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stac-geoparquet-spec.md

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STAC GeoParquet Specification

Overview

This document specifies how to map a set of STAC Items into GeoParquet. It is directly inspired by the STAC GeoParquet library, but aims to provide guidance for anyone putting STAC data into GeoParquet.

Use cases

  • Provide a STAC GeoParquet that mirrors a static Collection as a way to query the whole dataset instead of reading every specific GeoJSON file.
  • As an output format for STAC API responses that is more efficient than paging through thousands of pages of GeoJSON.
  • Provide efficient access to specific fields of a STAC item, thanks to Parquet's columnar format.

Guidelines

Each row in the Parquet Dataset represents a single STAC item. Most all the fields in a STAC Item should be mapped to a column in GeoParquet. We embrace Parquet structures where possible, mapping from JSON into nested structures. We do pull the properties to the top level, so that it is easier to query and use them. The names of most of the fields should be the same in STAC and in GeoParquet.

Field GeoParquet Type Required Details
type String Optional This is just needed for GeoJSON, so it is optional and not recommended to include in GeoParquet
stac_extensions List of Strings Required This column is required, but can be empty if no STAC extensions were used
id String Required Required, should be unique within each collection
geometry Binary (WKB) Required For GeoParquet 1.0 this must be well-known Binary
bbox Struct of Floats Required Can be a 4 or 6 value struct, depending on dimension of the data. It must conform to the "Bounding Box Columns" definition of GeoParquet 1.1.
links List of Link structs Required See Link Struct for more info
assets An Assets struct Required See Asset Struct for more info
collection String Optional The ID of the collection this Item is a part of. See notes below on 'Collection' and 'Collection JSON' in the Parquet metadata
property columns varies - Each property should use the relevant Parquet type, and be pulled out of the properties object to be a top-level Parquet field
  • Must be valid GeoParquet, with proper metadata. Ideally the geometry types are defined and as narrow as possible.
  • Strongly recommend to only have one GeoParquet per STAC 'Collection'. Not doing this will lead to an expanded GeoParquet schema (the union of all the schemas of the collection) with lots of empty data
  • Any field in 'properties' of the STAC item should be moved up to be a top-level field in the GeoParquet.
  • STAC GeoParquet does not support properties that are named such that they collide with a top-level key.
  • datetime columns should be stored as a native timestamp, not as a string
  • The Collection JSON should be included in the Parquet metadata. See Collection JSON below.
  • Any other properties that would be stored as GeoJSON in a STAC JSON Item (e.g. proj:geometry) should be stored as a binary column with WKB encoding. This simplifies the handling of collections with multiple geometry types.

Link Struct

The GeoParquet dataset can contain zero or more Link Structs. Each Link Struct has 2 required fields and 2 optional ones:

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.

See Link Object for more.

Asset Struct

The GeoParquet dataset can contain zero or more Asset Structs. Each Asset Struct can have the following fields:

Field Name Type Description
href string REQUIRED. URI to the asset object. Relative and absolute URI are both allowed.
title string The displayed title for clients and users.
description string A description of the Asset providing additional details, such as how it was processed or created. CommonMark 0.29 syntax MAY be used for rich text representation.
type string Media type of the asset. See the common media types in the best practice doc for commonly used asset types.
roles [string] The semantic roles of the asset, similar to the use of rel in links.

Each struct has each full asset key and object as a sub-struct, it's a direct mapping from the JSON to Parquet

To take advantage of Parquet's columnar nature and compression, the assets should be uniform so they can be represented by a simple schema, which in turn means every item should probably come from the same STAC collection.

See Asset Object for more.

Including a STAC Collection JSON in a STAC Geoparquet Collection

To make a stac-geoparquet file a fully self-contained representation, you can include the Collection JSON in the Parquet metadata. If present in the Parquet file metadata, the key must be stac:collection and the value must be a JSON string with the Collection JSON.

Referencing a STAC Geoparquet Collections in a STAC Collection JSON

A common use case of stac-geoparquet is to create a mirror of a STAC collection. To refer to this mirror in the original collection, use an Asset Object at the collection level of the STAC JSON that includes the application/vnd.apache.parquet Media type and collection-mirror Role type to describe the function of the Geoparquet STAC Collection Asset.

For example:

Field Name Type Value
href string s3://example/uri/to/file.parquet
title string An example STAC GeoParquet.
description string Example description.
type string application/vnd.apache.parquet
roles [string] [collection-mirror]*

*Note the IANA has not approved the new Media type application/vnd.apache.parquet yet, it's been submitted for approval.

The description should ideally include details about the spatial partitioning method.

Mapping to other geospatial data formats

The principles here can likely be used to map into other geospatial data formats (GeoPackage, FlatGeobuf, etc), but we embrace Parquet's nested 'structs' for some of the mappings, so other formats will need to do something different. The obvious thing to do is to dump JSON into those fields, but that's outside the scope of this document, and we recommend creating a general document for that.