Meaning of double centering

dou'ble cen'tering

Pronunciation: [key]
— Survey. Survey.
  1. a method of extending a survey line by taking the average of two foresights, one with the telescope direct and one with it inverted, made each time by transiting the telescope after a backsight.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.