The Towfish Layback is the distance the towfish is being towed behind the vessel. It is important to remenber that the sonar image data is not being generated at the tow vessel but at the towfish. Also remember that the GPS fix being generated is where the GPS antenna is located on the tow vessel not where the towfish is. The lay back correction can be calculated fairly accurately by using the Pythagorean theorem. The information you need is as follows.
Measure the amount of cable that has been deployed (the hypotenuse).
Establishing the depth of the towfish (one leg of the right triangle).
This can be obtained by subtracting the height of the towfish off the bottom (a measurement made on the sonar record) from the total depth of the water (ships depth sounder).
