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| [[File:Cartesian-coordinate-system.svg|thumb|right|250px|Illustration of a Cartesian coordinate plane. Four points are marked and labeled with their coordinates: (2, 3) in green, (−3, 1) in red, (−1.5, −2.5) in blue, and the origin (0, 0) in purple.]]
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| A '''Cartesian coordinate system''' is a [[coordinate system]] that specifies each [[point (geometry)|point]] uniquely in a [[plane (geometry)|plane]] by a pair of [[number|numerical]] '''coordinates''', which are the [[positive and negative numbers|signed]] distances from the point to two fixed [[perpendicular]] directed lines, measured in the same [[unit length|unit of length]]. Each reference line is called a ''coordinate axis'' or just ''axis'' of the system, and the point where they meet is its ''[[Origin (mathematics)|origin]]'', usually at ordered pair (0, 0). The coordinates can also be defined as the positions of the [[orthogonal projection|perpendicular projections]] of the point onto the two axes, expressed as signed distances from the origin.
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| One can use the same principle to specify the position of any point in three-[[dimension]]al [[space (mathematics)|space]] by three Cartesian coordinates, its signed distances to three mutually perpendicular planes (or, equivalently, by its perpendicular projection onto three mutually perpendicular lines). In general, ''n'' Cartesian coordinates (an element of [[real n-space|real ''n''-space]]) specify the point in an ''n''-dimensional [[Euclidean space]] for any [[dimension (mathematics)|dimension]] ''n''. These coordinates are equal, up to [[sign (mathematics)|sign]], to distances from the point to ''n'' mutually perpendicular [[hyperplane]]s.
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| [[File:Cartesian-coordinate-system-with-circle.svg|thumb|right|250px|Cartesian coordinate system with a circle of radius 2 centered at the origin marked in red. The equation of a circle is (''x'' − ''a'')<sup>2</sup> + (''y'' − ''b'')<sup>2</sup> = ''r''<sup>2</sup> where ''a'' and ''b'' are the coordinates of the center (''a'', ''b'') and ''r'' is the radius.]]
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| The invention of Cartesian coordinates in the 17th century by [[René Descartes]] ([[Latinisation (literature)|Latinized]] name: ''Cartesius'') revolutionized mathematics by providing the first systematic link between [[Euclidean geometry]] and [[algebra]]. Using the Cartesian coordinate system, geometric shapes (such as [[curve]]s) can be described by '''Cartesian equations''': algebraic [[equations]] involving the coordinates of the points lying on the shape. For example, a circle of radius 2 may be described as the set of all points whose coordinates ''x'' and ''y'' satisfy the equation ''x''<sup>2</sup> + ''y''<sup>2</sup> = 4.
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| Cartesian coordinates are the foundation of [[analytic geometry]], and provide enlightening geometric interpretations for many other branches of mathematics, such as [[linear algebra]], [[complex number|complex analysis]], [[differential geometry]], multivariate [[calculus]], [[group theory]], and more. A familiar example is the concept of the [[graph of a function|graph]] of a [[function (mathematics)|function]]. Cartesian coordinates are also essential tools for most applied disciplines that deal with geometry, including [[astronomy]], [[physics]], [[engineering]], and many more. They are the most common coordinate system used in [[computer graphics]], [[computer-aided geometric design]], and other [[computational geometry|geometry-related data processing]].
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| ==History==
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| The adjective ''Cartesian'' refers to the French [[mathematician]] and [[philosopher]] [[René Descartes]] (who used the name ''Cartesius'' in [[Latin]]).
