ROS 2 Lyrical Luth · Lesson 53 of 86

Odometry estimation

Dead reckoning for a differential drive, drift and nav_msgs/Odometry.

Estimated time · 60–90 min

Learning goals

What you will understand

Explain odometry estimation in your own words
Use it in a small robotics example
Verify the result instead of assuming it worked

Understand the idea first

The mental model

Wheel odometry integrates velocities: x += v cos θ dt, y += v sin θ dt, θ += ω dt. With encoders, v = (v_right + v_left)/2 and ω = (v_right − v_left)/wheel_separation. It is smooth but drifts with wheel slip and small measurement errors.

Publish the result as nav_msgs/msg/Odometry (pose plus twist and covariances) on /odom and broadcast the transform odom → base_link. Fuse with an IMU or lidar localization to reduce drift. The diff_drive_controller already does this for a real or simulated robot.

Guided lesson

Work through these steps

1.
Source the ROS 2 Lyrical environment in a fresh terminal.
2.
Read the odometry estimation example and identify what each line does.
3.
Build only the affected package, source the workspace overlay, and run the example.
4.
Inspect the result from a second terminal with the ros2 CLI or RViz2 and compare it with the expected result.

Worked example

Read it before you run it

double v = 0.5, w = 0.0, dt = 0.1;
// Linear and angular velocity for this step.
x += v * std::cos(theta) * dt;
// Advance along the current heading (X).
y += v * std::sin(theta) * dt;
// Advance along the current heading (Y).
theta += w * dt;
// Turn by the angular velocity.
nav_msgs::msg::Odometry odom;
// The message to publish.
odom.pose.pose.position.x = x;
// Write the position into the message.
odom.pose.pose.orientation.z = std::sin(theta / 2); odom.pose.pose.orientation.w = std::cos(theta / 2);
// Convert yaw to a quaternion.

What the example is doing

1.The ros2 commands query the running graph; they do not prove that a node's internal behavior is correct.
2.Build from the workspace root, then source install/setup.bash so the shell can discover the new package.
3.Use a second sourced terminal to inspect nodes and interfaces while the example runs.

Watch it run · Mobile robot

See what this code does

The terminal types this lesson's practice solution and runs it. The scene shows the kind of robot behaviour that code drives. It plays by itself and loops.

Practice code

Now type it yourself

Copy the starter into practice.cpp and replace every TODO with your own code. Build and run it: g++ -std=c++17 -Wall -Wextra -Wpedantic practice.cpp -o practice -pthread && ./practice

#include <cmath>
#include <cstdio>

int main() {
  double x = 0.0, y = 0.0, theta = 0.0;
  const double dt = 0.1;
  auto step = [&](double v, double w) {
    // TODO: Euler update: x += v*cos(theta)*dt, y += v*sin(theta)*dt, theta += w*dt
  };
  for (int i = 0; i < 20; ++i) step(0.5, 0.0);        // drive 1 m
  for (int i = 0; i < 10; ++i) step(0.0, M_PI / 2);   // turn 90 degrees in 1 s
  for (int i = 0; i < 20; ++i) step(0.5, 0.0);        // drive 1 m
  std::printf("x=%.2f y=%.2f theta=%.2f\n", x + 0.0, y, theta);
  return 0;
}

Expected output

x=1.00 y=1.00 theta=1.57

Stuck? Show the full solution (try it yourself first)

#include <cmath>
#include <cstdio>

int main() {
  double x = 0.0, y = 0.0, theta = 0.0;
  const double dt = 0.1;
  auto step = [&](double v, double w) {
    x += v * std::cos(theta) * dt;
    y += v * std::sin(theta) * dt;
    theta += w * dt;
  };
  for (int i = 0; i < 20; ++i) step(0.5, 0.0);        // drive 1 m
  for (int i = 0; i < 10; ++i) step(0.0, M_PI / 2);   // turn 90 degrees in 1 s
  for (int i = 0; i < 20; ++i) step(0.5, 0.0);        // drive 1 m
  std::printf("x=%.2f y=%.2f theta=%.2f\n", x + 0.0, y, theta);
  return 0;
}

Expected result

How you know it worked

ROS_DISTRO is lyrical in every terminal used for the exercise.
The complete package from the exercise builds and the expected graph entity appears in a second terminal.

Practical exercise

Apply the lesson

Integrate odometry for a robot that drives 1 m, turns 90 degrees left, then drives 1 m. Compare your result with /odom from the simulated diff drive and explain any difference.

Before you continue

ROS_DISTRO reports lyrical in every terminal you used
The example builds and runs without errors
You checked the result with a ros2 command, RViz2 or a test, not just by reading the code

If something goes wrong

ros2 cannot find a package or executable.

Source /opt/ros/lyrical/setup.bash, build from the workspace root, then source install/setup.bash in that terminal.

Nodes run but cannot discover each other.

Confirm both terminals use Lyrical, the same ROS_DOMAIN_ID, and compatible network/discovery settings.