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.