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# ARAnchor

An object that specifies the position and orientation of an item in the physical environment.

```
class ARAnchor
```

## Overview

To track the static positions and orientations of real or virtual objects relative to the camera, create anchor objects and use the [`add(anchor:)`](/documentation/ARKit/ARSession/add(anchor:)) method to add them to your AR session.

> Tip:
> Adding an anchor to the session helps ARKit to optimize world-tracking accuracy in the area around that anchor, so that virtual objects appear to stay in place relative to the real world. If a virtual object moves, remove the corresponding anchor from the old position and add one at the new position.

Some ARKit features automatically add special anchors to a session. World-tracking sessions can add [`ARPlaneAnchor`](/documentation/ARKit/ARPlaneAnchor), [`ARObjectAnchor`](/documentation/ARKit/ARObjectAnchor), and [`ARImageAnchor`](/documentation/ARKit/ARImageAnchor) objects if you enable the corresponding features; face-tracking sessions add [`ARFaceAnchor`](/documentation/ARKit/ARFaceAnchor) objects.

### Subclassing Notes

In addition to creating your own `ARAnchor` instances to track the real-world positions of your virtual content, you can also subclass `ARAnchor` to associate custom data with anchors you create. Ensure that your anchor classes behave correctly when ARKit updates frames or saves and loads anchors in an [`ARWorldMap`](/documentation/ARKit/ARWorldMap):

- Anchor subclasses must fullfill the requirements of the [`ARAnchorCopying`](/documentation/ARKit/ARAnchorCopying) protocol. ARKit calls [`init(anchor:)`](/documentation/ARKit/ARAnchorCopying/init(anchor:)) (on a background thread) to copy instances of your anchor class from each [`ARFrame`](/documentation/ARKit/ARFrame) to the next. Your implementation of this initializer should copy the values of any custom properties your subclass adds.
- Anchor subclasses must also adopt the <doc://com.apple.documentation/documentation/Foundation/NSSecureCoding> protocol. Override <doc://com.apple.documentation/documentation/Foundation/NSCoding/encode(with:)> and <doc://com.apple.documentation/documentation/Foundation/NSCoding/init(coder:)> to save and restore the values your subclass’ custom properties when ARKit saves and loads them in a world map.
- Anchors are considered equal based on their [`identifier`](/documentation/ARKit/ARAnchor/identifier) property.
- Only anchors that do not adopt [`ARTrackable`](/documentation/ARKit/ARTrackable) are included when you save a world map.

## Topics

### Creating Anchors

[`init(transform:)`](/documentation/ARKit/ARAnchor/init(transform:))

Creates a new anchor object with the specified transform.

[`init(name:transform:)`](/documentation/ARKit/ARAnchor/init(name:transform:))

Creates a new anchor object with the specified transform and a descriptive name.

[`name`](/documentation/ARKit/ARAnchor/name)

A descriptive name for the anchor.

### Tracking Anchors

[`identifier`](/documentation/ARKit/ARAnchor/identifier)

A unique identifier for the anchor.

[`sessionIdentifier`](/documentation/ARKit/ARAnchor/sessionIdentifier)

The unique identifier of the session that owns this anchor.

[`transform`](/documentation/ARKit/ARAnchor/transform)

A matrix encoding the position, orientation, and scale of the anchor relative to the world coordinate space of the AR session the anchor is placed in.


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