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| The idea of this system was developed in 1637 in writings by Descartes and independently by [[Pierre de Fermat]], although Fermat also worked in three dimensions, and did not publish the discovery.<ref>{{cite encyclopedia|title=analytic geometry|encyclopedia=Encyclopædia Britannica|year=2008|edition=Encyclopædia Britannica Online|accessdate=2 August 2008}}</ref> Both authors used a single axis in their treatments and have a variable length measured in reference to this axis. The concept of using a pair of axes was introduced later, after Descartes' ''[[La Géométrie]]'' was translated into Latin in 1649 by [[Frans van Schooten]] and his students. These commentators introduced several concepts while trying to clarify the ideas contained in Descartes' work.<ref>{{harvnb|Burton|2011|loc=p. 374}}</ref>
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| The development of the Cartesian coordinate system would play an intrinsic role in the development of the [[calculus]] by [[Isaac Newton]] and [[Gottfried Wilhelm Leibniz]].<ref>A Tour of the Calculus, David Berlinski</ref>
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| [[Nicole Oresme#Mathematics|Nicole Oresme]], a French cleric and friend of the [[Dauphin of France|Dauphin]] (later to become King Charles V) of the 14th Century, used constructions similar to Cartesian coordinates well before the time of Descartes and Fermat.
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| Many other coordinate systems have been developed since Descartes, such as the [[polar coordinate system|polar coordinates]] for the plane, and the [[spherical coordinates|spherical]] and [[cylindrical coordinates]] for three-dimensional space.
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| ==Definitions==
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| ===Number line===
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| {{Main|Number line}}
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| Choosing a Cartesian coordinate system for a one-dimensional space—that is, for a straight line—means choosing a point ''O'' of the line (the origin), a unit of length, and an orientation for the line. An orientation chooses which of the two half-lines determined by ''O'' is the positive, and which is negative; we then say that the line "is oriented" (or "points") from the negative half towards the positive half. Then each point ''p'' of the line can be specified by its distance from ''O'', taken with a + or − sign depending on which half-line contains ''p''.
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| A line with a chosen Cartesian system is called a '''number line'''. Every real number, whether integer, rational, or irrational, has a unique location on the line. Conversely, every point on the line can be interpreted as a [[number]] in an ordered continuum which includes the real numbers.
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| ===Cartesian coordinates in two dimensions===
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| The modern Cartesian coordinate system in two dimensions (also called a rectangular coordinate system) is defined by an [[ordered pair]] of [[perpendicular]] lines (axes), a single unit of length for both axes, and an orientation for each axis. (Early systems allowed "oblique" axes, that is, axes that did not meet at right angles.) The lines are commonly referred to as the ''x'' and ''y''-axes where the '''x-axis''' is taken to be horizontal and the '''y-axis''' is taken to be vertical. The point where the axes meet is taken as the origin for both, thus turning each axis into a number line. For a given point ''P'', a line is drawn through ''P'' perpendicular to the ''x''-axis to meet it at ''X'' and second line is drawn through ''P'' perpendicular to the ''y''-axis to meet it at ''Y''. The coordinates of ''P'' are then ''X'' and ''Y'' interpreted as numbers ''x'' and ''y'' on the corresponding number lines. The coordinates are written as an ordered pair (''x'', ''y'').
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| The point where the axes meet is the common origin of the two number lines and is simply called the ''origin''. It is often labeled ''O'' and if so then the axes are called ''Ox'' and ''Oy''. A plane with ''x'' and ''y''-axes defined is often referred to as the Cartesian plane or ''xy'' plane. The value of ''x'' is called the ''x''-coordinate or [[abscissa]] and the value of ''y'' is called the ''y''-coordinate or [[ordinate]].
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| The choices of letters come from the original convention, which is to use the latter part of the alphabet to indicate unknown values. The first part of the alphabet was used to designate known values.
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| In the Cartesian plane, reference is sometimes made to a [[unit circle]] or a [[unit hyperbola]].
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| ===Cartesian coordinates in three dimensions===
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| [[File:Coord system CA 0.svg|thumb|240px|A three dimensional Cartesian coordinate system, with origin ''O'' and axis lines ''X'', ''Y'' and ''Z'', oriented as shown by the arrows. The tick marks on the axes are one length unit apart. The black dot shows the point with coordinates ''X'' = 2, ''Y'' = 3, and ''Z'' = 4, or (2,3,4).]]
