LiDAR-based SLAM (gmapping, cartographer) in ROS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to launch the gmapping node in ROS.

ROS

roslaunch gmapping slam_gmapping[1]

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A.yaml

B.xml

C.launch

D.config

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong file extension like .xml or .yaml for launch files.

The gmapping node is launched using a .launch file in ROS.

2fill in blank

medium

Complete the code to subscribe to the LiDAR scan topic in a ROS node.

ROS

ros::Subscriber sub = nh.subscribe("[1]", 1000, scanCallback);

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A/scan

B/odom

C/cmd_vel

D/camera/depth/points

Attempts:

3 left

💡 Hint

Common Mistakes

Subscribing to the wrong topic that does not provide LiDAR data.

The LiDAR scan data is usually published on the /scan topic in ROS.

3fill in blank

hard

Fix the syntax error in the cartographer Lua configuration for IMU data.

ROS

TRAJECTORY_BUILDER_2D.use_imu_data = [1];

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Atrue

Bfalse

CTrue

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using capitalized booleans like True or False which cause syntax errors.

In Cartographer's Lua config, boolean values are lowercase true or false. Here, disabling IMU data requires false.

4fill in blank

hard

Fill both blanks to configure the ROS node to publish the map and set the correct frame.

ROS

node_handle.param<std::string>("[1]", map_frame, "[2]");

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Amap_frame

Bodom_frame

Cmap

Dodom

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing odom and map frames.

The parameter name is map_frame and the default frame is map for publishing the map in ROS.

5fill in blank

hard

Fill all three blanks to create a ROS launch argument for selecting the SLAM method.

ROS

<arg name="slam_method" default="[1]" doc="Select SLAM method: gmapping or cartographer" />
<include file="$(find [2])/launch/[3].launch" />

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Agmapping

Bcartographer

Cslam_gmapping

Dcartographer_ros

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing package names and launch file names incorrectly.

The default SLAM method is cartographer, the package is cartographer_ros, and the launch file is cartographer.launch.

Practice

1. What is the main purpose of LiDAR-based SLAM in ROS?

easy

A. To process camera images for object detection

B. To control robot motors directly

C. To help robots create maps and know their location using laser scans

D. To send commands to robot arms

Solution

1. Step 1: Understand SLAM basics

   SLAM stands for Simultaneous Localization and Mapping, which means building a map and knowing the robot's position at the same time.

2. Step 2: Identify LiDAR role

   LiDAR uses laser scans to sense the environment, which helps SLAM create accurate maps and localize the robot.

3. Final Answer:

   To help robots create maps and know their location using laser scans -> Option C

4. Quick Check:

   LiDAR-based SLAM = mapping + localization with lasers [OK]

Hint: SLAM = map + location using sensors like LiDAR [OK]

Common Mistakes:

• Confusing SLAM with motor control
• Thinking SLAM processes camera images only
• Assuming SLAM sends commands to robot arms

2. Which ROS launch command correctly starts the gmapping SLAM node?

easy

A. rosstart gmapping slam.launch

B. roslaunch gmapping slam_gmapping.launch

C. rosrun gmapping slam_gmapping.py

D. roslaunch cartographer slam.launch

Solution

1. Step 1: Recall ROS launch syntax

   ROS uses roslaunch to start nodes with launch files, specifying package and launch file name.

2. Step 2: Match gmapping launch file

   The gmapping package provides slam_gmapping.launch to start the SLAM node.

3. Final Answer:

   roslaunch gmapping slam_gmapping.launch -> Option B

4. Quick Check:

   Use roslaunch + package + launch file [OK]

Hint: Use roslaunch with package and launch file name [OK]

Common Mistakes:

• Using rosrun instead of roslaunch for launch files
• Typing rosstart which is invalid
• Confusing cartographer launch with gmapping

3. Given this ROS launch snippet for Cartographer:

<launch>
  <include file="$(find cartographer_ros)/launch/demo_backpack_2d.launch" />
</launch>

What will happen when you run this launch file?

medium

A. It launches a robot simulation without SLAM

B. It starts gmapping SLAM with a 3D map

C. It causes a syntax error due to missing parameters

D. It starts Cartographer SLAM with a 2D LiDAR backpack demo configuration

Solution

1. Step 1: Understand the include tag

   The <include> tag loads another launch file from the cartographer_ros package.

2. Step 2: Identify the included launch file

   demo_backpack_2d.launch is a Cartographer demo for 2D SLAM using a backpack LiDAR sensor.

3. Final Answer:

   It starts Cartographer SLAM with a 2D LiDAR backpack demo configuration -> Option D

4. Quick Check:

   Include demo_backpack_2d.launch = Cartographer 2D SLAM [OK]

Hint: Include demo_backpack_2d.launch means Cartographer 2D SLAM [OK]

Common Mistakes:

• Confusing Cartographer with gmapping
• Assuming missing parameters cause errors here
• Thinking it launches simulation without SLAM

4. You try to run gmapping with this command:

roslaunch gmapping slam_gmapping.launch

But get an error: "Could not find node of type slam_gmapping in package gmapping". What is the likely cause?

medium

A. The gmapping package is not installed or sourced properly

B. You used rosrun instead of roslaunch

C. The launch file name is misspelled

D. Your LiDAR sensor is not connected

Solution

1. Step 1: Analyze the error message

   The error says it cannot find the node type in the gmapping package, meaning the package or node is missing.

2. Step 2: Check common causes

   This usually happens if the gmapping package is not installed or the ROS environment is not sourced to find it.

3. Final Answer:

   The gmapping package is not installed or sourced properly -> Option A

4. Quick Check:

   Missing package or source = node not found error [OK]

Hint: Check package installed and sourced if node not found error [OK]

Common Mistakes:

• Assuming rosrun vs roslaunch causes this error
• Thinking a typo in launch file name causes node type error
• Blaming LiDAR connection for node not found

5. You want to create a 3D map using Cartographer with a LiDAR sensor on a robot. Which approach is best?

hard

A. Use Cartographer's 3D SLAM launch files and configure sensor topics properly

B. Use gmapping with default 2D settings and add a 3D camera

C. Run gmapping and Cartographer simultaneously for better accuracy

D. Use Cartographer's 2D demo launch files and convert output to 3D later

Solution

1. Step 1: Understand SLAM capabilities

   Gmapping supports only 2D SLAM, so it cannot create 3D maps directly.

2. Step 2: Choose Cartographer for 3D

   Cartographer supports 3D SLAM and has launch files designed for 3D mapping with LiDAR sensors.

3. Step 3: Configure sensor topics

   Properly setting sensor topics in Cartographer launch files ensures correct data flow for 3D mapping.

4. Final Answer:

   Use Cartographer's 3D SLAM launch files and configure sensor topics properly -> Option A

5. Quick Check:

   3D map needs Cartographer 3D launch + sensor config [OK]

Hint: 3D mapping needs Cartographer 3D launch and sensor setup [OK]

Common Mistakes:

• Trying to use gmapping for 3D maps
• Running both SLAM systems at once
• Using 2D demos and expecting 3D output