８．シングルビームデータ処理（Single Beam Processing）
Q. シングルビームにおけるロール、ピッチ補正について(Pitch and Roll Corrections in Sbmax)[英語]
When a single beam survey is done in calm water conditions, there isn’t much need to measure the pitch and roll of the boat. Because there is none. But as water conditions become rougher, sounding errors due to boat motion begin to increase and pitch and roll measurements can significantly reduce those errors.
In HYPACK®, pitch and roll compensation is done in the editing stage of post-processing – the Sbmax edit program. A number of people have asked: how exactly does Sbmax correct for roll and pitch. Hmm, sounds like a good topic for Sounding Better…
We consider the simple case of the GPS antenna mounted directly above the transducer. When there is no pitch or roll, the horizontal position of the antenna is exactly the same as the transducer and there is no sounding error.
When the boat rolls, the antenna is going to move one horizontal direction and the transducer is going to move the opposite direction – the pivot point of rotation being directly above the keel at the waterline. Without compensation, this will lead to a position error. Additionally, roll will cause the transducer to move slightly upward, changing it’s draft and causing a slight depth error.
The little stick diagram illustrates the roll rotation and it’s effect on the position of the transducer relative to the GPS antenna. Although not shown, pitch rotation will have a similar effect, with the pivot in this case at the boat center of mass.
Sbmax pitch and roll compensation uses standard mathematical formulas to correctly transform the transducer position relative to the antenna. The end result is that the transducer position is corrected to give the correct sounding position and the transducer draft change is compensated giving the correct depth. Note that in order for this to all work properly, the Hypack boat origin should be specified at the boat center of mass (approximately).
Pitch and roll corrections are enabled with the check box in Sbmax Advanced Read Parameters: Apply Pitch and Roll Corrections.
If you are interested in what happens when you check “Steer Sounding Beam”, refer to Sounding Better, February 2001: Beam Steering in the Single Beam Editor.