations to support a HATEOAS-style traversable interface and a RESTful API providing additional browse and search interfaces. Typically, several STAC specifications are composed together to create an implementation. The Item, Catalog, and Collection specifications define a minimal core of the most frequently used JSON object types. Because of the hierarchical structure between these objects, a STAC catalog can be implemented in a completely ‘static’ manner as a group of hyperlinked Catalog, Collection, and Item URLs, enabling data publishers to expose their data as a browsable set of files. If more complex query abilities are desired, such as spatial or temporal predicates, the STAC API specification can be implemented as a web service interface to query over a group of STAC objects, usually held in a database.

To the greatest extent possible, STAC uses and extends existing specifications. The most important object in STAC is an Item, which is simply a GeoJSON Feature with a well-defined set of additional attributes (“foreign members”). The STAC API extends the OGC API – Features – Part 1: Core with additional web service endpoints and object attributes.