Great-circle navigation

Great-circle navigation or orthodromic navigation (related to orthodromic course; ) is the practice of navigating a vessel (a ship or aircraft) along a great circle. Such routes yield the shortest distance between two points on the globe.

Course

The great circle path may be found using spherical trigonometry; this is the spherical version of the ''inverse geodetic problem''.
If a navigator begins at ''P''₁ = (φ₁,λ₁) and plans to travel the great circle to a point at point ''P''₂ = (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
:$\begin \tan\alpha_1&=\frac,\\ \tan\alpha_2&=\frac,\\ \end$
where λ₁₂ = λ₂ − λ₁In the article on great-circle distances,
the notation Δλ = λ₁₂
and Δσ = σ₁₂ is used. The notation in this article is needed to
deal with differences between other points, e.g., λ₀₁.
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
:$\tan\sigma_=\frac.$
(The numerator of this formula contains the quantities that were used to determine
tan α₁.)
The distance along the great circle will then be ''s''₁₂ = ''R''σ₁₂, where ''R'' is the assumed radius
of the Earth and σ₁₂ is expressed in radians.
Using the mean Earth radius, ''R'' = ''R''₁ ≈  yields results for
the distance ''s''₁₂ which are within 1% of the geodesic length for the WGS84 ellipsoid; see Geodesics on an ellipsoid for details.

Relation to geocentric coordinate system

Detailed evaluation of the optimum direction is possible if the sea surface is approximated by a sphere surface. The standard computation places the ship at a geodetic latitude and geodetic longitude , where is considered positive if north of the equator, and where is considered positive if east of Greenwich. In the geocentric coordinate system centered at the center of the sphere, the Cartesian components are

::$=R\left(\begin \cos\varphi_s \cos\lambda_s \\ \cos\varphi_s \sin\lambda_s \\ \sin\varphi_s \end\right)$
and the target position is

::$=R\left(\begin \cos\varphi_t \cos\lambda_t \\ \cos\varphi_t \sin\lambda_t \\ \sin\varphi_t \end\right).$
The North Pole is at
::$=R\left(\begin 0 \\ 0 \\ 1 \end\right).$
The minimum distance is the distance along a great circle that runs through and . It is calculated in a plane that contains the sphere center and the great circle,
::$d_=R\theta_$
where is the angular distance of two points viewed from the center of the sphere, measured in radians. The cosine of the angle is calculated by the dot product of the two vectors
::$\mathbf\cdot \mathbf = R^2\cos \theta_ = R^2(\sin\varphi_s\sin\varphi_t+\cos\varphi_s\cos\varphi_t\cos(\lambda_t-\lambda_s))$
If the ship steers straight to the North Pole, the travel distance is
::$d_ = R\theta_ = R(\pi/2-\varphi_s)$
If a ship starts at and sails straight to the North Pole, the travel distance is
::$d_ = R\theta_ =R(\pi/2-\varphi_t)$

Derivation

The ''cosine formula'' of spherical trigonometry yields for the
angle between the great circles through that point to the North on one hand and to on the other hand
::$\cos\theta_ = \cos\theta_\cos\theta_+\sin\theta_\sin\theta_\cos p.$
::$\sin\varphi_t = \cos\theta_\sin\varphi_s +\sin\theta_\cos\varphi_s\cos p.$
The ''sine formula'' yields
::$\frac = \frac.$
Solving this for and insertion in the previous formula gives an expression for the tangent of the position angle,
::$\sin\varphi_t = \cos\theta_\sin\varphi_s +\frac\cos\varphi_t\cos\varphi_s\cos p;$
::$\tan p = \frac.$

Further details

Because the brief derivation gives an angle between 0 and which does not reveal the sign (west or east of north ?), a more explicit derivation is desirable which yields separately the sine and the cosine of such that use of the correct branch of the inverse tangent allows to produce an angle in the full range .

The computation starts from a construction of the great circle between and . It lies in the plane that contains the sphere center, and and is constructed rotating by the angle around an axis . The axis is perpendicular to the plane of the great circle and computed by the normalized vector cross product of the two positions:
::$\mathbf = \frac\mathbf\times \mathbf = \frac\left(\begin \cos\varphi_s\sin\lambda_s\sin\varphi_t -\sin\varphi_s\cos\varphi_t\sin\lambda_t \\ \sin\varphi_s\cos\lambda_t\cos\varphi_t -\cos\varphi_s\sin\varphi_t\cos\lambda_s \\ \cos\varphi_s\cos\varphi_t\sin(\lambda_t-\lambda_s) \end\right).$
A right-handed tilted coordinate system with the center at the center of the sphere is given by the
following three axes: the
axis , the axis
::\mathbf_\perp = \omega \times \frac\mathbf =
\frac
\left(\begin
\cos\varphi_t\cos\lambda_t(\sin^2\varphi_s+\cos^2\varphi_s\sin^2\lambda_s)-\cos\lambda_s(\sin\varphi_s\cos\varphi_s\sin\varphi_t+\cos^2\varphi_s\sin\lambda_s\cos\varphi_t\sin\lambda_t)\\
\cos\varphi_t\sin\lambda_t(\sin^2\varphi_s+\cos^2\varphi_s\cos^2\lambda_s)-\sin\lambda_s(\sin\varphi_s\cos\varphi_s\sin\varphi_t+\cos^2\varphi_s\cos\lambda_s\cos\varphi_t\cos\lambda_t)\\
\cos\varphi_s cos\varphi_s\sin\varphi_t-\sin\varphi_s\cos\varphi_t\cos(\lambda_t-\lambda_s)\end\right)

