Geospatial Parquet format

Overview

The Apache Parquet provides a standardized open-source columnar storage format. This specification defines how geospatial data should be stored in parquet format, including the representation of geometries and the required additional metadata.

Version

This is version 0.4.0 of the geoparquet specification.

Geometry columns

Geometry columns are stored using the BYTE_ARRAY parquet type. They are encoded as WKB. See the encoding section below for more details.

Metadata

geoparquet files include additional metadata at two levels:

1. File metadata indicating things like the version of this specification used
2. Column metadata with additional metadata for each geometry column

These are both stored under a "geo" key in the parquet metadata (the FileMetaData::key_value_metadata) as a JSON-encoded UTF-8 string.

File metadata

All file-level metadata should be included under the "geo" key in the parquet metadata.

At this level, additional implementation-specific fields (e.g. library name) are allowed, and thus readers should be robust in ignoring those.

Additional file metadata information

primary_column

This indicates the "primary" or "active" geometry for systems that can store multiple geometries, but have a default geometry used for geospatial operations.

version

Version of the geoparquet spec used, currently 0.4.0

Column metadata

Each geometry column in the dataset must be included in the columns field above with the following content, keyed by the column name:

crs

The Coordinate Reference System (CRS) is an optional parameter for each geometry column defined in geoparquet format.

The CRS must be provided in PROJJSON format, which is a JSON encoding of WKT2:2019 / ISO-19162:2019, which itself implements the model of OGC Topic 2: Referencing by coordinates abstract specification / ISO-19111:2019. Apart from the difference of encodings, the semantics are intended to match WKT2:2019, and a CRS in one encoding can generally be represented in the other.

If CRS is not provided, all coordinates in the geometry must use longitude, latitude based on the WGS84 datum to store their data.

If an implementation is CRS-aware and needs a CRS representation of the data it should assume a default value is OGC:CRS84, which is equivalent to the well-known EPSG:4326 but changes the axis from latitude-longitude to longitude-latitude.

Due to the large number of CRSes available and the difficulty of implementing all of them, we expect that a number of implementations will start without support for the optional crs field. Users are recommended to store their data in longitude, latitude (OGC:CRS84 or not including the crs field) for it to work with the widest number of tools. Data that are more appropriately represented in particular projections may use an alternate coordinate reference system. We expect many tools will support alternate CRSes, but encourage users to check to ensure their chosen tool supports their chosen CRS.

See below for additional details about representing or identifying OGC:CRS84.

The value of this key may be explicitly set to null to indicate that there is no CRS assigned to this column (CRS is undefined or unknown).

epoch

In a dynamic CRS, coordinates of a point on the surface of the Earth may change with time. To be unambiguous, the coordinates must always be qualified with the epoch at which they are valid.

The optional "epoch" field allows to specify this in case the "crs" field defines a a dynamic CRS. The coordinate epoch is expressed as a decimal year (e.g. 2021.47). Currently, this specification only supports an epoch per column (and not per geometry).

encoding

This is the binary format that the geometry is encoded in. The string 'WKB', signifying Well Known Binary is the only current option, but future versions of the spec may support alternative encodings. This should be the "OpenGIS® Implementation Specification for Geographic information - Simple feature access - Part 1: Common architecture" WKB representation (using codes for 3D geometry types in the [1001,1007] range). This encoding is also consistent with the one defined in the "ISO/IEC 13249-3:2016 (Information technology - Database languages - SQL multimedia and application packages - Part 3: Spatial)" standard.

Note that the current version of the spec only allows for a subset of WKB: 2D or 3D geometries of the standard geometry types (the Point, LineString, Polygon, MultiPoint, MultiLineString, MultiPolygon, and GeometryCollection geometry types). This means that M values or non-linear geometry types are not yet supported.

Coordinate axis order

The axis order of the coordinates in WKB stored in a geoparquet follows the de facto standard for axis order in WKB and is therefore always (x, y) where x is easting or longitude and y is northing or latitude. This ordering explicitly overrides the axis order as specified in the CRS. This follows the precedent of GeoPackage, see the note in their spec.

geometry_type

This field captures the geometry type(s) of the geometries in the column, when known. Accepted geometry types are: "Point", "LineString", "Polygon", "MultiPoint", "MultiLineString", "MultiPolygon", "GeometryCollection".

