For more information on how to methods to get AprilTag data, look {ref}`here <docs/programming/photonlib/getting-target-data:Getting AprilTag Data From A Target>`.
PhotonLib includes a `PhotonPoseEstimator` class, which allows you to combine the pose data from all tags in view in order to get a field relative pose. For each camera, a separate instance of the `PhotonPoseEstimator` class should be created.
`AprilTagFieldLayout` is used to represent a layout of AprilTags within a space (field, shop at home, classroom, etc.). WPILib provides a JSON that describes the layout of AprilTags on the field which you can then use in the AprilTagFieldLayout constructor. You can also specify a custom layout.
The API documentation can be found in here: [Java](https://github.wpilib.org/allwpilib/docs/release/java/edu/wpi/first/apriltag/AprilTagFieldLayout.html), [C++](https://github.wpilib.org/allwpilib/docs/release/cpp/classfrc_1_1_april_tag_field_layout.html), and [Python](https://robotpy.readthedocs.io/projects/apriltag/en/stable/robotpy_apriltag/AprilTagFieldLayout.html#robotpy_apriltag.AprilTagFieldLayout).
Another necessary argument for creating a `PhotonPoseEstimator` is the `Transform3d` representing the robot-relative location and orientation of the camera. A `Transform3d` contains a `Translation3d` and a `Rotation3d`. The `Translation3d` is created in meters and the `Rotation3d` is created with radians. For more information on the coordinate system, please see the {ref}`Coordinate Systems <docs/apriltag-pipelines/coordinate-systems:Coordinate Systems>` documentation.
The PhotonPoseEstimator has a constructor that takes an `AprilTagFieldLayout` (see above), `PoseStrategy`, `PhotonCamera`, and `Transform3d`. `PoseStrategy` has nine possible values:
- Calculates a new robot position estimate by combining all visible tag corners. Recommended for all teams as it will be the most accurate.
- Must configure the AprilTagFieldLayout properly in the UI, please see {ref}`here <docs/apriltag-pipelines/multitag:multitag localization>` for more information.
- Use distance data from best visible tag to compute a Pose. This runs on the RoboRIO in order
to access the robot's yaw heading, and MUST have addHeadingData called every frame so heading
data is up-to-date. Based on a reference implementation by [FRC Team 6328 Mechanical Advantage](https://www.chiefdelphi.com/t/frc-6328-mechanical-advantage-2025-build-thread/477314/98).
Python still takes a `PhotonCamera` in the constructor, so you must create the camera as shown in the next section and then return and use it to create the `PhotonPoseEstimator`.
The final prerequisite to using your `PhotonPoseEstimator` is creating a `PhotonCamera`. To do this, you must set the name of your camera in Photon Client. From there you can define the camera in code.
Calling `update()` on your `PhotonPoseEstimator` will return an `EstimatedRobotPose`, which includes a `Pose3d` of the latest estimated pose (using the selected strategy) along with a `double` of the timestamp when the robot pose was estimated.
You should be updating your [drivetrain pose estimator](https://docs.wpilib.org/en/latest/docs/software/advanced-controls/state-space/state-space-pose-estimators.html) with the result from the `PhotonPoseEstimator` every loop using `addVisionMeasurement()`.
