The across-track width covered on each line. For multibeam it depends on the angular sector and the water depth: a 120 degree sector covers about 3.5 times the depth, so swaths widen in deep water and shrink in the shallows. Usable swath is often narrower than the brochure figure, because outer beams suffer from refraction, low grazing angles and noise and get trimmed in processing.
We're only getting a 30 metre swath in here, the line plan's going to need double the lines.
Plan line spacing from the swath at the shoalest depth on the line, not the average depth, or the shallow end fills with holidays.
Also heard as
- swath width
Related terms
MBES
Multibeam echo sounder: a sonar that measures a fan of depths across the track on every ping, covering a wide swath.
Overlap
The share of each swath that is also covered by the neighbouring line, planned in to guard against gaps.
Line plan
The set of pre-planned survey lines, with spacing and orientation, that a vessel follows to achieve the required coverage.
Nadir
The point on the seabed directly beneath the sonar, where the beam looks straight down.
Mowing the lawn
Running long parallel survey lines back and forth across an area, like cutting grass in strips.