> For the complete documentation index, see [llms.txt](https://cityweft.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://cityweft.gitbook.io/docs/basics/coordinate-systems-crs.md).

# Coordinate Systems (CRS)

The Cityweft API supports three coordinate reference systems for returned geometry, and any projected EPSG system for file exports. Choose the appropriate CRS based on your use case

There are two places a coordinate system applies:

* **`settings.crs`** — the coordinate system of the geometry returned in the JSON response: `local`, `EPSG:3857`, or `UTM`.
* **`export.crs`** — for file exports only: any supported projected EPSG system, including national grids and US State Plane. See [#export-coordinate-systems](#export-coordinate-systems).

| CRS                                                                                                   | Description                   | Use Case                                 | Coordinates          |
| ----------------------------------------------------------------------------------------------------- | ----------------------------- | ---------------------------------------- | -------------------- |
| <p>local<br><br><a href="#local-coordinate-system">#local-coordinate-system</a></p>                   | Relative to origin point      | Small sites, game engines, visualization | Meters from origin   |
| <p>EPSG:3857<br><br><a href="#web-mercator-epsg-3857">#web-mercator-epsg-3857</a></p>                 | Web Mercator                  | Web mapping, global applications         | Global Web Mercator  |
| <p>UTM<br><br><a href="#universal-transverse-mercator-utm">#universal-transverse-mercator-utm</a></p> | Universal Transverse Mercator | GIS, surveying, precise mapping          | UTM zone coordinates |

## Local Coordinate System

Parameter: `crs: "local"` (default)

#### Description

The local coordinate system positions all geometry relative to a specified origin point. This creates a localized coordinate space with the origin at (0,0,0).

#### Characteristics

* Origin: User-defined point or first polygon vertex
* Units: Meters
* Coordinate Space: X-right, Y-forward/back, Z-up (depending on upAxis parameter)
* Range: Suitable for sites up to \~10km radius

#### When to Use

* ✅ Game engines (Unity, Unreal)
* ✅ 3D visualization applications
* ✅ Small to medium-sized sites
* ✅ When you need simple, relative coordinates
* ❌ Large geographical areas (>10km)
* ❌ When integrating with GIS systems

#### Output format:

```json
{
  "spatialReference": {
    "crs": "local",
    "origin": [52.5200, 13.4050]
  },
  "geometry": [...] // Coordinates relative to origin
}
```

### Web Mercator (EPSG:3857)

Parameter: `crs: "EPSG:3857"`

#### Description

Web Mercator is the standard projection used by most web mapping services (Google Maps, Mapbox). Coordinates are in the global Web Mercator coordinate system.

#### Characteristics

* Origin: Global (0,0) at the intersection of the equator and the prime meridian
* Units: Meters (keep in mind the Mercator distortion)
* Projection: Spherical Mercator

#### When to Use

* ✅ Web mapping applications
* ✅ Integration with web map services
* ✅ Global or continental-scale projects
* ✅ When working with existing Web Mercator data
* ❌ High-precision surveying
* ❌ Polar regions (>85° latitude)

#### Output format:

```json
{
  "spatialReference": {
    "crs": "EPSG:3857"
  },
  "geometry": [...] // Global Web Mercator coordinates
}
```

### Universal Transverse Mercator (UTM)

Parameter: `crs:"UTM"`

#### Description

UTM divides the world into 60 zones, each 6° wide (with some exceptions like Norway mainland and Svalbard). The system automatically detects the appropriate UTM zone based on your polygon's location and provides high-precision coordinates.

#### Characteristics

* Origin: Zone-specific (500,000m Easting, 0m or 10,000,000m Northing)
* Units: Meters
* Projection: Transverse Mercator per zone
* Accuracy: Sub-meter precision within zone
* Auto-detection: Zone calculated from the polygon center

#### When to Use

* ✅ GIS applications (QGIS, ArcGIS)
* ✅ Surveying and engineering
* ✅ High-precision mapping
* ✅ CAD integration
* ✅ Construction and infrastructure projects
* ❌ Cross-zone projects (spanning >6° longitude)
* ❌ Simple visualization needs

\
**Output format**

```json
{
  "spatialReference": {
    "crs": "UTM",
    "utmZoneEPSG": "EPSG:32633",
    "zoneString": "33N",
    "hemisphere": "N",
    "zone": 33,
    "letter": "N"
  },
  "geometry": [...] // UTM zone coordinates
}
```

## Export Coordinate Systems

{% hint style="success" %}
**New:** file exports can be delivered in any supported projected EPSG system — national grids, UTM zones, and US State Plane (metre, international-foot, and US-survey-foot variants).
{% endhint %}

When using the [Exports](/docs/basics/exports.md) parameter, set `export.crs` to a projected EPSG code:

```json
"export": {
  "format": "dxf",
  "crs": "EPSG:27700",
  "placement": "absolute"
}
```

* Geometry is reprojected **per vertex** into the target system, so the export is grid-north aligned and free of projection scale distortion — two nearby exports in the same CRS register exactly.
* `placement: "anchor"` (default) keeps coordinates origin-relative and stores the georeference in the format's native anchor; `placement: "absolute"` bakes real grid coordinates into the file for GIS workflows.
* For foot-based systems (e.g. US State Plane in US survey feet), absolute exports carry coordinates in the grid's own unit.
* `export.crs` is only valid when `settings.crs` is `local` (the default).

## Integration Compatibility

| Software Type | Recommended CRS                            |
| ------------- | ------------------------------------------ |
| Game Engines  | local                                      |
| Web Maps      | EPSG:3857                                  |
| GIS Software  | UTM (or an export with absolute placement) |
| CAD Software  | UTM or local                               |
| 3D Modeling   | local                                      |

#### Error Handling

* Invalid CRS values default to 'local'
* UTM zones are auto-calculated; manual specification is not required
* Cross-zone polygons use the zone of the polygon centroid

#### Performance Notes

* Local & EPSG:3857: Fastest processing, minimal coordinate transformations
* UTM: Slightly slower, requires zone calculation and transformation