Merge pull request #884 from radiantearth/more-rel-types

More rel types

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 ## [Unreleased]
 
 ### Added
+ - 'via' and 'canonical' rel types as options in items.
 
 - Added clarification about how collection-level assset object properties do not remove the need for item-level asset object properties in the `item-assets` extension ([#880](https://github.com/radiantearth/stac-spec/pull/880))
 - Added [processing extension](extensions/processing/README.md)

item-spec/item-spec.md

@@ -134,6 +134,8 @@ The following types are commonly used as `rel` types in the Link Object of an It
 | collection   | STRONGLY RECOMMENDED. URL to a [Collection](../collection-spec/README.md). *Absolute* URLs should be used whenever possible. The referenced Collection is STRONGLY RECOMMENDED to implement the same STAC version as the Item. |
 | derived_from | URL to a STAC Item that was used as input data in the creation of this Item. |
 | alternate    | It is recommended that STAC Items are also available as HTML, and should use this rel with `"type" : "text/html"` to tell clients where they can get a version of the Item to view in a browser. See [STAC on the Web in Best Practices](../best-practices.md#stac-on-the-web) for more information. |
+| canonical    | The URL of the [canonical](https://en.wikipedia.org/wiki/Canonical_link_element) version of the Item. API responses and copies of catalogs should use this to inform users that they are direct copy of another STAC Item, using the canonical rel to refer back to the primary location. |
+| via          | The URL of the metadata that this STAC Item is created from. Used similarly to canonical, but refers back to a non-STAC record (Landsat MTL, Sentinel tileInfo.json, etc) |
 
 A more complete list of possible 'rel' types can be seen at the [IANA page of Link Relation Types](https://www.iana.org/assignments/link-relations/link-relations.xhtml).