Although a specific area of hunginess system is useful for numerical calculations in a given space, the space itself is considered to exist independently of any particular choice of coordinates. From this point of view, a coordinate on a space is simply a function from the space (or a subset of the space) to the scalars. When the space has additional structure, one restricts attention to the functions which are compatible with this structure. Examples include:

• Continuous functions on topological spaces;
• Smooth functions on smooth manifolds;
• Measurable functions on measure spaces;
• Rational functions on algebraic varieties;
• Linear functionals on vector spaces.

The coordinates on a space transform naturally (by pullback) under the group of automorphisms of the space, and the set of all coordinates is a commutative ring called the coordinate ring of the space.

In informal usage, coordinate systems can have singularities: these are points where one or more of the coordinates is not well-defined. For example, the origin in the polar coordinate system (r,θ) on the plane is singular, because although the radial coordinate has a well-defined value (r = 0) at the origin, θ can be any angle, and so is not a well-defined function at the origin.

Examples

The prototypical example of a coordinate system is the Cartesian coordinate system, which describes the position of a point P in the Euclidean space Rⁿ by an n-tuple

P = (r₁, ..., r_n)

r₁, ..., r_n.

If a subset S of a Euclidean space is mapped continuously onto another topological space, this defines coordinates in the image of S. That can be called a parametrization of the image, since it assigns numbers to points. That correspondence is unique only if the mapping is bijective.

The system of assigning longitude and latitude to geographical locations is a coordinate system. In this case the parametrization fails to be unique at the north and south poles.

Defining a coordinate system based on another one

Transformations

With every bijection from the space to itself two coordinate transformations can be associated:

• such that the new coordinates of the image of each point are the same as the old coordinates of the original point (the formulas for the mapping are the inverse of those for the coordinate transformation)
• such that the old coordinates of the image of each point are the same as the new coordinates of the original point (the formulas for the mapping are the same as those for the coordinate transformation)

For example, in 1D, if the mapping is a translation of 3 to the right, the first moves the origin from 0 to 3, so that the coordinate of each point becomes 3 less, while the second moves the origin from 0 to -3, so that the coordinate of each point becomes 3 more.

Systems commonly used

• The Cartesian coordinate system (also called the "rectangular coordinate system"), which, for three-dimensional flat space, uses three numbers representing distances.
• Curvilinear coordinates are a generalization of coordinate systems generally; the system is based on the intersection of curves.
• The polar coordinate systems:
• Circular coordinate system (commonly referred to as the polar coordinate system) represents a point in the plane by an angle and a distance from the origin.
• Cylindrical coordinate system represents a point in space by an angle, a distance from the origin and a height.
• Spherical coordinate system represents a point in space with two angles and a distance from the origin.
• Plücker coordinates are a way of representing lines in 3D Euclidean space using a six-tuple of numbers as homogeneous coordinates.
• Generalized coordinates are used in the Lagrangian treatment of mechanics.
• Canonical coordinates are used in the Hamiltonian treatment of mechanics.
• Parallel coordinates visualise a point in n-dimensional space as a polyline connecting points on n vertical lines.

While not coordinate systems, there are ways of describing curves using intrinsic equations that use invariant quantities such as curvature and arc length. These include:

• Whewell equation relates arc length and tangential angle.
• Cesàro equation relates arc length and curvature.

A list of orthogonal coordinate systems

Geographical systems

The Global Positioning System uses the WGS84 coordinate system.

The Universal Transverse Mercator (UTM) and Universal Polar Stereographic (UPS) coordinate systems both use a metric-based cartesian grid laid out on a conformally projected surface to locate positions on the surface of the Earth. The UTM system is not a single map projection but a series of map projections, one for each of sixty zones. The UPS system is used for the polar regions, which are not covered by the UTM system.

During medieval times, the stereographic coordinate system was used for navigation purposes. The stereographic coordinate system was superseded by the latitude-longitude system, and more recently, the Global Positioning System.

Although no longer used in navigation, the stereographic coordinate system is still used in modern times to describe crystallographic orientations in the field of materials science.

Astronomical systems

See also

• Active and passive transformation
• Frame of reference
• Galilean transformation
• Coordinate-free approach

References and notes