(dā’ t∂m ∂ng) 1. The arbitrary flattening and straightening of a particular reflection for use as a reference on a cross-section, i.e., using a paleo-datum; flattening (see Figure F-12) thereby assuming that everything was horizontal at the time of the paleo-datum. The result emphasizes differences between this and other reflections. 2. Changing the reference plane of a wavefield.