Aids to Navigation - Special Purpose Marks

ATON’s - IALA Special Purpose Buoys & Marks

Special Purpose B uoy A buoy or mark used to indicate something notable in the area it is located in. It has no lateral significance or directional meaning.

The yellow color of the buoy is a reminder that caution should be exercised until you have determined the reason that any “Special Purpose Buoy or Mark” has been placed in a particular location. If you cannot determine the reason the mark is placed in its location, it should be given a wide berth where possible or the area should be transited with caution.

As defined by the International Association of Marine Aids and Lighthouse Authorities (IALA), the Special Purpose Mark is authorized for usage in both IALA Region A and IALA Region B.


Below is a list of where you may encounter the Special Purpose Buoy:

  • General or Special Anchorage Areas
  • Military Exercise Areas (firing ranges)
  • Locations of Underwater Structures
  • Race Courses
  • Seaplane Bases
  • Areas without a safe channel for passage
  • Spoil Areas
  • Quarantine Anchorages
  • Traffic Separation Schemes
  • Swimming Areas
  • Surface and Sub-surface Pipelines
  • Dredging Operations
  • Survey Operations
  • Marine Construction Areas
  • Weather Reporting



The color of the Special Purpose Buoy consist of the solid color Yellow, making it easy to recognize during daylight hours.



The Special Purpose Mark can be found in any one of the (5) basic buoy configurations: A Can Buoy, a Nun Buoy, a Pillar Buoy (Combination Buoy), a Spherical Buoy, or a Spar Buoy.

Special Purpose Mark Shapes

However, the most common shapes found will be the Can, Nun, and Pillar (Combination Buoy) shown below.

Special Can Buoy


Markings - If marked, Special Purpose Buoys will only be lettered. Numbers are not authorized on other than ODAS weather buoys.


If a topmark is fitted, the Special Purpose Mark will only display the shape of a “St. Andrews Cross.” It is easily identifiable as it closely resembles an "X."

St. Andrews Cross

The St. Andrews Cross topmark will always be Yellow.

Light Characteristics

If the Special Purpose Mark is lighted, it will display the following characteristics:


It will only show a "Yellow" light.

ATON Yellow Light


Any light phase can be used with the exception of those prescribed for cardinal marks (combinations of Quick Flashing or Very Quick Flashing), Isolated Danger Marks (Flashing (2), or Safe Water Marks (Isophase, Occulting, Long Flash 10 seconds, or Morse “A”).


The period for the light is optional; meaning any period may be used.

Chart Display

Below is a chart excerpt showing (2) Special Purpose Buoys at the beginning of the Traffic Separation Scheme at the mouth of the Delaware Bay in the U.S.

Special Purpose Chart

The southeastern most buoy - Y “D” Fl Y 6s RACON (▬ ● ▬) surrounded by a Magenta Circle. and marks the actual beginning of the Traffic Separation Scheme.

The chart notation indicates that this is a yellow “Y” buoy marked with the letter “D” displaying a flashing yellow light every 6 seconds “Fl Y 6s.” It is equipped with a “RACON” displaying the Morse code equivalent of the letter “K” (▬ ● ▬) dash-dot-dash and is also equipped with an AIS transponder (indicated by the magenta circle around the mark.

The second mark, located just to the northwest, Y ODAS “44009” Fl (4) Y 20s is a Weather Buoy.

(Y ODAS “44009” Fl (4) Y 20s)

The chart notation indicates that this is a yellow “Y” Ocean Data Acquisition Systems “ODAS” buoy marked with the numbers “44009” displaying a group flashing (4) yellow light every 20 seconds “Fl (4) Y 20s.”


There are a number of publications available to help in identifying Aids to Navigation including: The USCG Light List, NGA List of Lights, and Chart #1. You can download these for free on our website or you can obtain the printed versions through many links on our website. Regardless, they should be carried on your vessel as part of your navigation library.

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