A charted man-made or unusual hazard on the seabed: old piles, concrete blocks, lost containers, fish havens and debris. Obstructions are charted with a least depth when known or as depth unknown when not, and areas strewn with many small items may be charted as foul ground. Surveys find them with side-scan and multibeam and develop each one for height and extent.
Side-scan shows an obstruction 2 metres tall off the old ferry slip, looks like a sunken pontoon.
Also heard as
- obstn
Related terms
Wreck
The remains of a sunken vessel, charted as a hazard or a feature depending on its depth and size.
Least depth
The shallowest depth over a wreck, rock or other feature, which is the value that gets charted.
Contact
A target picked out of side-scan or multibeam data that may be an object worth investigating.
Development
Extra survey work over a feature or shoal to pin down its least depth, shape and extent.