![[ROS Q&A] 076 – How to get the closest obstacle with rotating laser](https://www.theconstruct.ai/wp-content/uploads/2017/12/How-to-get-the-closest-obstacle-with-rotating-laser-screen.png)
Q: How to get the distance to obstacles given that the laser is rotating?
Original question: https://answers.ros.org/question/229865/how-to-incorporate-moveable-sensor-lidar-into-navigation-stack
A: By using TF to transform the obstacle detected to the base_link of the robot.
![[ROS Q&A] 075 – 3rd time getting stuck at “Building a ROS Package”](https://www.theconstruct.ai/wp-content/uploads/2017/12/3rd-time-getting-stuck-at-Building-a-ROS-Package-screen.png)
In this question we see how to solve the problem when running catkin_make on a Catkin Workspace: The specified base path /home/user/catkin_ws contains a CMakeLists.txt but catkin_make must be invoked in the root of workspace
Q: https://answers.ros.org/question/277210/3rd-time-getting-stuck-at-building-a-ros-package/
A: The problem is because the user had a CMakeLists.txt on the root folder of the Workspace (~/catkin_ws/CMakeLists.txt). The file must be on the src folder: ~/catkin_ws/src/CMakeLists.txt
![[ROS Q&A] 074 – Aerodynamic parameters of lift-drag plugin part2](https://www.theconstruct.ai/wp-content/uploads/2017/12/Aerodynamic-parameters-of-lift-drag-plugin-part2-screen.png)
In this video we complete the answer given previously ( https://www.youtube.com/watch?v=I-dhwqxV7QM ) with a more complex shape. Here is the question: http://answers.gazebosim.org/question/15331/need-information-on-aerodynamic-parameters-of-lift-drag-plugin/#17793
Q: Need information on aerodynamic parameters of lift-drag plugin
A: You have to calculate the area for the new shapes and also decide the axis for the front and upwards forces.
![[ROS Q&A] 073 – How to add a simple wall to a Gazebo world](https://www.theconstruct.ai/wp-content/uploads/2017/12/How-to-add-a-simple-wall-to-a-Gazebo-world-screen.png)
Q: I created a custom gazebo world for my turtleBot containing several walls and saved it as a .world file. However when I run the world file the walls that i created do not load.
A: Here you can see a simple example of how to launch a simple Gazebo world with a wall in it.
Related ROS Answers Question: https://answers.ros.org/question/277170/custom-gazebo-world-doesnt-load/
![[ROS Q&A] 072 – How to check your complex URDF](https://www.theconstruct.ai/wp-content/uploads/2017/12/How-to-check-your-complex-URDF-screen.png)
When developing for ROS, you will have to create the URDF of your robot. If your robot is complex and contains lots of different joints, you would like to test how is it going and if you are building the model properly. Here we provide you with a method to check the status of your robot URDF model.
Related ROS Answers Question: https://answers.ros.org/question/187815/how-do-i-display-my-own-urdf-in-rviz/
Related courses:
1- Understanding TF: https://www.theconstruct.ai/construct-learn-develop-robots-using-ros/robotigniteacademy_learnros/ros-courses-library/tf-ros-101/
2- URDF creation in detail: https://www.theconstruct.ai/construct-learn-develop-robots-using-ros/robotigniteacademy_learnros/ros-courses-library/robot-creation-with-urdf/
TF ROS Course
![[ROS Q&A] 071 – How to use ApproximateTimeSynchronizer in Python](https://www.theconstruct.ai/wp-content/uploads/2017/12/How-to-use-ApproximateTimeSynchronizer-in-Python-screen.png)
Q: How to use ApproximateTimeSynchronizer in Python?
Original question: https://answers.ros.org/question/81126/how-to-use-approximatetime-in-python/
Q: How to check that two topics are synced?
Original question: https://answers.ros.org/question/274858/how-to-check-that-two-topics-are-synced/
A: Here you can see a very simple script that uses ApproximateTimeSynchronizer in Python.