- 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. The exception to this is that proj:shape and
proj:transform can be overridden at the asset level, where an asset has a different shape and transform from the
default, which should be specified at the asset level, while those assets that use the defaults can remain unspecified.
| Field Name | Type | Description |
|---|---|---|
| proj:epsg | integer |null | REQUIRED. EPSG code of the datasource |
| 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:shape | [integer] | Number of pixels in Y and X directions for the default grid |
| proj:transform | [number] | The affine transformation coefficients for the default grid |
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: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:epsg, proj:projjson or proj:wkt2 fields (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, proj:projjson or proj:wkt2 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.
proj:shape - An array of integers that represents the number of pixels in the most common pixel grid used by the item's assets. The number of pixels should be specified in Y, X order. If the shape is defined in an item's properties it is used as the default shape for all assets that don't have an overriding shape.
proj:transform - Linear mapping from pixel coordinate space (Pixel, Line) to projection coordinate space (Xp, Yp). It is a 3x3 matrix stored as a flat array of 9 elements in row major order. Since the last row is always 0,0,1 it can be omitted, in which case only 6 elements are recorded. This mapping can be obtained from GDAL(GetGeoTransform) or the Rasterio (Transform).
[Xp] [a0, a1, a2] [Pixel]
[Yp] = [a3, a4, a5] * [Line ]
[1 ] [0 , 0, 1] [1 ]If the transform is defined in an item's properties it is used as the default transform for all assets that don't have an overriding transform.
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. |