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| Choosing a Cartesian coordinate system for a three-dimensional space means choosing an ordered triplet of lines (axes), any two of them being perpendicular; a single unit of length for all three axes; and an orientation for each axis. As in the two-dimensional case, each axis becomes a number line. The coordinates of a point ''p'' are obtained by drawing a line through ''p'' perpendicular to each coordinate axis, and reading the points where these lines meet the axes as three numbers of these number lines.
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| Alternatively, the coordinates of a point ''p'' can also be taken as the (signed) distances from ''p'' to the three planes defined by the three axes. If the axes are named ''x'', ''y'', and ''z'', then the ''x'' coordinate is the distance from the plane defined by the ''y'' and ''z'' axes. The distance is to be taken with the + or − sign, depending on which of the two [[Half-space (geometry)|half-space]]s separated by that plane contains ''p''. The ''y'' and ''z'' coordinates can be obtained in the same way from the (''x'',''z'') and (''x'',''y'') planes, respectively.
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| [[File:Cartesian coordinate surfaces.png|thumb|240px|right| The [[Coordinate system#Coordinate surface|coordinate surfaces]] of the Cartesian coordinates (''x'', ''y'', ''z''). The ''z''-axis is vertical and the ''x''-axis is highlighted in green. Thus, the red plane shows the points with ''x''=1, the blue plane shows the points with ''z''=1, and the yellow plane shows the points with ''y''=−1. The three surfaces intersect at the point '''P''' (shown as a black sphere) with the Cartesian coordinates (1, −1, 1).]]
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| ===Generalizations===
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| One can generalize the concept of Cartesian coordinates to allow axes that are not perpendicular to each other, and/or different units along each axis. In that case, each coordinate is obtained by projecting the point onto one axis along a direction that is parallel to the other axis (or, in general, to the hyperplane defined by all the other axes). In those '''oblique coordinate systems''' the computations of distances and angles is more complicated than in standard Cartesian systems, and many standard formulas (such as the Pythagorean formula for the distance) do not hold.
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| == Notations and conventions ==<!-- [[Abscissa]] redirects here -->
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| The Cartesian coordinates of a point are usually written in parentheses and separated by commas, as in (10,5) or (3,5,7). The origin is often labelled with the capital letter ''O''. In analytic geometry, unknown or generic coordinates are often denoted by the letters ''x'' and ''y'' on the plane, and ''x'', ''y'', and ''z'' in three-dimensional space. ''w'' is often used for four-dimensional space, but the rarity of such usage precludes concrete convention here. This custom comes from an old convention of algebra, to use letters near the end of the alphabet for unknown values (such as were the coordinates of points in many geometric problems), and letters near the beginning for given quantities.
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| These conventional names are often used in other domains, such as physics and engineering. However, other letters may be used too. For example, in a graph showing how a [[pressure]] varies with [[time]], the graph coordinates may be denoted ''t'' and ''P''. Each axis is usually named after the coordinate which is measured along it; so one says the '''<nowiki></nowiki>''x''-axis''', the '''<nowiki></nowiki>''y''-axis''', the '''<nowiki></nowiki>''t''-axis''', etc.
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| Another common convention for coordinate naming is to use subscripts, as in ''x''<sub>1</sub>, ''x''<sub>2</sub>, ... ''x''<sub>''n''</sub> for the ''n'' coordinates in an ''n''-dimensional space; especially when ''n'' is greater than 3, or variable. Some authors (and many [[programmer]]s) prefer the numbering ''x''<sub>0</sub>, ''x''<sub>1</sub>, ... ''x''<sub>''n''−1</sub>. These notations are especially advantageous in [[computer programming]]: by storing the coordinates of a point as an [[Array data type|array]], instead of a [[record (computer science)|record]], one can use [[Iteration|iterative command]]s or [[subroutine|procedure]] [[parameter]]s instead of repeating the same commands for each coordinate.
