The purpose of the Geographic Notice is to transmit information that pertains to a region or area, for example a security zone, an area of fog, or dredging operations. The areas that are being defined can be circles, rectangles, polygons, or sectors. They can also be defined as a simple point or series of points (polyline). The Geographic Notice message can be made up of multiple subareas in which case the total area is the union of the subareas. This message can also be used to convey advisory lines or tracks (using the polyline subarea); however, the Route Information message should be used for recommended or directed routes.
A Geographic Notice should be used to broadcast dynamic information (i.e., information that is time-dependent). These messages are to be used for a specific time period, and will automatically timeout at the end of the period. If the Geographic Notice must be in place longer, then a new Geographic Notice must be transmitted with a new start and end time. It should be only used to convey pertinent time-critical navigation-safety information to mariners or authorities, and not as a means to convey information already provided by official nautical charts or publications.

1. The information is time-dependent (i.e., has start date/time and duration). If a Geographic Notice (except for a cancellation message – Area Type 126) is received without a valid start date/time and duration then it should be discarded.
2. When the current month is December and the notice start month is January, the notice start year shall be the current year plus one; the notice start year shall be the current year in all other cases.
3. The message may be transmitted prior to the start time/date to allow for advance notice. To avoid confusion, it should not be transmitted more than one day in advance.
4. The message should not be transmitted beyond the designated end date/time except for a cancellation message. A cancellation message can be transmitted before the designated end date/time using the same Message Linkage ID with an Area Type of 126 (cancellation), a Duration = 0, and start time fields all set to “not available.”
5. Presentation software should automatically remove the Geographic Notice from the display after the end date/time or upon receipt of a cancellation message.
6. Up to 5-slot messages can be created, but messages with more than three slots should be avoided. Messages with more slots are less likely to be received due to RF noise or packet collision.
7. A circular sub-area (Type 0) with a zero radius (scale factor should also be set to 0) is a point that can be used as a node in a polygon. This is used when more precision is needed than is possible using the points in the polygon subarea (the trade-off is more subareas and a longer message).
8. Polygon sub-areas (Type 4) must follow immediately after a circle/point sub-area (Type 0 sub-area with 0 radius) in the same Geographic Notice message. The point defines the start of the line segments. If more than five points are needed for a polygon, then additional polygon sub-areas can be used. However, they must follow immediately after the first polygon sub-area and be contained in the same Geographic Notice message.
9. A rectangular sub-area (Type 1) with a zero for both North and East dimensions (scale factor and orientation should also be set to 0) is a point that can be used as a waypoint or to define a line. This is used when more precision is needed than is possible using the points in the polyline subarea (the trade-off is more subareas and a longer message).
10. Waypoints can be specified using the polyline/waypoint sub-area (Type 3). If more precision is needed then multiple rectangle/point sub-areas (Type 1 sub-area with 0 North and East dimensions) can be used (e.g., one for each waypoint).
11. When waypoints are specified using polyline or rectangle/point sub-areas, they should be numbered/used in the order that they appear in the message.
12. Polyline sub-areas (Type 3) must follow immediately after a rectangle/point sub-area (Type 1 sub-area with 0 for North and East dimensions) in the same Geographic Notice message. The point defines the start of the line segments. If more than five points are needed for a polyline, then additional polyline sub-areas can be used. However, they must follow immediately after the first polyline sub-area and be contained in the same Geographic Notice message.
13. The polygon sub-area (Area Shape 4) should be used to create polygon areas. However, if more precision is needed to specify the points in the polygon then the circle/point sub-area (Type 0 with radius set to zero) can be used, one sub-area per point. All points (sub-areas of Type 0) must occur in sequence and be contained within the same message. The polygon is formed by connecting the points and closing the shape from the last point back to the first.
14. Distances and bearings between points in the Geographic Notice should be calculated using Rhumb lines not Great Circles.
15. The Message Linkage ID and the source MMSI can be used to link additional text (e.g., a separate Linked Text message). This information must be included in both the Geographic Notice and additional Linked Text message.
16. The total area defined by one Geographic Notice (one Message Linkage ID) is the union of all of the sub-areas contained in the message.
17. If the same Message Linkage ID is retransmitted with different sub-areas and/or times the presentation software should replace the old Area with the new.
18. The Message Linkage ID must be unique across all ASMs to which it applies. In this way, the Message Linkage ID and Source MMSI are connected to the same text message.
19. A message version number is encoded as part of the message; If the received version number is different than what the display system has been programmed for, a message should be displayed to the operator indicating the mismatch in the version.
20. All directions are relative to True North, all positions are WGS-84 Datum, and all distance calculations should be in accordance with IEC 61993-2 Annex G.