compass
The bow, GPS arrow and lighthouse bearing point three ways
North
Magnetic north still differs from geographic north.
East
Compass points divide the circle into usable steering commands.
South
A stable reading can still be affected by the ship’s motion.
West
Heading must be recorded together with speed and time.
Heading describes the bow, not a guaranteed direction of travel
Heading is the direction of the vessel's fore-and-aft axis, normally measured clockwise from true, magnetic or compass north. A magnetic compass, gyrocompass or electronic heading sensor can supply it. A recorded heading must retain its north reference: 090°T, 090°M and 090°C are not interchangeable labels for one number.
A bow pointing 090° does not guarantee motion due east. Crosswind can create leeway, current moves the surrounding water mass, and waves and steering make the bow yaw. Heading is an orientation of the vessel, not a GPS trail and not the direction toward a target.
- Heading: direction of the fore-and-aft axis
- Reference north: true, magnetic, compass or grid
- Sources: magnetic compass, gyrocompass or electronic heading sensor
Course is intended or steered; track is the path actually followed
Course can describe a planned leg or a direction to steer, such as the line between two waypoints or a course to steer chosen to offset wind and current. It expresses intent. Track is the path traced by positions over time. It may be an irregular line or be summarized by a direction made good across an interval.
A route drawn on the chart, a course ordered to the helmsman and the track reconstructed afterward can therefore all differ. Planned track is the objective, course to steer is an input, and track made good is the result. Calling all three simply course hides whether the difference arose in planning, steering or the environment.
COG is ground-referenced motion; GPS does not necessarily know the bow
Course over ground is calculated from changes in ground-referenced position and describes motion relative to the seabed or Earth. GNSS receivers and AIS report COG. AIS can also broadcast true heading from shipboard heading equipment. The two fields exist side by side because they are not the same measurement.
COG and heading can differ greatly while a vessel moves slowly, goes astern, crabs sideways or holds its bow against a strong current. At very low speed, position noise can make COG jump. Sound interpretation uses speed, time interval, heading source and conditions instead of inferring bow direction from one GPS arrow.
Bearing points from the observer to an object
Bearing answers where an object lies from an observation point. A lighthouse bearing of 045°T points from the vessel toward the light with true north as zero. A relative bearing of 045° lies forty-five degrees to starboard of the bow. With heading 090°T, that relative target is approximately 135°T. Change the reference and the same numeral changes meaning.
A bearing can fix position, monitor a target or help assess collision risk, but it does not automatically become a safe course. Steering directly along a lighthouse bearing may cross a shoal. Labels such as BRG, relative, true and magnetic preserve more information than an angle alone.
One diagram separates five directions and reveals wind, current and error
Suppose the planned track is 000°T and current sets east. A navigator might order a course to steer of 350°T, the average heading may yaw around it, and GPS may show COG 002°T. A lighthouse northeast of the vessel can simultaneously bear 045°T. Every value can be correct because each describes a different object.
A useful record names the object, north reference and time: HDG 350°T, CTS 350°T, COG 002°T and BRG lighthouse 045°T, with planned track drawn separately. The differences can then be assigned to current, leeway, compass error, steering or observation noise instead of being concealed beneath one word.
Questions
Continue exploring this subject
Why are heading and COG different?
Heading is bow direction; COG is motion over the ground. Wind, current, leeway, low-speed drift and going astern can separate them.
Are course and heading the same?
Not necessarily. Course may be planned or ordered, while heading is where the bow actually points at that moment. Steering and environmental effects create differences.
How do track and course over ground differ?
Track is the actual path. COG is normally the direction of that ground track at a moment or over a time window. A curved track has changing COG.
Is a bearing the direction the vessel should steer?
No. A bearing only locates an object from the observer. Depth, hazards, routing and conditions determine whether steering toward it is safe.
Why do nautical angles carry T, M or C?
They identify true, magnetic or compass north. Variation and compass deviation make the numeric readings differ for one physical direction.
Sources
Continue the research
- S-32 Hydrographic DictionaryInternational Hydrographic Organization
- Bearing definition, S-32International Hydrographic Organization
- Course made good definition, S-32International Hydrographic Organization
- How AIS WorksU.S. Coast Guard Navigation Center
- Class A AIS Position ReportsU.S. Coast Guard Navigation Center
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