The sea is constantly changing: shipwrecks create hazards, jetties are constructed, buoys relocated. Most mariners aboard ships have limited access to the Internet, so for navigation, they rely on paper charts. But how can they be sure their charts, which may have been published a decade or more ago, are still accurate?

That’s where NGA’s Notice to Mariners publication, or NtM, comes in. Working with the National Ocean Service, U.S. Coast Guard and international partners, NGA’s Maritime Safety Office compiles safety alerts and changes to navigational charts and books for all mariners into one weekly publication, approximately 100 pages.

NGA predecessor agency the U.S. Navy Hydrographic Office published its first NtM in 1869 and has issued NtMs weekly since 1886. While how the information is compiled, organized and disseminated has evolved, the NtM mission to provide mariners with accurate navigation information has remained the same.

In honor of its 150th anniversary, here’s an inside look at an NtM, how it’s created, and how mariners use it.

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Check here to view the full issue of Notice to Mariners No. 34. Want more Notice to Mariners? Visit msi.nga.mil .