Homework 01

Your task will be to explore a bagfile containing data from a laser scanner (obtained from a real turtlebot). Once you have familiarized yourself with the contents of the bagfile, you should create a node that will collect and process the laser scanner data. After collecting the specified number of samples, the node will plot the collected data. You can find a more detailed description of the individual sub-tasks below.

Reading messages from the bagfile

  1. Download the bagfile from the labs web page. Link should be next to the link to this document or:
    $ wget https://cw.fel.cvut.cz/b202/_media/courses/aro/tutorials/2019-02-20-11-00-56.zip
    $ unzip 2019-02-20-11-00-56.zip
  2. Check the contents of the bagfile:
    rosbag info <bag_name>
    rqt_bag <bag_name>
  3. Find topic name and message type corresponding to data from a laser scanner. This is a device that takes range measurements using laser ranging technique. The laser is incrementally (but very quickly) rotated and a measurement is taken after each increment. Thus, a single scan contains multiple range measurements taken at different angles.Based on the data (ROS message) type, try to find documentation for the laser scanner data (e.g. http://wiki.ros.org/sensor_msgs).
  4. Create a node that will listen for the messages from the laser scanner and process them in the following way:
    1. Filter out possibly erroneous data (i.e. outside of the normal value range – see the message documentation)
    2. Discard measurements taken at an angle greater than 30° or lower than -30° (i.e. abs(angle) < 30°)
    3. Compute the mean of the remaining values and store it into a buffer (you can use Python list or numpy array)
    4. Store the message timestamp as well (you can store it as a single float using the to_sec() function)
    5. If the number of stored values had reached the number specified in the plot_length global parameter:
      1. Stop accumulating the data
      2. Plot the data
        1. on the X-axis: the timestamps minus the value in the global parameter start_time – that is, each value should represent the number of seconds from the beginning of the bagfile.
        2. on the Y-axis: the computed means
  5. Run all the nodes and then play the bagfile (rosbag play).


Plot the data using the plot(x, y) function of the pyplot module from the matplotlib package. You might also need to use the show() function to show the plot.

Setting up the parameters

  1. Next, you will need to set a couple of parameters.
  2. First, set a global parameter with the name plot_length specifying the number of values that shall be collected and plotted.
  3. Set a second global parameter called start_time that will hold the starting timestamp of the ROS bag recording.


  • You can find most of the necessary information within the presentations from Lab 01.
  • The rest could be found using the mentioned commands or by following the provided links.
  • If you have problem with plotting, search for matplotlib plot examples or documentation
  • When dealing with more complex nodes and persistent variables, it is useful to create a class that will handle all of the data processing. In the case of the listener node, the structure of your script should look like this:
    #!/usr/bin/env python2
    import rospy
    import numpy as np
    # TODO other imports
    class ScanCollector():
        def __init__(self):
            # Initialize the node here
            # retrieve the necessary parameters from the parameter server
            # and store them into variables
            # create the listener object and assign a class method as the callback
            # possibly do some additional stuff
        def scan_callback(self, msg):
            # process the message
            # if enough data has been collected, plot the data
    if __name__ == '__main__':
        sc = ScanCollector()


Here is an example of the resulting plot (extracted from the bag with plot_length set to 500):

Your plot might differ a little, depending on the parameters. You also don't need to tune value ranges of the plot axis and the labels (but the plot will look nicer if you do!)


Please, submit your solution to the upload system. Upload only the folder of your package (e.g., “example_package”, that contains a “src” folder, and the config files “CMakeList.txt”, “package.xml”). Please, do not upload the entire workspace.

  • The solution must work as described above.
  • The submitted package must be “buildable” by catkin build in a ROS Melodic distro
courses/aro/tutorials/homework01.txt · Last modified: 2021/02/18 08:28 by skovirad