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Coordinates for Sailors and Marine Navigation

Marine navigation uses DDM — the same format as aviation, because 1 minute of latitude = 1 nautical mile.

Marine navigation shares much with aviation in its use of DDM coordinates. Chart plotters, NOAA charts, and waypoint systems all revolve around Degrees Decimal Minutes. Understanding this format and how it connects to your GPS is essential seamanship.

DDM in Marine Navigation

All nautical charts use DDM for waypoint notation. NOAA charts show coordinates as N 47°36.374' W 122°19.926'. The minute-based system is ideal for marine navigation because 1 minute of latitude = 1 nautical mile. Measuring distance on a chart is as simple as counting latitude minutes.

Chart Plotter Coordinate Entry

Garmin, Raymarine, Furuno, and Navionics chart plotters default to DDM. To enter a waypoint: navigate to Waypoint → New Waypoint → Edit Position. Enter degrees, then decimal minutes, and the N/S and E/W direction.

Reading Chartbook Waypoints

Cruising guides and chartbooks list anchorage and hazard waypoints in DDM. Copy these directly to your chart plotter. If you find a waypoint in a different format (DD or DMS), use this converter before entering.

AIS and GMDSS Coordinates

AIS (Automatic Identification System) broadcasts vessel positions in DD internally, but most displays show DDM. GMDSS distress coordinates are transmitted in DD. EPIRB and SART transmissions report location in DD to MRCC.

Coordinate Converter

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