How do I visualize the contents of a bag

Webviz is an advanced online visualization tool

I have spent many hours tracking down a tricky tf tool and in doing that came across some new techniques for troubleshooting. In this FAQ I introduce using rosbag with Webviz

Rosbag

This CLI from ROS monitors all topic publications and records their data in a timestamped file called a bag or rosbag. This bag can be used for many purposes. For example the recording can be "played back" which allows you to test the effects that a certain run of the robot would have had. You can select which topics to collect.

One other use is to analyze the rosbag moment to moment.

Webiz

Is an online tool (https://webviz.io). You need to understand topics and messages to use it and the UI is a little bit obscure. But it's easy enough. You supply your bag file (drag and drop) and then arrange a series of panes to visualize all of them in very fancy ways.