Normally, the extent of a local government area ends at the low water mark. In a small number of cases, a local government area has been extended by legislation around a port or harbour into the surrounding sea beyond the low water mark. The largest of these seaward extensions is Yell Sound in the Shetland Islands, and there are also substantial extensions at Aberdeen and Greenock. We use the Ordnance Survey depiction of these seaward extensions.