Appendix G: Frame Identifiers (Informative Reference)

SpatialDDS represents reference frames using the FrameRef structure:

The normative IDL for FrameRef resides in Appendix A (Core profile). This appendix is descriptive/informative and restates the usage guidance for reference frames.

struct FrameRef {
  string uuid;                       // globally unique frame ID
  string fqn;                        // normalized fully-qualified name, e.g. "earth-fixed/map/cam_front"
  boolean has_coord_convention;      // 1.6: optional axis convention
  CoordConvention coord_convention;  // see §2.12; absent ⇒ assume ENU
};

The optional coord_convention field (added in 1.6) specifies the axis convention for poses expressed in this frame — see §2.12 for the full convention table (ENU, CV, GRAPHICS, UNITY_LH, NED, OTHER) and chaining rules. When has_coord_convention is false, consumers MUST assume ENU. Chaining poses across frames with different conventions requires an intervening axis-swap transform.

UUID Rules

• uuid is authoritative for identity.
• fqn is an optional human-readable alias.
• Implementations MUST treat uuid uniqueness as the identity key.
• Deployments SHOULD establish well-known UUIDs for standard roots (e.g., earth-fixed, map, body) and document them for participants.

Name and Hierarchy Rules

• fqn components are slash-delimited.
• Reserved roots include earth-fixed, map, body, anchor, local.
• A FrameRef DAG MUST be acyclic.

Constructing FQNs from External Data (Informative)

Datasets and frameworks that use flat frame identifiers (e.g., nuScenes calibrated_sensor.token, ROS TF frame_id) must construct hierarchical FQNs when publishing to SpatialDDS.

Recommended approach: 1. Choose a root corresponding to the vehicle/robot body: fqn = "ego" or fqn = "<vehicle_id>". 2. Append the sensor channel as a child: fqn = "ego/cam_front", fqn = "ego/lidar_top". 3. Use the original flat token as the uuid field. 4. For earth-fixed references, use the reserved root fqn = "earth-fixed".

Example nuScenes mapping:

calibrated_sensor.token = "a1b2c3..."
sensor.channel = "CAM_FRONT"
-> FrameRef { uuid: "a1b2c3...", fqn: "ego/cam_front" }

Manifest References

Manifest entries that refer to frames MUST use a FrameRef object rather than raw strings. Each manifest MAY define local frame aliases resolvable by fqn.

Notes

Derived schemas (e.g. GeoPose, Anchors) SHALL refer to the Appendix A definition by reference and MUST NOT re-declare frame semantics. The conventions in §2.1 and the coverage/discovery rules in §3.3 reference this appendix for their frame requirements.