Flight routes

Best map projection for showing flight routes

To show the shortest path on a sphere, use the gnomonic projection: every displayed great-circle segment is straight on it. The map covers less than a hemisphere, though, so a route to the other side of the world is better shown on an azimuthal equidistant map centred on the departure city.

GnomonicRecommended for this job

Neither equal-area nor conformal

MercatorWhat to avoid

Conformal

These previews show projection shapes and do not share a scale. The lab opens a view configured for this job.

The shortest route between two places

Best choice: Gnomonic

Neither equal-area nor conformal, perspective azimuthal

Why

The shortest path on a sphere is a great circle. On a gnomonic map every great circle is a straight line, which is why navigators planned long routes on it. The map covers less than a hemisphere, though.

Area: distorted
Shape: distorted
Distances: distorted
Angles and directions: distorted
Continuity: partially

Show the route in the route lab Projection guide

Also good

Azimuthal EquidistantEquidistant
Centred on the departure city: the route is straight and the distance from the start is true to scale.
Open in the map lab Projection guide

What to avoid

Mercator

It draws the rhumb line, a route of constant bearing relative to geographic north, as straight. The shortest spherical path therefore often looks like a detour.

See the problem on the map Compare features: Gnomonic vs Mercator

Why planes fly in an arc

The shortest path between two points on a sphere follows the shorter arc of a great circle. On a Mercator map it is usually curved, but it is straight along the equator and meridians. For many routes between Europe and North America, the arc on this map is a shorter spherical path than the straight rhumb line between the same points.

The shortest route from London to Tokyo climbs to about 71° north, although both cities lie below 52°. It is also about 1,700 km shorter than the constant-bearing route. Real flight paths depart from the great circle because of winds, air corridors and regulations, but the great circle remains the starting point of planning.

Gnomonic or azimuthal equidistant

Each of these draws the shortest path as a straight line, but over a different range.

Gnomonic: every great circle is straight, so you can draw the route with a ruler. Scale grows quickly toward the edge, and the map covers less than a hemisphere.
Azimuthal equidistant centred on the departure city: every great circle leaving that city is straight, and the distance from it is true to scale. The map covers the whole world.
Mercator: the straight line is the rhumb line, a route of constant bearing relative to geographic north. It is usually longer than the great-circle route. Along the equator or a meridian, the two can coincide.

How navigators used it

Navigators drew the great circle on a gnomonic chart and then transferred it in segments to a Mercator chart. On each segment they sailed or flew a constant course, which combined a short route with simple steering.

Frequently asked questions

Because the shortest path on the sphere between these cities follows a great circle that rises toward the Arctic. On a Mercator map it looks like an arc, yet it is shorter than the straight rhumb line between those points. A real flight may follow a different route. The route lab shows the geometric comparison.

No. The gnomonic projection projects the globe from the Earth's centre onto a plane, so it covers less than a hemisphere and its scale grows without limit toward the edge. In the route lab the map centres on the middle of the chosen route.

In the spherical model, the shortest distance over the surface is the length of the great-circle route. The route lab computes it for a radius of 6,371 km and compares it with the rhumb-line length. This approximates the distance on Earth.

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