A robot that says “I am at (3.2, 1.7)” has told you little. A robot that says “I am at (3.2, 1.7), give or take 4 cm sideways and 12 cm along the corridor” has told you how much to trust it.
You will compute that “give or take” as the robot drives, shrink it with a landmark measurement, and learn to test whether the filter’s claims are honest.