cesiumjs-spatial-math

# CesiumJS Spatial Math & Transforms

Version baseline: CesiumJS v1.142 (2026-06-01)

Mathematical foundation for every CesiumJS application: coordinate types, unit conversions, ellipsoid geometry, reference frame transforms, bounding volumes, intersection tests, and projections.

## Core Concepts

CesiumJS uses a right-handed Earth-Centered Earth-Fixed (ECEF) coordinate system:

- **Cartesian3** -- ECEF (x, y, z) in meters. Internal representation for all 3D positions.
- **Cartographic** -- (longitude, latitude, height). Angles are **radians**, height in meters above ellipsoid.

All angular values in core math are radians. Use `Math.toRadians()` / `Math.toDegrees()`. Math types use a **static-method-with-result** pattern: pass a `result` parameter to reuse allocations.

## Cartesian3 -- Positions and Vectors

```js
import { Cartesian3, Math as CesiumMath } from "cesium";

// From lon/lat degrees -- most common entry point
const pos = Cartesian3.fromDegrees(-105.0, 40.0);
const elevated = Cartesian3.fromDegrees(-105.0, 40.0, 1500.0); // with height

// Batch creation: [lon, lat, lon, lat, ...]
const ring = Cartesian3.fromDegreesArray([-105, 40, -100, 40, -100, 35]);

// With heights: [lon, lat, h, lon, lat, h, ...]
const wall = Cartesian3.fromDegreesArrayHeights([-105, 40, 500, -100, 40, 1000]);

// From raw ECEF or from radians
const raw = new Cartesian3(-1275096.0, -4797180.0, 4075270.0);
const fromRad = Cartesian3.fromRadians(-1.8326, 0.6981, 1500.0);

// Constants
Cartesian3.ZERO;   // (0,0,0)
Cartesian3.UNIT_X; // (1,0,0)
Cartesian3.UNIT_Y; // (0,1,0)
Cartesian3.UNIT_Z; // (0,0,1)
```

> **Breaking change (1.139, #8359):** `Cartesian2`, `Cartesian3`, and `Cartesian4`
> are now ES6 classes. Calling `new` on a static **factory** method now throws --
> `new Cartesian3.fromArray([...])` and `new Cartesian3.fromDegrees(...)` are
> errors. Drop `new` for factory methods (`Cartesian3.fromArray([...])`); keep it
> only for the real constructor (`new Cartesian3(x, y, z)`). More classes are
> migrating to ES6 classes, so apply this rule everywhere.

### Vector Operations

```js
const a = new Cartesian3(1.0, 2.0, 3.0);
const b = new Cartesian3(4.0, 5.0, 6.0);
const r = new Cartesian3(); // reusable scratch

Cartesian3.add(a, b, r);                // a + b
Cartesian3.subtract(a, b, r);           // a - b
Cartesian3.multiplyByScalar(a, 2.0, r); // a * 2
Cartesian3.negate(a, r);                // -a
Cartesian3.cross(a, b, r);              // cross product
Cartesian3.normalize(a, r);             // unit vector
Cartesian3.lerp(a, b, 0.5, r);         // linear interpolation
Cartesian3.midpoint(a, b, r);           // midpoint

const dot = Cartesian3.dot(a, b);       // dot product
const len = Cartesian3.magnitude(a);    // ||a||
const dist = Cartesian3.distance(a, b); // Euclidean distance
const distSq = Cartesian3.distanceSquared(a, b); // faster for comparisons
const angle = Cartesian3.angleBetween(a, b);     // radians
```

## Cartographic -- Geographic Coordinates

```js
import { Cartographic, Cartesian3, Math as CesiumMath } from "cesium";

const carto = Cartographic.fromDegrees(-105.0, 40.0, 1500.0);
const cartoRad = Cartographic.fromRadians(-1.8326, 0.6981, 1500.0);

// Cartesian3 <-> Cartographic
const position = Cartesian3.fromDegrees(-105.0, 40.0, 1500.0);
const geo = Cartographic.fromCartesian(position);
const lonDeg = CesiumMath.toDegrees(geo.longitude); // -105.0
const latDeg = CesiumMath.toDegrees(geo.latitude);  // 40.0
const backToCart = Cartographic.toCartesian(geo);
```

## CesiumMath Utilities

```js
import { Math as CesiumMath } from "cesium";

// Degree/radian conversion
const rad = CesiumMath.toRadians(90.0);    // PI/2
const deg = CesiumMath.toDegrees(Math.PI); // 180

// Constants: PI, TWO_PI, PI_OVER_TWO, PI_OVER_FOUR, RADIANS_PER_DEGREE
// EPSILON1 (0.1) through EPSILON21 (1e-21)

const clamped = CesiumMath.clamp(value, 0.0, 1.0);
const interp = CesiumMath.lerp(0.0, 100.0, 0.5);          // 50
const norm = CesiumMath.negativePiToPi(angle);              // [-PI, PI]
const pos = CesiumMath.zeroToTwoPi(angle);                  // [0, 2*PI]
const safeLon = CesiumMath.convertLongitudeRange(angle);    // [-PI, PI)
const eq = CesiumMath.equalsEpsilon(a, b, CesiumMath.EPSILON7); // float compare
```

## Ellipsoid

```js
import { Ellipsoid, Cartesian3, Cartographic } from "cesium";

// Built-in ellipsoids
Ellipsoid.WGS84;       // Earth (default)
Ellipsoid.UNIT_SPHERE;  // radius 1
Ellipsoid.MOON;         // lunar sphere
Ellipsoid.MARS;         // Mars (v1.133+)

// Change default (affects Ellipsoid.default everywhere)
Ellipsoid.default = Ellipsoid.MOON;

// Conversions on a specific ellipsoid
const cart = Ellipsoid.WGS84.cartographicToCartesian(
  Cartographic.fromDegrees(-75.0, 40.0, 100.0),
);
const carto = Ellipsoid.WGS84.cartesianToCartographic(cart);

// Surface normal at a position
const normal = Ellipsoid.WGS84.geodeticSurfaceNormal(cart, new Cartesian3());

// Project point onto ellipsoid surface
const onSurface = Ellipsoid.WGS84.scaleToGeodeticSurface(cart, new Cartesian3());
```

## Transforms -- Reference Frames

`Transforms` builds 4x4 matrices relating local frames to ECEF. The most commonly used function is `eastNorthUpToFixedFrame`.

### East-North-Up (ENU)

ENU: X = east, Y = north, Z = up. Standard frame for placing models on the globe.

```js
import { Cartesian3, Transforms, Matrix4 } from "cesium";

const origin = Cartesian3.fromDegrees(-105.0, 40.0);
const enuMatrix = Transforms.eastNorthUpToFixedFrame(origin);
// Columns: [east, north, up, origin] in ECEF
```

### Heading-Pitch-Roll Model Matrix

Standard way to position and orient a 3D model.

```js
import { Cartesian3, Transforms, HeadingPitchRoll, Math as CesiumMath } from "cesium";

const position = Cartesian3.fromDegrees(-105.0, 40.0, 0.0);
const hpr = new HeadingPitchRoll(
  CesiumMath.toRadians(90.0), // heading: 90 deg east
  0.0,                         // pitch: level
  0.0,                         //