In addition, the following rules are used:

• In case of 3D geometries, a " Z" suffix gets added (e.g. "Point Z").
• The value can be a single string or an array of strings in case multiple geometry types are present (e.g. ["Polygon", "MultiPolygon"]).
• Additionally the value "Unknown" is accepted to explicitly signal that the geometry type is not known.

It is expected that this field is strictly correct. For example, if having both polygons and multipolygons, it is not sufficient to specify "MultiPolygon", but it is expected to specify ["Polygon", "MultiPolygon"]. Or if having 3D points, it is not sufficient to specify "Point", but it is expected to list "Point Z".

orientation

This attribute indicates the winding order of polygons. The only available value is:

• "counterclockwise": All vertices of exterior polygon rings MUST be ordered in the counterclockwise direction and all interior rings MUST be ordered in the clockwise direction.

If no value is set, no assertions are made about winding order or consistency of such between exterior and interior rings or between individual geometries within a dataset. Readers are responsible for verifying and if necessary re-ordering vertices as required for their analytical representation.

Writers are encouraged but not required to set orientation="counterclockwise" for portability of the data within the broader ecosystem.

It is recommended to always set the orientation (to counterclockwise) if edges is 'spherical' (see below).

edges

This attribute indicates how to interpret the edges of the geometries: whether the line between two points is a straight cartesian line or the shortest line on the sphere (geodesic line). Available values are:

• planar: use a flat cartesian coordinate system.
• spherical: use a spherical coordinate system and radius derived from the spheroid defined by the coordinate reference system.

If no value is set, the default value to assume is 'planar'.

Note if edges is 'spherical' then it is recommended that orientation is always ensured to be 'counterclockwise'. If it is not set, it is not clear how polygons should be interpreted within spherical coordinate systems, which can lead to major analytical errors if interpreted incorrectly. In this case, software will typically interpret the rings of a polygon such that it encloses at most half of the sphere (i.e. the smallest polygon of both ways it could be interpreted). But the specification itself does not make any guarantee about this.

bbox

Bounding boxes are used to help define the spatial extent of each geometry column. Implementations of this schema may choose to use those bounding boxes to filter partitions (files) of a partitioned dataset.

The bbox, if specified, must be encoded with an array containing the minimum and maximum values of each dimension: [<xmin>, <ymin>, <xmax>, <ymax>]. This follows the GeoJSON specification (RFC 7946, section 5).

Additional information

You can find an example in the examples folder.

OGC:CRS84 details

The PROJJSON JSON object for OGC:CRS84 is:

{
    "$schema": "https://proj.org/schemas/v0.4/projjson.schema.json",
    "type": "GeographicCRS",
    "name": "WGS 84 longitude-latitude",
    "datum": {
        "type": "GeodeticReferenceFrame",
        "name": "World Geodetic System 1984",
        "ellipsoid": {
            "name": "WGS 84",
            "semi_major_axis": 6378137,
            "inverse_flattening": 298.257223563
        }
    },
    "coordinate_system": {
        "subtype": "ellipsoidal",
        "axis": [
        {
            "name": "Geodetic longitude",
            "abbreviation": "Lon",
            "direction": "east",
            "unit": "degree"
        },
        {
            "name": "Geodetic latitude",
            "abbreviation": "Lat",
            "direction": "north",
            "unit": "degree"
        }
        ]
    },
    "id": {
        "authority": "OGC",
        "code": "CRS84"
    }
}

For implementations that operate entirely with longitude, latitude coordinates and are not CRS-aware or do not have easy access to CRS-aware libraries that can fully parse PROJJSON, it may be possible to infer that coordinates conform to the OGC:CRS84 CRS based on elements of the crs field. For simplicity, Javascript object dot notation is used to refer to nested elements.

The CRS is likely equivalent to OGC:CRS84 for a GeoParquet file if the id element is present:

• id.authority = "OGC" and id.code = "CRS84"
• id.authority = "EPSG" and id.code = 4326 (due to longitude, latitude ordering in this specification)

It is reasonable for implementations to require that one of the above id elements are present and skip further tests to determine if the CRS is functionally equivalent with OGC:CRS84.

Note: EPSG:4326 and OGC:CRS84 are equivalent with respect to this specification because this specification specifically overrides the coordinate axis order in the crs to be longitude-latitude.