Cesium Impl Aec Georef

CesiumJS AEC Georeferencing

Overview

BIM, CAD, and city models use a LOCAL engineering coordinate system: meters from a project origin. CesiumJS renders in ECEF (Earth-Centered Earth-Fixed) WGS84 meters. A model loaded with no transform draws near the CENTER of the Earth, invisible from any normal camera. Georeferencing is building the Matrix4 that maps the model's local frame to its real position on the globe and assigning it to modelMatrix.

Core principle: the placement math (Transforms, Matrix4, HeadingPitchRoll, modelMatrix) is official CesiumJS API. Getting CityGML or IFC INTO a loadable format is an EXTERNAL workflow with no CesiumJS API. This skill keeps the two strictly separate.

This skill is technology-specific: CesiumJS 1.124+, WebGL2 only.

When to Use This Skill

• Placing a BIM, CAD, or city model at a real-world location.
• A loaded model is invisible, or appears at the center of the Earth.
• A model sits at the wrong place, or faces the wrong direction.
• A model floats above or sinks below terrain.
• Getting an IFC or CityGML model into CesiumJS.
• Rendering CAD-style edges on a model or tileset.

Official API vs External Workflow

This separation is CRITICAL. NEVER present a file-conversion pipeline as a CesiumJS API call.

CesiumJS has NO native CityGML loader and NO native IFC loader. Those formats MUST be converted first, outside CesiumJS.

Quick Reference: Georeferencing API

The Placement Problem and modelMatrix

modelMatrix is a Matrix4 on both Model and Cesium3DTileset, default Matrix4.IDENTITY. With the identity matrix the asset is drawn in raw ECEF WGS84 coordinates, so a CAD model built around local (0, 0, 0) lands at the Earth's core.

ALWAYS assign a modelMatrix that maps the model's local origin to a real Cartesian3 position on the globe.

Placing a Model at a Location

Transforms.eastNorthUpToFixedFrame(origin) returns the Matrix4 that maps a local east-north-up frame (local +X east, +Y north, +Z up) at origin into ECEF. Assign it to modelMatrix.

const origin = Cesium.Cartesian3.fromDegrees(4.9041, 52.3676, 12.0);
const model = await Cesium.Model.fromGltfAsync({
  url: "/models/building.glb",
  modelMatrix: Cesium.Transforms.eastNorthUpToFixedFrame(origin),
});
viewer.scene.primitives.add(model);

Orienting a Model

When the model must be rotated (a building not aligned to north, a tilted element), use Transforms.headingPitchRollToFixedFrame(origin, hpr). The HeadingPitchRoll constructor takes RADIANS; ALWAYS use HeadingPitchRoll.fromDegrees when working in degrees.

const origin = Cesium.Cartesian3.fromDegrees(4.9041, 52.3676, 12.0);
const hpr = Cesium.HeadingPitchRoll.fromDegrees(135.0, 0.0, 0.0);
const modelMatrix = Cesium.Transforms.headingPitchRollToFixedFrame(origin, hpr);
const model = await Cesium.Model.fromGltfAsync({
  url: "/models/building.glb",
  modelMatrix,
});
viewer.scene.primitives.add(model);

heading rotates about the local up axis from north, pitch about the east axis, roll about the north axis.

Placing a Tileset

Cesium3DTileset.modelMatrix works the same way and is mutable after the tileset loads.

ALWAYS check whether the tileset is ALREADY georeferenced before assigning a modelMatrix. A tileset produced by the ion tiler from georeferenced source data carries its real-world transform in its root tile; its modelMatrix must stay Matrix4.IDENTITY. Assign a modelMatrix ONLY to a tileset whose content sits in a local CAD frame.

// Non-georeferenced tileset: place it.
const tileset = await Cesium.Cesium3DTileset.fromUrl("/tiles/plant/tileset.json");
tileset.modelMatrix = Cesium.Transforms.eastNorthUpToFixedFrame(
  Cesium.Cartesian3.fromDegrees(4.9041, 52.3676, 0.0)
);
viewer.scene.primitives.add(tileset);

Custom Local Frames

A glTF or BIM export whose axes do not match east-north-up needs either a local correction matrix composed onto the ENU frame, or a custom frame function from Transforms.localFrameToFixedFrameGenerator(firstAxis, secondAxis), where each axis is one of east, north, up, west, south, down.

// A model authored north-west-up instead of east-north-up.
const nwuToFixed = Cesium.Transforms.localFrameToFixedFrameGenerator("north", "west");
const modelMatrix = nwuToFixed(origin);

Compose a correction with Matrix4.multiply(enuFrame, localCorrection, result).

External Workflow: CityGML and IFC

These steps run OUTSIDE CesiumJS. They produce a file CesiumJS can load; they are NOT CesiumJS API.

• CityGML: convert to 3D Tiles with the Cesium ion tiler or external tooling, then stream the result with Cesium3DTileset.fromUrl or fromIonAssetId.
• IFC: export to glTF with ThatOpen Components or IfcOpenShell, then load the glTF with Model.fromGltfAsync and georeference it with a modelMatrix.

The georeference step (the modelMatrix) is the only CesiumJS-side work; the conversion is external workflow.

CAD-style Edges

EdgeDisplayMode (CesiumJS 1.142+) renders the crisp edges architects expect. The edgeDisplayMode property on Model and Cesium3DTileset takes EdgeDisplayMode.SURFACES_ONLY (default), SURFACES_AND_EDGES, or EDGES_ONLY (CAD wireframe).

// Requires CesiumJS 1.142 or newer.
model.edgeDisplayMode = Cesium.EdgeDisplayMode.SURFACES_AND_EDGES;

Edges are read from the EXT_mesh_primitive_edge_visibility glTF extension, so the glTF MUST carry edge data; an exporter that omits the extension shows no edges. The extension loads since 1.135; the EdgeDisplayMode control is 1.142+.

Cross-Tech Companion Notes

Georeferencing usually sits inside a larger AEC toolchain. Each tool below has its own skill package; this skill covers only the CesiumJS step.

• QGIS: prepares and inspects the geospatial source data (survey points, imagery, terrain) that defines where a model belongs.
• ThatOpen Components and IfcOpenShell: the IFC-to-glTF bridge feeding Model.fromGltfAsync.
• Speckle: federates multi-discipline BIM data; export to glTF or 3D Tiles for streaming into CesiumJS.

Common Mistakes

Full root-cause analysis is in references/anti-patterns.md.

Reference Files

• references/methods.md : verified signatures for Transforms, Matrix4, HeadingPitchRoll, TranslationRotationScale, modelMatrix, and EdgeDisplayMode.
• references/examples.md : runnable placement, orientation, tileset, and custom-frame snippets.
• references/anti-patterns.md : georeferencing failure modes, each with symptom, root cause, prevention, and recovery.

Related Skills

• cesium-core-coordinates : Cartesian3, Cartographic, the ellipsoid, ENU frames.
• cesium-syntax-gltf-model : Model.fromGltfAsync and model loading.
• cesium-syntax-3d-tiles : Cesium3DTileset loading and tile formats.
• cesium-impl-cesium-ion : the ion tiler that converts source data to 3D Tiles.
• cesium-errors-coordinates : NaN positions and off-globe geometry.