What is a self centering video test? I need to carry a tape judge that is not self centering...

I need to carry a tape judge that is not self centering and I am not correct what self-centering means.
How can I located the buried PVC pipes...
Answers:    It have two scales, one full sized and one half-sized.

Using the full-sized scale, you nick the measurement, utter 23 1/4" Then you simply look up 23 1/4" on the half size go up and you have the center of your work
Tin Foil on Grill?
A centering benchmark tape tell you the 1/2 measure of the thickness of your board.....Lets say your board is 11" broad, the centering tape will show you that the center of the boards is 51/2" most tape are not self centering
Good luck