GIS Tools - Great Circle Route

What Is a Great Circle Route?

A great circle route is the shortest path between two points on the surface of a sphere. On Earth, the great circle through any two points is the intersection of Earth's surface with a plane passing through both points and the center of the Earth. Because of this geometric property, great-circle routes are the fundamental reference for efficient long-distance navigation — airlines, ocean shipping, intercontinental cables, and satellite ground tracks all rely on great circles as the baseline for route design.

The counterintuitive visual quirk of great circles is that they look curved on a Mercator map. A flight from Los Angeles to London appears to swoop north over Greenland rather than take a straight east line, and that swoop is not airlines wasting fuel — it is the flat Mercator projection making a geometrically straight path look bent. Unprojected or on a globe, the great circle is obviously the shortest route. The Great Circle Route tool generates that path as a dense GeoJSON LineString, ready to display on any map, compute distances along, or export for use in flight planning, radio propagation, or educational visualizations.

Mathematically the tool samples the great-circle arc at many points (typically 100–500) using spherical interpolation (slerp) or the ellipsoidal equivalent. The resulting LineString is indistinguishable from the true great circle at normal map zooms and handles antimeridian crossings cleanly by splitting the line at ±180° where appropriate.

How Great Circle Route Generation Works

Slerp (Spherical Linear Interpolation)

Given two points A and B on a unit sphere, slerp computes intermediate points along the shortest arc between them:

P(t) = (sin((1−t)·Ω) / sin(Ω)) · A + (sin(t·Ω) / sin(Ω)) · B

where Ω is the angle between A and B (the angular distance) and t goes from 0 to 1. This produces uniformly-spaced points along the great circle.

Densification

The tool picks a sample count based on route length — longer routes get more samples to avoid visible kinks on the map. A 10,000 km route might use 500 samples; a 100 km route needs only 20.

Antimeridian Splitting

If the great circle crosses ±180° longitude, the resulting LineString would have a horizontal segment spanning the entire map in naive renderers. The tool detects this and splits the LineString into a MultiLineString so renderers draw it correctly.

Ellipsoidal vs Spherical

Spherical great circles are accurate to about 0.5%. For high-precision aviation or geodetic work, the tool optionally uses GeographicLib-style ellipsoidal geodesics which compute the shortest path on WGS84 to sub-meter accuracy.

Key Parameters and Options

Start and End Points

Provide the two endpoints as latitude/longitude pairs. Accepts decimal degrees, DMS (via the Coordinate Format Converter), or any format supported by the site.

Sample Count

Pick the number of intermediate points along the arc. More samples produce a smoother curve but a larger file. 100 samples is a good default for most routes.

Method

Spherical (fast, ±0.5% accuracy) or ellipsoidal (slower, sub-meter accuracy).

Output Format

Single LineString or split MultiLineString for antimeridian-crossing routes. The tool defaults to split mode for safer rendering.

Practical Applications

Airline Route Planning and Visualization

Flight dispatchers and airline enthusiasts use great-circle routes to compute the shortest feasible path between airports, which becomes the baseline for actual flight plans after adjusting for jet stream winds, ATC restrictions, ETOPS diversion rules, and no-fly zones. Route mapping websites display great circles to demonstrate why a London-to-Tokyo flight goes over Siberia.

Shipping and Maritime Navigation

Great-circle sailing is the standard for transoceanic voyages. Captains compute the great circle at departure, break it into rhumb-line legs of a few hundred miles each (so they can steer a constant compass bearing), and re-compute at each waypoint. The route tool produces the initial great circle ready for that subdivision.

Radio Propagation and Amateur Radio

HF radio signals propagate along great circles (with ionospheric refraction). Amateur radio operators use great-circle maps to aim directional antennas and predict skip zones. The tool generates the exact great-circle path from station to station for antenna pointing.

Undersea Cables and Transcontinental Infrastructure

Undersea fiber optic cables are planned roughly along great-circle paths, adjusted for seabed topography, avoidance zones, and landing station locations. The initial engineering study begins with a great-circle reference line, which this tool can produce in seconds.

Education and Geographic Literacy

Students struggle to understand why a flight from New York to Hong Kong crosses the Arctic when they look at a flat world map. Generating and displaying a great circle on top of their familiar Mercator view is one of the best ways to teach projection distortion. The tool is a favorite in GIS and cartography classrooms.

Satellite Ground Track Planning

Low Earth orbit satellites sweep out paths that, over short time windows, approximate great circles. Ground station operators use great-circle routes to plan antenna tracking, pass prediction, and handoffs between stations.

