@pixelfed You should definitely check out the ActivityStreams specification: https://www.w3.org/TR/activitystreams-vocabulary/#dfn-location, which describes the standard location property.

You can also take a look at the Vernissage extensions: https://joinvernissage.org/ns#geonameId, where we've added geonameId to uniquely identify places, and FEP-ee3a: https://codeberg.org/fediverse/fep/src/branch/main/fep/ee3a/fep-ee3a.md, which describes how EXIF metadata (including GPS coordinates from the camera) can be shared via ActivityPub.