Coordinate Frames And Tf

name: coordinate-frames-and-tf description: 'Understand ROS 2 coordinate frames (REP 105). Use when working with map, odom, base_link, base_footprint, or sensor frames.'

Coordinate Frames and TF for Navigation

REP 105 Standard Frames

ROS defines four standard frames for mobile robot navigation:

The TF Chain

map
 └── odom                    (published by AMCL or global EKF)
      └── base_link          (published by wheel odom or local EKF)
           ├── base_footprint
           ├── laser_frame    (published by robot_state_publisher from URDF)
           ├── imu_link       (published by robot_state_publisher from URDF)
           ├── camera_link    (published by robot_state_publisher from URDF)
           └── ...other sensor frames

Who Publishes What

The Critical Rule: One Publisher Per Transform

Only ONE node may publish each transform. If two nodes both publish odom → base_link, TF2 receives conflicting data and downstream consumers (Nav2, costmaps) will see oscillation, warnings, or crashes.

Common Violation

Your wheel odometry node publishes odom → base_link via tf_broadcaster. You also run robot_localization EKF with publish_tf: true, which also publishes odom → base_link.

Fix: Set publish_tf: false in the raw odometry publisher, or don't broadcast TF from the raw odometry node at all. Let the EKF be the sole publisher.

# In your odometry node:
# self.tf_broadcaster.sendTransform(t)  # REMOVE THIS if using robot_localization

Or in the EKF config:

publish_tf: true   # EKF publishes odom → base_link

And in the raw odometry node:

publish_tf: false   # Raw odom does NOT publish TF

REP 103 Coordinate Conventions

All frames follow right-hand rule:

• X → forward
• Y → left
• Z → up

Rotations:

• Roll → rotation about X (tilt left/right)
• Pitch → rotation about Y (tilt forward/backward)
• Yaw → rotation about Z (turn left/right)

Positive yaw is counterclockwise when viewed from above.

Sensor Frame Alignment

Every sensor frame in your URDF must conform. If a physical sensor has a different convention (e.g., camera: z-forward, x-right, y-down), the URDF joint must include an appropriate rotation:

<joint name="camera_joint" type="fixed">
  <parent link="base_link"/>
  <child link="camera_link"/>
  <origin xyz="0.1 0.0 0.3" rpy="0 0 0"/>
</joint>
<!-- camera_optical_frame follows camera convention: z-forward -->
<joint name="camera_optical_joint" type="fixed">
  <parent link="camera_link"/>
  <child link="camera_optical_frame"/>
  <origin xyz="0 0 0" rpy="${-pi/2} 0 ${-pi/2}"/>
</joint>

Verifying the TF Tree

List All Transforms

ros2 run tf2_tools view_frames

Generates a PDF of the complete TF tree with publishers and rates.

Check a Specific Transform

ros2 run tf2_ros tf2_echo map base_link

Shows the live transform between two frames. If this fails with "Could not transform", the chain is broken somewhere.

Monitor for Problems

ros2 run tf2_ros tf2_monitor

Reports frequency and delay for all published transforms. Look for:

• Transforms at 0 Hz — broken publisher
• High delay — timestamps too far from current time
• Multiple publishers — conflicting sources

map → odom: The Localization Correction

AMCL (or a global EKF) publishes map → odom. This transform encodes the accumulated drift correction:

true_robot_pose_in_map = map→odom * odom→base_link

When AMCL updates, map → odom may change discontinuously (the correction "jumps"). This is expected and correct — it means the odom frame has drifted and the map→odom transform compensates.

Without AMCL

If you only run the local EKF (no AMCL), the map → odom transform is never published. Nav2 requires it. Options:

1. Run AMCL (recommended)
2. Publish a static identity transform: map → odom (works if odom drift is negligible for your use case)

Node(
    package='tf2_ros',
    executable='static_transform_publisher',
    arguments=['0', '0', '0', '0', '0', '0', 'map', 'odom'],
)

Launch File: Complete Frame Publishers

from launch import LaunchDescription
from launch_ros.actions import Node

def generate_launch_description():
    return LaunchDescription([
        # URDF → sensor frames
        Node(
            package='robot_state_publisher',
            executable='robot_state_publisher',
            parameters=[{'robot_description': robot_description_content}],
        ),

        # EKF → odom → base_link
        Node(
            package='robot_localization',
            executable='ekf_node',
            name='ekf_filter_node',
            parameters=[ekf_config],  # publish_tf: true
        ),

        # AMCL → map → odom
        Node(
            package='nav2_amcl',
            executable='amcl',
            name='amcl',
            parameters=[amcl_config],
        ),
    ])

Troubleshooting