Shifting offshore sandbanks pose unique navigation hazard

Sandbanks can shift position during winter storms, but sonar mapping means charts can now be updated immediately

Offshore sandbanks are a particular navigation hazard because, unlike rocks and reefs, they have a habit of shifting position during winter storms.

The Goodwin Sands is a 10-mile (16km) bank off the coast of Deal in Kent, close to the busy shipping lanes of the Dover strait. The sands have claimed about 2,000 vessels over the years. In 1634, two lighthouses were set up on South Foreland for sailors to follow a safe route through.