Furuno CI-35 Operator's Manual page 16

Tide Differential
Tide differential is a relative movement of tides at different depths, layer (A) and layer (B).
To calculate tide differential, the following two data are used:
1 Ship's speed and course based on layer (A)
2 Ship's speed and course based on layer (B)
Tide differential between two layers is, then, given as a difference of these two speed vectors.
Tide Difference = 1 - 2 (Movement of layer B based on layer A)
1-4
or
= 2 - 1 (Movement of layer A based on layer B)
q
w
Layer (A)
Layer (B)
:Base of measurement