Lines run at a large angle, ideally perpendicular, to the mainscheme so the same seabed is measured with different geometry and usually at a different stage of tide. Differences at the crossings expose tide, draft, sound speed and motion errors that a single line direction hides. Crossline mileage is commonly a few percent of mainscheme for multibeam and more for singlebeam, and the comparison statistics go into the survey report.
Crosslines agree with mainscheme to 8 centimetres, well inside Order 1a, so the tides look clean.
Compare crosslines using nadir and near-nadir beams; outer beams carry their own refraction and roll error and blur the result.
Also heard as
- check lines
- cross lines
- tie lines
Related terms
Mainscheme lines
The primary set of parallel survey lines that delivers the required depth coverage over a survey area.
Bust
A vertical or horizontal mismatch between overlapping lines or surveys that exposes an error somewhere.
TPU
Total propagated uncertainty: the combined estimate of how far each sounding could be from the truth, at 95 percent confidence.
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.