The vertical direction below the transducer and the seabed spot it hits. In multibeam data the nadir beams return a strong, mirror-like echo and carry the least refraction and roll error, so they anchor quality checks such as crossline comparisons. In side-scan data nadir is the blind zone beneath the towfish, where the sonar records only water until the first seabed echo arrives.
Compare the crosslines at nadir only, the outer beams are smiling.
Also heard as
- nadir beam
Related terms
Nadir gap
The unimaged strip directly under a side-scan towfish, roughly twice as wide as the fish's height above the seabed.
Swath
The strip of seabed measured or imaged across the track on one line of a multibeam or side-scan survey.
Bottom detection
The method a multibeam uses to pick the instant the seabed echo arrives in each beam and turn it into a depth.
Crosslines
Survey lines run across the mainscheme lines so the two data sets can be compared as an independent check.