Layback is the distance from a ship's GPS antenna, and the instrument towed behind.

It is important to precisely locate the collected data, and to insure that search lines are completely covered.  The shurvey ship must be on the line, traveling in a straight line, when the towfish gets to the starting waypoint, and must remain on the line until the towfish gets to the final waypoint.  The towfish will get to any targets after the survey vessel has already passed them.

The layback is primarily affected by the length of towing cable let out, but the speed and towfish characteristics also affect it.

For instruments that must fly a certain height above the bottom, or to avoid running the towfish into the bottom, the layback will be adjusted by changing the cable length. 

Last revision 4/7/2016