Great Circle vs Rhumb Line Calculator compares shortest-path great-circle routing with constant-bearing rhumb-line routing between two coordinates. It is useful for passage planning, voyage briefings, and assessing tradeoffs between distance efficiency and steering simplicity. The tool reports distance, initial bearing, and ETA for both methods at a selected speed. Use it to frame route strategy, then validate with weather routing and operational constraints.
Enter values in the left panel, keep units explicit, run the calculation, then copy or share the result. Invalid fields are highlighted immediately.
How to use this tool
Enter departure and destination latitude/longitude coordinates.
Set expected vessel speed and run the route comparison.
Compare great-circle and rhumb-line distance, bearing, and ETA to choose planning baseline.
Route Inputs
Result
Great-circle distance: 4467.935 nm
Great-circle initial bearing: 303.36°
GC ETA: 319.14 h
Rhumb-line distance: 4713.825 nm
Rhumb bearing: 268.47°
Rhumb ETA: 336.70 h
Worked example
Comparing route distance before a passage brief
Start: 37.7749, -122.4194
End: 34.0522, -118.2437
Vessel speed: 12 kn
Result: The calculator compares great-circle and rhumb-line distance, bearing, and ETA at the entered speed.
A shorter geometric path may still be operationally unsuitable if weather, traffic separation, exclusion zones, or routing guidance require a different track.
How to interpret the result
Route geometry is a planning baseline; the operational route still depends on navigation constraints, weather, traffic, and vessel capability.
Great-circle routing is the shortest path on a spherical Earth approximation.
Rhumb-line routing keeps a constant bearing, which can be simpler to steer or discuss in some planning contexts.
Differences are usually more important over longer distances and higher latitudes.
The output does not include weather routing, currents, hazards, traffic schemes, or regulatory constraints.
Common mistakes
Treating the shorter geometric distance as the best operational route.
Forgetting that initial great-circle bearing changes along the route.
Entering latitude/longitude signs incorrectly for north/south or east/west.
Using ETA output without checking speed assumptions, current, wind, and route constraints.
Review note and limitations
Method - spherical great-circle and rhumb-line planning formulas.
Uses a spherical Earth approximation and does not include ellipsoidal geodesic refinements.
Does not account for currents, wind, traffic separation schemes, exclusion zones, ice, depth, routing guidance, or port approaches.
Navigation planning support only. Verify routes with official charts, weather routing, notices to mariners, local rules, and bridge-team procedures.
FAQ
What is the difference between great-circle and rhumb-line routing?
A great circle is the shortest path on a sphere. A rhumb line follows a constant bearing, which can be easier to plot but may be longer.
Does this include weather or current routing?
No. It compares geometric route types only. Weather, current, traffic, hazards, and local rules must be checked separately.
Why can the bearing differ between methods?
A great-circle route changes bearing along the path, while a rhumb line keeps a constant bearing.
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