Notice to mariners

A notice to mariners (NtM or NOTMAR,) advises mariners of important matters affecting navigational safety, including new hydrographic information, changes in channels and aids to navigation, and other important data.

Over 60 countries which produce nautical charts also produce a notice to mariners. About one third of these are weekly, another third are bi-monthly or monthly, and the rest irregularly issued according to need. For example, the United Kingdom Hydrographic Office issues weekly updates.