Great-circle distance for pilots
A great-circle route follows the shortest path between two points on the surface of a sphere. Because Earth is curved, the shortest route between two distant airports can look curved when drawn on a flat map. That is a map-projection effect, not a detour.
Latitude and longitude inputs
Great-circle calculations use the latitude and longitude of the departure and destination. The coordinate differences are converted to angular separation around the Earth, then multiplied by an Earth-radius model to estimate distance.
Why long routes curve on charts
A flat map must distort some combination of area, shape, distance, or direction. On many common projections, a great-circle route bends toward higher latitudes even though it is the shortest surface path. The effect becomes more obvious on long flights.
Initial bearing
A great-circle calculation can also return an initial bearing from the first point toward the second. The bearing generally changes as you move along a great-circle route, so the initial value is not necessarily one constant heading to hold from departure to destination.
Great-circle versus rhumb line
A rhumb-line route crosses meridians at a constant angle, which makes it easier to represent as a constant course on some charts. A great circle minimizes distance on a sphere. On short general-aviation legs, the difference may be small; on long-distance routes it can become significant.
Distance is only one part of route planning
The mathematically shortest line may not be the usable flight route. Airways, restricted airspace, terrain, weather, winds aloft, navigation coverage, fuel stops, procedures, and dispatch requirements can all change the practical route.
Coordinate precision
The quality of the result depends on the coordinates. A sign error on west longitude or a DMS conversion mistake can move a point hundreds or thousands of miles. Always verify coordinate format and hemisphere.
Related PilotDesk tools
DMS / decimal converter · Route planner · Time / speed / distance
One degree north and east
On a spherical Earth, moving from 0° latitude, 0° longitude to 1° latitude, 1° longitude spans about 84.9 NM along the great-circle path. One degree of latitude is about 60 NM; the east-west distance for one degree of longitude is smaller away from the equator. The great-circle calculation accounts for both directions together.
This rounded example is for checking the scale of a result. Use current charted or approved navigation data for route planning; actual routes may need to avoid airspace, terrain, weather or other restrictions.