and the axis .
A position along the great circle is
::$\mathbf(\theta) = \cos\theta \mathbf+\sin\theta \mathbf_\perp,\quad 0\le\theta\le 2\pi.$
The compass direction is given by inserting the two vectors and and computing the gradient of the vector with respect to at .
::$\frac\mathbf_=\mathbf_\perp.$
The angle is given by splitting this direction along two orthogonal directions in the plane tangential to the sphere at the point . The two directions are given by the partial derivatives of with respect to and with respect to , normalized to unit length:
::$\mathbf_N = \left( \begin -\sin\varphi_s\cos\lambda_s\\ -\sin\varphi_s\sin\lambda_s\\ \cos\varphi_s \end\right);$
::$\mathbf_E = \left(\begin -\sin\lambda_s\\ \cos\lambda_s\\ 0 \end \right);$
::$\mathbf_N\cdot \mathbf = \mathbf_E\cdot \mathbf_N =0$
points north and points east at the position .
The position angle projects
into these two directions,
::$\mathbf_\perp = \cos p \,\mathbf_N+\sin p\, \mathbf_E$,
where the positive sign means the positive position angles are defined to be north over east. The values of the cosine and sine of are computed by multiplying this equation on both sides with the two unit vectors,
::\cos p = \mathbf_\perp \cdot \mathbf_N
=\frac cos\varphi_s\sin\varphi_t - \sin\varphi_s\cos\varphi_t\cos(\lambda_t-\lambda_s)

::\sin p = \mathbf_\perp \cdot \mathbf_E
=\frac cos\varphi_t\sin(\lambda_t-\lambda_s)

Instead of inserting the convoluted expression of , the evaluation may employ that the triple product is invariant under a circular shift
of the arguments:
::$\cos p = (\mathbf\times \frac\mathbf)\cdot \mathbf_N = \omega\cdot(\frac\mathbf\times \mathbf_N).$

If atan2 is used to compute the value, one can reduce both expressions by division through
and multiplication by ,
because these values are always positive and that operation does not change signs; then effectively
::$\tan p = \frac.$

Finding way-points

To find the way-points, that is the positions of selected points on the great circle between
''P''₁ and ''P''₂, 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
:$\tan\alpha_0 = \frac .$
Let the angular distances along the great circle from ''A'' to ''P''₁ and ''P''₂ be σ₀₁ and σ₀₂ respectively. Then using Napier's rules we have
:$\tan\sigma_ = \frac \qquad$(If φ₁ = 0 and α₁ = π, use σ₀₁ = 0).

This gives σ₀₁, whence σ₀₂ = σ₀₁ + σ₁₂.

The longitude at the node is found from

:$\begin \tan\lambda_ &= \frac,\\ \lambda_0 &= \lambda_1 - \lambda_. \end$

Finally, calculate the position and azimuth at an arbitrary point, ''P'' (see Fig. 2), by the spherical version of the ''direct geodesic problem''. Napier's rules give
:$\tan\phi = \frac ,$
:$\begin \tan(\lambda - \lambda_0) &= \frac ,\\ \tan\alpha &= \frac . \end$
The atan2 function should be used to determine
σ₀₁,
λ, and α.
For example, to find the
midpoint of the path, substitute σ = (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 = +π.
It may be convenient to parameterize the route in terms of the longitude using
:$\tan\phi = \cot\alpha_0\sin(\lambda-\lambda_0).$
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 $(\phi,\lambda)$
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.

Example

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°,λ₁₂
is reduced to the range minus;180°, 180°by adding or subtracting 360° as
necessary
α₁ = −94.41°,
α₂ = −78.42°, and
σ₁₂ = 168.56°. Taking the earth radius to be
''R'' = 6371 km, the distance is
''s''₁₂ = 18743 km.

To compute points along the route, first find
α₀ = −56.74°,
σ₀₁ = −96.76°,
σ₀₂ = 71.8°,
λ₀₁ = 98.07°, and
λ₀ = −169.67°.
Then to compute the midpoint of the route (for example), take
σ = (σ₀₁ + σ₀₂) = −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,

the results
are α₁ = −94.82°, α₂ = −78.29°, and
''s''₁₂ = 18752 km. The midpoint of the geodesic is
φ = −7.07°, λ = −159.31°,
α = −57.45°.

Gnomonic chart

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

See also

* 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

Notes

References

{{reflist

External links

Great Circle – from MathWorld
Great Circle description, figures, and equations. Mathworld, Wolfram Research, Inc. c1999

Great Circle Map
Interactive tool for plotting great circle routes on a sphere.

Great Circle Mapper
Interactive tool for plotting great circle routes.


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

Navigation
Circles
Spherical curves