LAF/GrAF (ISO 24612)
Overview
LAF is the ISO standard for linguistic annotation interchange. It defines a three-layer architecture: (1) a media layer containing primary data, (2) an anchoring layer mapping annotations to regions of the primary data, and (3) an annotation layer containing labeled nodes and edges forming a directed graph. GrAF is the concrete XML serialization of LAF. The model is theory-neutral and representation-neutral by design.
Architectural Mapping
LAF Three-Layer Architecture
| LAF Layer | Layers Equivalent | Notes |
|---|---|---|
| Media layer (primary data) | pub.layers.expression.expression + pub.layers.media.media | The expression record holds text; media records hold audio/video/image. |
| Anchoring layer (regions) | pub.layers.defs#anchor (polymorphic) | LAF's anchoring to regions of primary data maps to Layers's polymorphic anchor type: span (UTF-8 byte offsets), tokenRef, tokenRefSequence, temporalSpan, spatioTemporalAnchor, pageAnchor, externalTarget. |
| Annotation layer (graph) | pub.layers.annotation.annotationLayer + pub.layers.graph | Labeled nodes and edges. Layers provides both within-layer graph structures (annotation with parentId/childIds/headIndex/arguments) and cross-layer/cross-document graph structures (pub.layers.graph.graphEdgeSet). |
LAF Data Model Primitives
| LAF Concept | Layers Equivalent | Notes |
|---|---|---|
| Region | pub.layers.defs#anchor | A contiguous area of primary data. LAF regions are defined by anchors into the media layer. Layers's span, temporalSpan, pageAnchor etc. serve the same purpose. |
| Node | pub.layers.annotation.defs#annotation | A labeled point in the annotation graph. In Layers, each annotation is a node with optional anchoring and feature structure. |
| Edge | annotation.headIndex, annotation.parentId, annotation.arguments, or pub.layers.graph.graphEdge | Directed connections between nodes. Within a single annotation layer, edges are represented via headIndex (dependency), parentId/childIds (constituency), or argumentRef (predicate-argument). Across layers, pub.layers.graph.graphEdge provides typed directed edges. |
| Feature Structure | pub.layers.defs#featureMap | LAF's attribute-value feature structures on nodes and edges map to Layers's featureMap (typed key-value pairs). |
| Annotation | pub.layers.annotation.defs#annotation | A node-feature structure pair. The label, value, features, and typed fields (tokenIndex, anchor, etc.) on a Layers annotation constitute its feature structure. |
| Annotation Space | pub.layers.annotation.annotationLayer | LAF annotation spaces (sets of nodes/edges from a single producer) map to annotation layers. Multiple layers can coexist over the same expression. |
| Annotation Document | pub.layers.annotation.annotationLayer (record) | A serialized annotation space. In Layers, each annotation layer is an ATProto record. |
| Link | annotation.anchor | The connection between a node and a region of primary data. |
GrAF XML Elements
| GrAF Element | Layers Equivalent | Notes |
|---|---|---|
<graph> | pub.layers.annotation.annotationLayer | Root element of an annotation document. |
<node xml:id="..."> | pub.layers.annotation.defs#annotation with uuid | Identified annotation node. |
<edge from="..." to="..."> | annotation.headIndex, argumentRef, or graphEdge | Directed edge between nodes. |
<link targets="..."> | annotation.anchor | Associates a node with regions of primary data. |
<region anchors="..."> | pub.layers.defs#span or other anchor type | Specifies offsets into primary data. |
<fs> (feature structure) | pub.layers.defs#featureMap | Attribute-value pairs on nodes. |
<f name="..." value="..."> | pub.layers.defs#feature | Single feature. |
<a> (annotation) | pub.layers.annotation.defs#annotation | Node + feature structure + optional link. |
LAF Standoff Principles
LAF mandates that annotations are stored separately from primary data, referencing it by anchors. Layers follows this principle exactly:
- Primary data is in
pub.layers.expression.expression(text) andpub.layers.media.media(audio/video/image) - Document structure (sections, sentences, paragraphs) is expressed via
pub.layers.expression.expressionrecords withparentRefchains, separate from primary data - Tokenization is in
pub.layers.segmentation.segmentationrecords, with optionalexpressionRefscoping each tokenization to a specific sub-expression - All annotations are in
pub.layers.annotation.annotationLayerrecords, referencing primary data viaanchorobjects - Multiple annotation layers from different producers coexist independently
LAF Compositionality
LAF requires that annotation graphs be composable: independent annotations from different sources can be combined. Layers achieves this through:
- Separate records: Each annotation layer is an independent ATProto record
- Shared anchoring: All layers reference the same expression via its AT-URI
- UUID cross-references: Annotations reference each other by UUID, enabling cross-layer composition
- No interference: Adding a new annotation layer never modifies existing layers