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| In mathematical illustrations of two-dimensional Cartesian systems, the first coordinate (traditionally called the [[abscissa]]) is measured along a [[horizontal plane|horizontal]] axis, oriented from left to right. The second coordinate (the [[ordinate]]) is then measured along a [[vertical direction|vertical]] axis, usually oriented from bottom to top.
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| However, in computer graphics and [[image processing]] one often uses a coordinate system with the ''y'' axis pointing down (as displayed on the computer's screen). This convention developed in the 1960s (or earlier) from the way that images were originally stored in [[display buffer]]s.
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| For three-dimensional systems, a convention is to portray the ''x-y'' plane horizontally, with the ''z'' axis added to represent height (positive up). Furthermore, there is a convention to orient "x" facing toward the viewer, biased either to the right or left. If a diagram ([[3D projection]] or [[Perspective (graphical)|2D perspective drawing]]) shows the ''x'' and ''y'' axis horizontally and vertically, respectively, then the ''z'' axis should be shown pointing "out of the page" towards the viewer or camera. In such a 2D diagram of a 3D coordinate system, the ''z'' axis would appear as a line or ray pointing down and to the left or down and to the right, depending on the presumed viewer or camera [[Perspective (graphical)|perspective]]. In any diagram or display, the orientation of the three axes, as a whole, is arbitrary. However, the orientation of the axes relative to each other should always comply with the [[right-hand rule]], unless specifically stated otherwise. All laws of physics and math assume this [[#Orientation and handedness|right-handedness]], which ensures consistency. For 3D diagrams, the names "abscissa" and "ordinate" are rarely used for ''x'' and ''y'', respectively. When they are, the ''z''-coordinate is sometimes called the '''applicate'''.
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| The words ''abscissa'', ''ordinate'' and ''applicate'' are sometimes used to refer to coordinate axes rather than values.<ref>[http://www.encyclopediaofmath.org/index.php/Cartesian_orthogonal_coordinate_system Springer online reference Encyclopedia of Mathematics]</ref>
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| ===Quadrants and octants===
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| {{Main|Octant (solid geometry)}}
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| {{Main|Quadrant (plane geometry)}}
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| <!-- This section is linked from [[Heraldry]] and from [[Quadrant (Cartesian coordinate system)]]-->
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| [[File:Cartesian coordinates 2D.svg|thumb|240px|The four quadrants of a Cartesian coordinate system.]]
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| The axes of a two-dimensional Cartesian system divide the plane into four infinite regions, called '''quadrants''', each bounded by two half-axes. These are often numbered from 1st to 4th and denoted by [[Roman numeral]]s: I (where the signs of the two coordinates are I (+,+), II (−,+), III (−,−), and IV (+,−). When the axes are drawn according to the mathematical custom, the numbering goes [[clockwise|counter-clockwise]] starting from the upper right ("northeast") quadrant.
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| Similarly, a three-dimensional Cartesian system defines a division of space into eight regions or '''octants''', according to the signs of the coordinates of the points. The convention used for naming a specific octant is to list its signs, e.g. (+ + +) or ( - + - ). The n-dimensional generalization of the quadrant and octant is the [[orthant]], and the same naming system applies.
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| ==Cartesian space==
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| A [[Euclidean plane]] with a chosen Cartesian system is called a '''Cartesian plane'''. Since Cartesian coordinates are unique and non-ambiguous, the points of a Cartesian plane can be identified with all possible pairs of [[real number]]s; that is with the [[Cartesian product]] <math>\R^2 = \R\times\R</math>, where <math>\R</math> is the set of all reals. In the same way one defines a '''Cartesian space''' of any dimension ''n'', whose points can be identified with the [[tuple]]s (lists) of ''n'' real numbers, that is, with <math>\R^n</math>.
