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To get approximate velocities out of a tf system you can do discrete differentiation.

You can compute the pose of the object in question at t0 and at t1, giving you p0 and p1. Use tf to get those poses in the coordinate frame of interest. Once they're in the same coordinate frame then the velocity is simply (p1 - p0)/(t1-t0) ala the change in pose divided by the change in time.

You can do the same with the transforms as well, they have slightly more information, but I find imagining the poses above to be easier.