What is the white line w/ x on the nav map?

[Liszt_]

When on the navigation map at periscope and when observing ships and after launching torpedos, I sometimes notice this white streak which ends with an X on the nav map.

What exactly does this represent? I have no clue.

[akdavis]

That's the TDC's calculated target position. It is visible on the attack map, not the nav map (at least that I have seen).

[SteamWake]

Indicates anticipated impact point and AOB

Its also a little innaccurate read here http://www.subsim.com/radioroom/showthread.php?t=109272