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Projection Extension Specification

  • Title: Projection
  • Identifier: projection
  • Field Name Prefix: proj
  • Scope: Item
  • Extension Maturity Classification: Proposal

This document explains the fields of the STAC Projection (proj) Extension to a STAC Item. Here proj is short for "projection", and not a reference to the use of the PROJ/PROJ4 formats.

The field names defined herein should be added as fields in the Item Properties object.

When specified on an Item, the values are assumed to apply to all Assets in that Item. For example, an Item may have several related Assets each representing a band or layer for the Item, and which typically all use the same CRS, e.g. a UTM Zone. However, there may also be assets intended for display, like a preview image or thumbnail, that have been reprojected to a different CRS, e.g., Web Mercator, to better accommodate that use case. This case of differing projections per Asset is not currently handled by this extension.

Examples

Schema

Item Properties fields

Field Name Type Description
proj:epsg integer|null Required EPSG code of the datasource
proj:proj4 string |null PROJ4 string representing the Coordinate Reference System (CRS) that the proj:geometry and proj:bbox fields represent
proj:wkt2 string |null WKT2 string representing the Coordinate Reference System (CRS) that the proj:geometry and proj:bbox fields represent
proj:projjson PROJJSON Object |null PROJJSON object representing the Coordinate Reference System (CRS) that the proj:geometry and proj:bbox fields represent
proj:geometry Polygon Object Defines the footprint of this Item.
proj:bbox [number] Bounding box of the Item in the asset CRS in 2 or 3 dimensions.
proj:centroid Centroid Object Coordinates representing the centroid of the Item in the asset CRS

proj:epsg - A Coordinate Reference System (CRS) is the data reference system (sometimes called a 'projection') used by the asset data, and can usually be referenced using an EPSG code. If the asset data does not have a CRS, such as in the case of non-rectified imagery with Ground Control Points, proj:epsg should be set to null. It should also be set to null if a CRS exists, but for which there is no valid EPSG code.

proj:proj4 - A Coordinate Reference System (CRS) is the data reference system (sometimes called a 'projection') used by the asset data. This value is a PROJ4 string. If the data does not have a CRS, such as in the case of non-rectified imagery with Ground Control Points, proj:proj4 should be set to null. It should also be set to null if a CRS exists, but for which a PROJ4 string does not exist.

proj:wkt2 - A Coordinate Reference System (CRS) is the data reference system (sometimes called a 'projection') used by the asset data. This value is a WKT2 string. If the data does not have a CRS, such as in the case of non-rectified imagery with Ground Control Points, proj:wkt2 should be set to null. It should also be set to null if a CRS exists, but for which a WKT2 string does not exist.

proj:projjson - A Coordinate Reference System (CRS) is the data reference system (sometimes called a 'projection') used by the asset data. This value is a PROJJSON object. If the data does not have a CRS, such as in the case of non-rectified imagery with Ground Control Points, proj:projjson should be set to null. It should also be set to null if a CRS exists, but for which a PROJJSON string does not exist. The schema for this object can be found here.

proj:geometry - A Polygon object representing the footprint of this item, formatted according the Polygon object format specified in RFC 7946, sections 3.1.6, except not necessarily in EPSG:4326 as required by RFC7946. Specified based on the proj:proj4 field (not necessarily EPSG:4326). Ideally, this will be represented by a Polygon with five coordinates, as the item in the asset data CRS should be a square aligned to the original CRS grid.

proj:bbox - Bounding box of the assets represented by this item in the asset data CRS. Specified as 4 or 6 coordinates based on the CRS defined in the proj:epsg and proj:proj4 fields. First two numbers are coordinates of the lower left corner, followed by coordinates of upper right corner, , e.g., [west, south, east, north], [xmin, ymin, xmax, ymax], [left, down, right, up], or [west, south, lowest, east, north, highest]. The length of the array must be 2*n where n is the number of dimensions. The array contains all axes of the southwesterly most extent followed by all axes of the northeasterly most extent specified in Longitude/Latitude or Longitude/Latitude/Elevation based on WGS 84. When using 3D geometries, the elevation of the southwesterly most extent is the minimum elevation in meters and the elevation of the northeasterly most extent is the maximum in meters.

proj:centroid - Coordinates representing the centroid of the item in the asset data CRS. Coordinates are defined in latitude and longitude, even if the data coordinate system does not use lat/long.

Centroid Object

This object represents the centroid of an item's geometry.

Field Name Type Description
lat number The latitude of the centroid.
lon number The longitude of the centroid.