How do I create and map pointcloud data?

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How do I generate point cloud data without actual cameras?

Well, I suppose you can use a randomized function to generate it but that wouldn't make sense if you are using that data for mapping. So I suggest you to use sample data available in the tutorials. In a real life scenario you will not have to have a node to produce point cloud data as that will be produced by the ROS driver for the camera / laser.

Then, you don't have to write a node to display it either. Rviz is a visualization tool. And it can visualize your results for you. At the end of the day, your mapping node will publish to a topic /map (similar to gmapping).

So all you have to do is get point cloud data (and odometer data would make mapping easier and more accurate) and publish it (rosbag play bagName.bag).

You said that you failed to execute gmapping with the sample data. I suggest you to look into it and see what's wrong before writing your own mapping (SLAM) algorithm.

Please edit your question / ask a new question with the issues you come across there.