June 3, 2026
Marine Navigation: How Sailors Use GPS Coordinates
Maritime GPS navigation has its own conventions — DDM, chart plotters, and NMEA sentences. Here's how sailors work with coordinates at sea.
At sea, GPS coordinates aren't just useful — they're a safety-critical system. Chart plotters, VHF radio distress calls, and electronic chart systems (ECDIS) all rely on accurate coordinate handling. Here's how maritime navigation uses coordinate formats in practice.
The Marine Standard: DDM
Like aviation, maritime navigation uses Degrees Decimal Minutes (DDM). Nautical charts mark positions in DDM, and chart plotters like Garmin GPSMap and Raymarine units default to DDM display. The reason is the same as aviation: one minute of latitude equals one nautical mile, making DDM natural for plotting courses and estimating time.
NMEA Sentences and Raw GPS Data
The NMEA 0183 protocol — the standard that connects GPS receivers to chart plotters, AIS transponders, and autopilots — encodes coordinates in a format called DDMM.MMMM (degrees and decimal minutes concatenated without a separator). For example, latitude 48°51.504′N appears as 4851.5040,N in an NMEA GGA sentence. Software parsing NMEA must split this correctly to extract DDM.
Distress Calls and GMDSS
Under GMDSS (Global Maritime Distress and Safety System), a DSC distress call transmits your position automatically in NMEA format. Rescue coordination centres receive and convert this to the format their local search and rescue team uses. Being able to read and verify your position in multiple formats is a core seamanship skill.
Waypoints and Route Planning
Waypoints in chart plotters are stored internally in DD or NMEA format and displayed in DDM. When importing routes from passage-planning software (Navionics, OpenCPN), coordinates may be in DD format — most plotters convert automatically on import.
Use our converter to translate any coordinate to DDM for manual entry into a chart plotter, or see our sailing coordinates guide for the full maritime picture.
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