How to read a navigational warning
Written by seahazards · updated 2026-09-24 · numbers recounted nightly
Navigational warnings are written for a watch officer with little time and a chart in front of them. Once you know the order the parts come in, any warning in this archive can be read in under a minute.
The heading: which series, which number
Every warning starts with its identity: a series and a number for the year, such as NAVAREA IV 586/26 or HYDROPAC 2436/23. The series tells you who broadcast it — the coordinator of one of the 21 NAVAREAs, or the U.S. National Geospatial-Intelligence Agency relaying another area's warning as a HYDROLANT, HYDROPAC or HYDROARC. The number runs from 1 each January.
The next line narrows the place: an ocean region, then a sea or a country, sometimes a chart number or a Digital Nautical Chart region. "WESTERN NORTH ATLANTIC. BRAZIL. DNC 01." is enough for a navigator to know whether to read on.
The body: what, where and until when
The body is one or more numbered paragraphs. The first says what the danger or activity is — a dangerous wreck, a rocket launch, a buoy off station, a vessel overdue. Positions follow in degrees and decimal minutes (49-40.8N 003-43.8W), either as a single point, a radius around a point, a list of corners introduced by "area bound by", or a line introduced by "track line joining".
Times are written as date-time groups in UTC: 201500Z APR 26 is 20 April 2026 at 15:00. Operations give daily windows ("0600Z TO 1000Z DAILY 21 THRU 25 SEP"); open-ended ones say "until further notice".
150,446 messages use it, 338 of them in force now; most often in South Atlantic Ocean, North Atlantic Ocean, North Pacific Ocean.
The instruction and the reporting line
Most warnings then tell the reader what to do. "Wide berth requested" and "two mile berth requested" ask ships to keep clear; "anchoring prohibited" and "navigation prohibited" are stronger. Search-and-rescue broadcasts instead ask every ship nearby to keep a sharp lookout, assist if possible and report to a named rescue centre.
These stock phrases are repeated thousands of times across the archive, which is what makes warnings quick to scan once you know them. The glossary explains each one with its own counts.
28,537 messages use it, 58 of them in force now; most often in South Atlantic Ocean, North Atlantic Ocean, Arabian Sea.
The ending: cancellations
The last paragraphs manage the warning's life. "CANCEL HYDROLANT 1085/12" withdraws an earlier message, usually one this new message replaces. "CANCEL THIS MSG 211600Z SEP 26" sets the warning's own expiry: after that time it is no longer in force, without any further broadcast.
Neither kind of cancellation says the danger has gone. A wreck warning may be cancelled because the wreck is now on the chart; a drifting container warning may simply stop being repeated. On seahazards the warning pages keep that distinction: a cancelled notice is described as cancelled, not as resolved.
95,350 messages use it, 72 of them in force now; most often in South Atlantic Ocean, North Atlantic Ocean, North Pacific Ocean.