Great-circle navigation

Figure 1. The great circle path between (φ₁, λ₁) and (φ₂, λ₂).

The great circle path may be found using spherical trigonometry; this is the spherical version of the inverse geodesic problem. If a navigator begins at P₁ = (φ₁,λ₁) and plans to travel the great circle to a point at point P₂ = (φ₂,λ₂) (see Fig. 1, φ is the latitude, positive northward, and λ is the longitude, positive eastward), the initial and final courses α₁ and α₂ are given by formulas for solving a spherical triangle

${\displaystyle {\begin{aligned}\tan \alpha _{1}&={\frac {\cos \phi _{2}\sin \lambda _{12}}{\cos \phi _{1}\sin \phi _{2}-\sin \phi _{1}\cos \phi _{2}\cos \lambda _{12}}},\\\tan \alpha _{2}&={\frac {\cos \phi _{1}\sin \lambda _{12}}{-\cos \phi _{2}\sin \phi _{1}+\sin \phi _{2}\cos \phi _{1}\cos \lambda _{12}}},\\\end{aligned}}}$

where λ₁₂ = λ₂ − λ₁[note 1] and the quadrants of α₁,α₂ are determined by the signs of the numerator and denominator in the tangent formulas (e.g., using the atan2 function). The central angle between the two points, σ₁₂, is given by

${\displaystyle \tan \sigma _{12}={\frac {\sqrt {(\cos \phi _{1}\sin \phi _{2}-\sin \phi _{1}\cos \phi _{2}\cos \lambda _{12})^{2}+(\cos \phi _{2}\sin \lambda _{12})^{2}}}{\sin \phi _{1}\sin \phi _{2}+\cos \phi _{1}\cos \phi _{2}\cos \lambda _{12}}}.}$[note 2][note 3]

(The numerator of this formula contains the quantities that were used to determine tanα₁.) The distance along the great circle will then be s₁₂ = Rσ₁₂, where R is the assumed radius of the earth and σ₁₂ is expressed in radians. Using the mean earth radius, R = R₁ ≈ 6,371 km (3,959 mi) yields results for the distance s₁₂ which are within 1% of the geodesic distance for the WGS84 ellipsoid.

To find the way-points, that is the positions of selected points on the great circle between P₁ and P₂, we first extrapolate the great circle back to its node A, the point at which the great circle crosses the equator in the northward direction: let the longitude of this point be λ₀ — see Fig 1. The azimuth at this point, α₀, is given by

${\displaystyle \tan \alpha _{0}={\frac {\sin \alpha _{1}\cos \phi _{1}}{\sqrt {\cos ^{2}\alpha _{1}+\sin ^{2}\alpha _{1}\sin ^{2}\phi _{1}}}}.}$[note 4]

Let the angular distances along the great circle from A to P₁ and P₂ be σ₀₁ and σ₀₂ respectively. Then using Napier's rules we have

${\displaystyle \tan \sigma _{01}={\frac {\tan \phi _{1}}{\cos \alpha _{1}}}\qquad }$(If φ₁ = 0 and α₁ = ​¹⁄₂π, use σ₀₁ = 0).

This gives σ₀₁, whence σ₀₂ = σ₀₁ + σ₁₂.

The longitude at the node is found from

${\displaystyle {\begin{aligned}\tan \lambda _{01}&={\frac {\sin \alpha _{0}\sin \sigma _{01}}{\cos \sigma _{01}}},\\\lambda _{0}&=\lambda _{1}-\lambda _{01}.\end{aligned}}}$

Figure 2. The great circle path between a node (an equator crossing) and an arbitrary point (φ,λ).

Finally, calculate the position and azimuth at an arbitrary point, P (see Fig. 2), by the spherical version of the direct geodesic problem.[note 5] Napier's rules give

${\displaystyle {\color {white}.\,\qquad )}\tan \phi ={\frac {\cos \alpha _{0}\sin \sigma }{\sqrt {\cos ^{2}\sigma +\sin ^{2}\alpha _{0}\sin ^{2}\sigma }}},}$[note 6]

${\displaystyle {\begin{aligned}\tan(\lambda -\lambda _{0})&={\frac {\sin \alpha _{0}\sin \sigma }{\cos \sigma }},\\\tan \alpha &={\frac {\tan \alpha _{0}}{\cos \sigma }}.\end{aligned}}}$

The atan2 function should be used to determine σ₀₁, λ, and α. For example, to find the midpoint of the path, substitute σ = ​¹⁄₂(σ₀₁ + σ₀₂); alternatively to find the point a distance d from the starting point, take σ = σ₀₁ + d/R. Likewise, the vertex, the point on the great circle with greatest latitude, is found by substituting σ = +​¹⁄₂π. It may be convenient to parameterize the route in terms of the longitude using

${\displaystyle \tan \phi =\cot \alpha _{0}\sin(\lambda -\lambda _{0}).}$[note 7]

Latitudes at regular intervals of longitude can be found and the resulting positions transferred to the Mercator chart allowing the great circle to be approximated by a series of rhumb lines. The path determined in this way gives the great ellipse joining the end points, provided the coordinates ${\displaystyle (\phi ,\lambda )}$ are interpreted as geographic coordinates on the ellipsoid.

These formulas apply to a spherical model of the earth. They are also used in solving for the great circle on the auxiliary sphere which is a device for finding the shortest path, or geodesic, on an ellipsoid of revolution; see the article on geodesics on an ellipsoid.

Compute the great circle route from Valparaíso, φ₁ = −33°, λ₁ = −71.6°, to Shanghai, φ₂ = 31.4°, λ₂ = 121.8°.

The formulas for course and distance give λ₁₂ = −166.6°,[note 8] α₁ = −94.41°, α₂ = −78.42°, and σ₁₂ = 168.56°. Taking the earth radius to be R = 6371 km, the distance is s₁₂ = 18743 km.

To compute points along the route, first find α₀ = −56.74°, σ₁ = −96.76°, σ₂ = 71.8°, λ₀₁ = 98.07°, and λ₀ = −169.67°. Then to compute the midpoint of the route (for example), take σ = ​¹⁄₂(σ₁ + σ₂) = −12.48°, and solve for φ = −6.81°, λ = −159.18°, and α = −57.36°.

If the geodesic is computed accurately on the WGS84 ellipsoid,[4] the results are α₁ = −94.82°, α₂ = −78.29°, and s₁₂ = 18752 km. The midpoint of the geodesic is φ = −7.07°, λ = −159.31°, α = −57.45°.

A straight line drawn on a gnomonic chart would be a great circle track. When this is transferred to a Mercator chart, it becomes a curve. The positions are transferred at a convenient interval of longitude and this is plotted on the Mercator chart.

• Compass rose
• Great circle
• Great-circle distance
• Great ellipse
• Geodesics on an ellipsoid
• Geographical distance
• Isoazimuthal
• Loxodromic navigation
• Map
  • Portolan map
• Marine sandglass
• Rhumb line
• Spherical trigonometry
• Windrose network

• Great Circle – from MathWorld Great Circle description, figures, and equations. Mathworld, Wolfram Research, Inc. c1999
• Great Circle Mapper Interactive tool for plotting great circle routes.
• Great Circle Calculator deriving (initial) course and distance between two points.
• Great Circle Distance Graphical tool for drawing great circles over maps. Also shows distance and azimuth in a table.
• Google assistance program for orthodromic navigation