How marine navigation charts are constantly corrected with the latest information.

Why correct marine navigation charts at all?

There is an old saying at sea, “A mariner sailing on an uncorrected chart is courting disaster”.

Marine navigation charts are in constant use by navigators, as we learnt in a previous article. But conditions at sea and under the sea keep changing and so we must have the latest information on them for safety. Is there a new wreck, and, if so, where exactly is it so we can avoid it? Has a buoy been destroyed in a storm, and, if so, do we know this so we can avoid colliding into it at night? Are their shallower depths reported in a particular location, and how do we know this? Since information is changing constantly, there has to be a foolproof, controlled and approved system of chart correction. Actually, although we will talk about only charts in this series, this system extends beyond charts and includes all hydrographic survey publications as well.

Legal implications

The chart in use must always be corrected to the latest notices to mariners on board. Failure to do so will be taken as negligence in the event of an accident. Sometimes Captains, managers and owners may become liable for damages too.

Who lets the mariner know when a chart needs correction, and how?

Hydrographic organisations that produce charts are also responsible for their correction. Probably the best known one is the Hydrographic Department in the UK, which publishes British Admiralty (BA) Charts. We will take this as an example in these articles.

The systemic way in which charts are corrected are as follows:

1. The Admiralty receives information about changes to chart information from hydrographic departments across the world

2. It collects this information, referencing it to which charts need to be corrected (there are thousands of charts that cover the globe; each has a unique identification number)

3. It sends weekly ‘Notices to Mariners’ to owners of vessels that carry BA charts; the owners then pass these on to the vessels at their next port. There are 52 such notices annually. These notices are also downloadable or can be sent electronically otherwise, and are a compilation of all corrections for the week; a typical booklet of these notices will be about fifteen sheets thick.

4. These notices contain full details of which charts (and publications) are affected by the corrections carried in the notice. Further, these notices detail what corrections need to be made on board. The Notices are usually accompanied by tracings for each chart affected; these tracings are overlaid temporarily on each chart and the correction ‘traced’ onto the chart for easy, accuracy and speedy chart correction.

5. These notices are also sent to suppliers of BA charts worldwide, who may have BA charts in stock but may not have sold them yet.

6. Second Officers on these ships (and BA agent employees ashore in the second case above) are responsible for chart correction. They receive the notices and cross reference them with which charts they have on board (or in the ‘ship’). They then correct them. Note that this means that whenever a chart is purchased, it is always correct to the nearest notice for large corrections only (more later)

7. At sea, the charts are corrected and a log is maintained indicating which chart has been corrected to which notice. This becomes a ready reference whenever required.

8. This is a continuous process, and a critical one. The safety of the crew, cargo and the ship depends on a systematic and accurate means of chart correction- and indeed, correction of all publications, information on which is also available in the same Notices to Mariners.

Temporary and Preliminary Corrections, Navigational warnings.

In addition to the ‘large’ corrections mentioned above, there are some ‘small’ corrections which may be required to be applied. Generally speaking, they fall into the following categories:

Temporary corrections: eg, a lighthouse is temporarily unlit.

Preliminary corrections: eg A jetty is being extended in a harbour. Although the entire length is not yet extended, works are in progress. Instead of a large correction, a preliminary notice may be sent, to be made permanent (or large) after the construction is complete.

Navigational warnings: eg, a Navy will be using a certain area for firing practice on a particular date and time. Obviously ships must be aware of this and avoid the area.

Generally speaking, all thecorrections in this section are made available to the ship in one or more of the following ways

-In the notices to mariners

-By VHF radio, warnings transmitted by shore based radio stations as voice on the internationally approved frequencies which ships must monitor 24 hrs a day.

-By Navtex receivers, or navigational telex

– Through satellite communications and others.

The Second Officer, or the watchkeeping officer at a time when these messages are received, will then correct the appropriate chart (if in use) in pencil. This is because, unlike large corrections that are made in ink, these small corrections are temporary in nature and only applicable for a brief period of time.


The importance of chart correction cannot be overstated. The next article will detail the use of the Notices to Mariners in greater detail.

For those interested, the ‘Mariner’s Handbook’ published by the British Admiralty contains more interesting information about chart correction, which is a systematic and intricate affair.

This post is part of the series: An overview of marine navigational charts and their correction at sea.

Charts at sea: how they are made and how they are kept current.
  1. A Brief Guide to Marine Navigational Charts Correction.
  2. How are Notices to Mariners Used in Correcting Navigational Charts?