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The great-circle distance, orthodromic distance, or spherical distance is the
distance Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
along a
great circle In mathematics, a great circle or orthodrome is the circular intersection of a sphere and a plane passing through the sphere's center point. Any arc of a great circle is a geodesic of the sphere, so that great circles in spherical geometry ...
. It is the shortest
distance Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
between two
points Point or points may refer to: Places * Point, Lewis, a peninsula in the Outer Hebrides, Scotland * Point, Texas, a city in Rains County, Texas, United States * Point, the NE tip and a ferry terminal of Lismore, Inner Hebrides, Scotland * Point ...
on the surface of a
sphere A sphere () is a Geometry, geometrical object that is a solid geometry, three-dimensional analogue to a two-dimensional circle. A sphere is the Locus (mathematics), set of points that are all at the same distance from a given point in three ...
, measured along the surface of the sphere (as opposed to a straight line through the sphere's interior). The distance between two points in
Euclidean space Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidean sp ...
is the length of a straight line between them, but on the sphere there are no straight lines. In spaces with curvature, straight lines are replaced by geodesics. Geodesics on the sphere are circles on the sphere whose centers coincide with the center of the sphere, and are called 'great circles'. The determination of the great-circle distance is part of the more general problem of great-circle navigation, which also computes the azimuths at the end points and intermediate way-points. Through any two points on a sphere that are not
antipodal point In mathematics, antipodal points of a sphere are those diametrically opposite to each other (the specific qualities of such a definition are that a line drawn from the one to the other passes through the center of the sphere so forms a true ...
s (directly opposite each other), there is a unique great circle. The two points separate the great circle into two arcs. The length of the shorter arc is the great-circle distance between the points. A great circle endowed with such a distance is called a
Riemannian circle In metric space theory and Riemannian geometry, the Riemannian circle is a great circle with a characteristic length. It is the circle equipped with the ''intrinsic'' Riemannian metric of a compact one-dimensional manifold of total length 2, or th ...
in
Riemannian geometry Riemannian geometry is the branch of differential geometry that studies Riemannian manifolds, smooth manifolds with a ''Riemannian metric'', i.e. with an inner product on the tangent space at each point that varies smoothly from point to po ...
. Between antipodal points, there are infinitely many great circles, and all great circle arcs between antipodal points have a length of half the
circumference In geometry, the circumference (from Latin ''circumferens'', meaning "carrying around") is the perimeter of a circle or ellipse. That is, the circumference would be the arc length of the circle, as if it were opened up and straightened out t ...
of the circle, or \pi r, where ''r'' is the
radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
of the sphere. The
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
is nearly spherical, so great-circle distance formulas give the distance between points on the surface of the Earth correct to within about 0.5%. The vertex is the highest-latitude point on a great circle.


Formulae

Let \lambda_1, \phi_1 and \lambda_2, \phi_2 be the geographical
longitude Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek let ...
and
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north po ...
of two points 1 and 2, and \Delta\lambda, \Delta\phi be their absolute differences; then \Delta\sigma, the central angle between them, is given by the spherical law of cosines if one of the poles is used as an auxiliary third point on the sphere: :\Delta\sigma = \arccos\bigl(\sin\phi_1\sin\phi_2 + \cos\phi_1\cos\phi_2\cos(\Delta\lambda)\bigr). The problem is normally expressed in terms of finding the central angle \Delta\sigma. Given this angle in radians, the actual
arc length ARC may refer to: Business * Aircraft Radio Corporation, a major avionics manufacturer from the 1920s to the '50s * Airlines Reporting Corporation, an airline-owned company that provides ticket distribution, reporting, and settlement services ...
''d'' on a sphere of radius ''r'' can be trivially computed as :d = r \, \Delta\sigma.


Computational formulas

On computer systems with low
floating point In computing, floating-point arithmetic (FP) is arithmetic that represents real numbers approximately, using an integer with a fixed precision, called the significand, scaled by an integer exponent of a fixed base. For example, 12.345 can be r ...
precision, the spherical law of cosines formula can have large
rounding error A roundoff error, also called rounding error, is the difference between the result produced by a given algorithm using exact arithmetic and the result produced by the same algorithm using finite-precision, rounded arithmetic. Rounding errors are ...
s if the distance is small (if the two points are a kilometer apart on the surface of the Earth, the cosine of the central angle is near 0.99999999). For modern 64-bit floating-point numbers, the spherical law of cosines formula, given above, does not have serious rounding errors for distances larger than a few meters on the surface of the Earth. The haversine formula is numerically better-conditioned for small distances: :\begin \Delta\sigma &= \operatorname\left( \operatorname\left(\Delta\phi\right) + \left(1 - \operatorname(\Delta\phi) - \operatorname(\phi_1 + \phi_2)\right)\cdot\operatorname\left(\Delta\lambda\right)\right) \\ &= 2\arcsin \sqrt. \end Historically, the use of this formula was simplified by the availability of tables for the haversine function: hav(''θ'') = sin2(''θ''/2). Although this formula is accurate for most distances on a sphere, it too suffers from rounding errors for the special (and somewhat unusual) case of antipodal points. A formula that is accurate for all distances is the following special case of the Vincenty formula for an ellipsoid with equal major and minor axes: :\Delta\sigma = \arctan \frac . Here the quadrant for \Delta\sigma should be governed by the signs of the numerator and denominator of the right hand side, e.g., using the atan2 function.