Step-by-Step Workflow in gis.tools

Open the Great Circle Route tool in your browser.
Enter the start and end coordinates, or click on a map to set them.
Optionally adjust sample count, method, and output split mode.
Click Generate. A dense LineString appears on the preview map, following the great-circle arc.
The tool reports the geodesic distance for reference (computed equivalently to the Distance Calculator).
Download the result as GeoJSON, KML, or CSV of waypoints.
For initial and final bearings along the route, pair with the Bearing & Destination tool.

Worked Example

A cartography student is building an infographic about global air travel and wants to show the great-circle route from New York (40.64 N, 73.78 W) to Beijing (40.08 N, 116.60 E). She enters the endpoints into the tool, uses 250 samples, and downloads the resulting GeoJSON. She drops the file into the Online Map Viewer (Multi-Layer) and sees the route arcing over Canada, Alaska, the Bering Strait, and eastern Russia — a visually striking path that looks completely wrong on a Mercator projection but is geometrically the shortest. The accompanying distance is 10,980 km, about 1,200 km less than a naive "straight line on Mercator" would suggest. The infographic, using this route as its centerpiece, makes the distortion concrete for readers.

Common Pitfalls and Gotchas

Apparent curvature: A great circle on Mercator looks curved. This is correct; it's the projection that distorts.
Antimeridian crossing: Lines spanning the dateline can draw incorrectly as a straight line across the whole map in naive renderers. The tool's split mode fixes this.
Low sample count for long routes: Fewer than ~50 samples on a 10,000 km route produces visible kinks at medium zoom levels.
Confusion with rhumb line: A great circle is not a line of constant bearing; don't expect a single compass course to follow it.
Assuming planar shortest path: The shortest path on a Mercator map is not the shortest path on Earth.
Endpoint precision: Sub-centimeter start/end coordinates give only marginally different routes from meter-precision ones; don't waste effort on precision you don't need.
Polar ambiguity: Endpoints near the poles can produce great circles with unstable bearing calculations.
Wrong CRS for distance: Use the geodesic distance, not planar Mercator distance, when reporting route length.

Tips for Best Results

Use the default sample count (100-250) for most routes; it's a good balance of smoothness and file size.
Always use split mode for routes that might cross ±180°.
Display great circles on a non-Mercator projection (Robinson, Natural Earth) to see their "true" shape.
For teaching, compare the great-circle route against a rhumb line to show the difference.
Combine with the Distance Calculator to verify route length.
Export as GeoJSON for maximum compatibility; use KML for Google Earth visualization.

Comparison with Other GIS Approaches

Turf.js turf.greatCircle() is functionally identical. ArcGIS has a Bearing Distance To Line tool. QGIS offers great circles through the MMQGIS plugin. Command-line users can build them with proj or GeographicLib. Our browser tool offers instant startup and a built-in map preview, which is unique among the free options.

Performance Considerations

Generating a 500-point great circle takes under a millisecond. The map preview is the limiting factor, not the math. Batches of hundreds of routes process in seconds.

Data Privacy and Browser-Based Processing

Route endpoints (sometimes sensitive — think diplomatic flights, customer addresses, military operations) are never uploaded. Everything runs in your browser tab and is gone when you close it.

Related GIS Concepts

Rhumb line (loxodrome): A line of constant bearing; straight on Mercator but longer than a great circle.
Geodesic: The generalization of "shortest path" to ellipsoidal surfaces.
Slerp: Spherical linear interpolation, the math behind great-circle sampling.
Antimeridian: The 180° meridian where longitude wraps from +180 to -180.
Rhomboidal quadrilateral: A four-sided region bounded by rhumb lines; common in old nautical charts.

Frequently Asked Questions

Why does a great circle look curved on a map?

Because flat maps (especially Mercator) distort straight geometric paths. The great circle is actually straight on a globe; the curve you see is the projection stretching it.

Is a great circle always the shortest path?

On an idealized sphere, yes. On the real ellipsoidal Earth, the shortest path is called a geodesic and is very slightly different (within 0.5%).

How do I export the route?

Download as GeoJSON, KML, or CSV of sampled waypoints.

Can I compute the distance at the same time?

Yes — the tool reports the route length using the same formula as the Distance Calculator.

What if my route crosses the dateline?

Enable split mode so the output is a MultiLineString that renders correctly in all map libraries.

Start Point

End Point

Example Routes

About Great Circle Routes