# How is the grid defined? Where is the opening in the wall? **URL:** \ **Category:** ROS2 Basics in 5 Days Humble (Python)\ **Created:** [January 14, 2024, 3:38pm UTC](https://get-help.theconstruct.ai/t/how-is-the-grid-defined-where-is-the-opening-in-the-wall/28391 "2024-01-14T15:38:37Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![derrell](https://get-help.theconstruct.ai/letter_avatar_proxy/v4/letter/d/3be4f8/32.png) [@derrell](https://get-help.theconstruct.ai/u/derrell)\ **Post date:** [January 14, 2024, 3:38pm UTC](https://get-help.theconstruct.ai/t/how-is-the-grid-defined-where-is-the-opening-in-the-wall/28391/1 "2024-01-14T15:38:38Z")
I believe I’m missing a basic piece of information. I am taking the ROS2 Basics in 5 Days (Python) course. In the Topics Quiz, section 3.7, I’m asked to “move forward until the robot is in parallel with the opening in the wall”. We have not yet learned about the definition of the World in the simulator, the simulator grid, or odometry. Thus, I have a few specific questions: - How is the grid defined? What is the size of each square? - How do I query the World to discover the location in the opening in the wall? - Do I need to do any transformations to correlate the grid coordinates with the World coordinates of the opening in the wall? Thank you ---
**Author:** ![ernest.cheong.ec](https://get-help.theconstruct.ai/letter_avatar_proxy/v4/letter/e/e9bcb4/32.png) [@ernest.cheong.ec](https://get-help.theconstruct.ai/u/ernest.cheong.ec)\ **Post date:** [January 15, 2024, 8:41am UTC](https://get-help.theconstruct.ai/t/how-is-the-grid-defined-where-is-the-opening-in-the-wall/28391/2 "2024-01-15T08:41:12Z")
Hi @derrell as a fellow learner who did this awhile ago I simply did it using the lidar data. It worked for me and I did not require odometry or knowledge of the World. Let me know if you need further help and I would be happy to assist. Regards, Ernest ---
**Author:** ![derrell](https://get-help.theconstruct.ai/letter_avatar_proxy/v4/letter/d/3be4f8/32.png) [@derrell](https://get-help.theconstruct.ai/u/derrell)\ **Post date:** [January 15, 2024, 5:42pm UTC](https://get-help.theconstruct.ai/t/how-is-the-grid-defined-where-is-the-opening-in-the-wall/28391/3 "2024-01-15T17:42:07Z")
Thank you, @ernest.cheong.ec. I may do it that way, but it’s not what this lesson is intending to teach, I think. I’d prefer to learn how to do this as directed, using odometry, first, and then later learn about using lidar data. Derrell ---
**Author:** ![albertoezquerro](https://get-help.theconstruct.ai/user_avatar/get-help.theconstruct.ai/albertoezquerro/32/7031_2.png) [@albertoezquerro](https://get-help.theconstruct.ai/u/albertoezquerro)\ **Post date:** [January 19, 2024, 10:10am UTC](https://get-help.theconstruct.ai/t/how-is-the-grid-defined-where-is-the-opening-in-the-wall/28391/4 "2024-01-19T10:10:38Z")
Hello @derrell , Each grid square is 1x1 meter. Also, if you check the robot’s odometry you will find out that the starting position is 0,0. With this data, you should be able to solve it. Best, ---
**Author:** ![derrell](https://get-help.theconstruct.ai/letter_avatar_proxy/v4/letter/d/3be4f8/32.png) [@derrell](https://get-help.theconstruct.ai/u/derrell)\ **Post date:** [January 19, 2024, 2:12pm UTC](https://get-help.theconstruct.ai/t/how-is-the-grid-defined-where-is-the-opening-in-the-wall/28391/5 "2024-01-19T14:12:11Z")
@albertoezquerro Thank you very much! Derrell ---
**Author:** ![system](https://get-help.theconstruct.ai/uploads/default/original/2X/9/9d3ce8305cc0d0d9b6e07d15767a6f245c526f1b.jpeg) [@system](https://get-help.theconstruct.ai/u/system)\ **Post date:** [January 24, 2024, 2:13pm UTC](https://get-help.theconstruct.ai/t/how-is-the-grid-defined-where-is-the-opening-in-the-wall/28391/6 "2024-01-24T14:13:12Z")
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