Vector version

Another representation of similar formulas, but using normal vectors instead of latitude and longitude to describe the positions, is found by means of 3D vector algebra, using the
dot product In mathematics, the dot product or scalar productThe term ''scalar product'' means literally "product with a scalar as a result". It is also used sometimes for other symmetric bilinear forms, for example in a pseudo-Euclidean space. is an alg ...
,
cross product In mathematics, the cross product or vector product (occasionally directed area product, to emphasize its geometric significance) is a binary operation on two vectors in a three-dimensional oriented Euclidean vector space (named here E), and i ...
, or a combination: :\begin \Delta\sigma &= \arccos \left(\mathbf n_1 \cdot \mathbf n_2\right) \\ &= \arcsin \left, \mathbf n_1 \times \mathbf n_2 \ \\ &= \arctan \frac \\ \end where \mathbf n_1 and \mathbf n_2 are the normals to the ellipsoid at the two positions 1 and 2. Similarly to the equations above based on latitude and longitude, the expression based on arctan is the only one that is well-conditioned for all angles. The expression based on arctan requires the magnitude of the cross product over the dot product.


From chord length

A line through three-dimensional space between points of interest on a spherical Earth is the
chord Chord may refer to: * Chord (music), an aggregate of musical pitches sounded simultaneously ** Guitar chord a chord played on a guitar, which has a particular tuning * Chord (geometry), a line segment joining two points on a curve * Chord ( ...
of the great circle between the points. The central angle between the two points can be determined from the chord length. The great circle distance is proportional to the central angle. The great circle chord length, C_h\,\!, may be calculated as follows for the corresponding unit sphere, by means of Cartesian subtraction: :\begin \Delta &= \cos\phi_2\cos\lambda_2 - \cos\phi_1\cos\lambda_1;\\ \Delta &= \cos\phi_2\sin\lambda_2 - \cos\phi_1\sin\lambda_1;\\ \Delta &= \sin\phi_2 - \sin\phi_1;\\ C &= \sqrt \end The central angle is: :\Delta\sigma=2\arcsin \frac .


Radius for spherical Earth

The shape of the Earth closely resembles a flattened sphere (a spheroid) with equatorial radius a of 6378.137 km; distance b from the center of the spheroid to each pole is 6356.7523142 km. When calculating the length of a short north-south line at the equator, the circle that best approximates that line has a radius of \frac (which equals the meridian's semi-latus rectum), or 6335.439 km, while the spheroid at the poles is best approximated by a sphere of radius \frac, or 6399.594 km, a 1% difference. So long as a spherical Earth is assumed, any single formula for distance on the Earth is only guaranteed correct within 0.5% (though better accuracy is possible if the formula is only intended to apply to a limited area). Using the mean earth radius, R_1 = \frac(2a + b) \approx 6371.009\text (for the
WGS84 The World Geodetic System (WGS) is a standard used in cartography, geodesy, and satellite navigation including GPS. The current version, WGS 84, defines an Earth-centered, Earth-fixed coordinate system and a geodetic datum, and also desc ...
ellipsoid) means that in the limit of small flattening, the mean square relative error in the estimates for distance is minimized.


See also

*
Air navigation The basic principles of air navigation are identical to general navigation, which includes the process of planning, recording, and controlling the movement of a craft from one place to another. Successful air navigation involves piloting an air ...
* Angular distance *
Circumnavigation Circumnavigation is the complete navigation around an entire island, continent, or astronomical body (e.g. a planet or moon). This article focuses on the circumnavigation of Earth. The first recorded circumnavigation of the Earth was the Magel ...
* Flight planning *
Geodesy Geodesy ( ) is the Earth science of accurately measuring and understanding Earth's figure (geometric shape and size), Earth rotation, orientation in space, and Earth's gravity, gravity. The field also incorporates studies of how these properti ...
* Geodesics on an ellipsoid * Geodetic system *
Geographical distance Geographical distance or geodetic distance is the distance measured along the surface of the earth. The formulae in this article calculate distances between points which are defined by geographical coordinates in terms of latitude and longitud ...
* Isoazimuthal * Loxodromic navigation *
Meridian arc In geodesy and navigation, a meridian arc is the curve between two points on the Earth's surface having the same longitude. The term may refer either to a segment of the meridian, or to its length. The purpose of measuring meridian arcs is to ...
* Rhumb line *
Spherical geometry 300px, A sphere with a spherical triangle on it. Spherical geometry is the geometry of the two-dimensional surface of a sphere. In this context the word "sphere" refers only to the 2-dimensional surface and other terms like "ball" or "solid sp ...
* Spherical trigonometry


References and notes


External links


GreatCircle
at
MathWorld ''MathWorld'' is an online mathematics reference work, created and largely written by Eric W. Weisstein. It is sponsored by and licensed to Wolfram Research, Inc. and was partially funded by the National Science Foundation's National Science ...
{{DEFAULTSORT:Great-Circle Distance Metric geometry Spherical trigonometry Distance Spherical curves