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| ==Cartesian formulas for the plane==
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| === Distance between two points ===
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| The [[Euclidean distance]] between two points of the plane with Cartesian coordinates <math>(x_1,y_1)</math> and <math>(x_2,y_2)</math> is
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| :<math>d = \sqrt{(x_2-x_1)^2 + (y_2-y_1)^2}.</math>
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| This is the Cartesian version of [[Pythagoras' theorem]]. In three-dimensional space, the distance between points <math>(x_1,y_1,z_1)</math> and <math>(x_2,y_2,z_2)</math> is
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| :<math>d = \sqrt{(x_2-x_1)^2 + (y_2-y_1)^2+ (z_2-z_1)^2}</math>
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| which can be obtained by two consecutive applications of Pythagoras' theorem.
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| ===Euclidean transformations===
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| The [[Euclidean plane isometry|Euclidean transformations]] or '''Euclidean motions''' are the ([[bijective]]) mappings of points of the [[Euclidean plane]] to themselves which preserve distances between points. There are four types of these mappings (also called isometries): [[Translation (geometry)|translations]], [[Rotation (mathematics)|rotations]], [[Reflection (mathematics)|reflections]] and [[glide reflection]]s.<ref>{{harvnb|Smart|1998|loc=Chap. 2}}</ref>
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| ==== Translation ====
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| [[translation (geometry)|Translating]] a set of points of the plane, preserving the distances and directions between them, is equivalent to adding a fixed pair of numbers (''a'',''b'') to the Cartesian coordinates of every point in the set. That is, if the original coordinates of a point are (''x'',''y''), after the translation they will be
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| :<math>(x',y') = (x + a, y + b).\,</math>
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| ====Rotation====
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| To [[rotation (geometry)|rotate]] a figure [[clockwise|counterclockwise]] around the origin by some angle <math>\theta</math> is equivalent to replacing every point with coordinates (''x'',''y'') by the point with coordinates (''x<nowiki>'</nowiki>'',''y<nowiki>'</nowiki>''), where
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| :<math>x'=x \cos \theta - y \sin \theta\,</math>
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| :<math>y'=x \sin \theta + y \cos \theta.\,</math>
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| Thus:
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| <math>(x',y') = ((x \cos \theta - y \sin \theta\,) , (x \sin \theta + y \cos \theta\,)).</math>
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| ====Reflection====
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| If (''x'', ''y'') are the Cartesian coordinates of a point, then (−''x'', ''y'') are the coordinates of its [[coordinate rotations and reflections|reflection]] across the second coordinate axis (the Y axis), as if that line were a mirror. Likewise, (''x'', −''y'') are the coordinates of its reflection across the first coordinate axis (the X axis). In more generality, reflection across a line through the origin making an angle <math>\theta</math> with the x-axis, is equivalent to replacing every point with coordinates (''x'',''y'') by the point with coordinates (''x<nowiki>'</nowiki>'',''y<nowiki>'</nowiki>''), where
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| :<math>x'=x \cos 2\theta + y \sin 2\theta\,</math>
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| :<math>y'=x \sin 2\theta - y \cos 2\theta.\,</math>
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| Thus:
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| <math>(x',y') = ((x \cos 2\theta + y \sin 2\theta\,) , (x \sin 2\theta - y \cos 2\theta\,)).</math>
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| ====Glide reflection====
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| A glide reflection is the composition of a reflection across a line followed by a translation in the direction of that line. It can be seen that the order of these operations does not matter (the translation can come first, followed by the reflection).
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| ====General matrix form of the transformations====
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| These [[Euclidean transformation]]s of the plane can all be described in a uniform way by using matrices. The result <math>(x', y')</math> of applying a Euclidean transformation to a point <math>(x,y)</math> is given by the formula
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| :<math>(x',y') = (x,y) A + b\,</math>
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| where ''A'' is a 2×2 orthogonal [[matrix (mathematics)|matrix]] and ''b'' = (''b''<sub>1</sub>, ''b''<sub>2</sub>) is an arbitrary ordered pair of numbers;<ref>{{harvnb|Brannan|Esplen|Gray|1998|loc=pg. 49}}</ref> that is,
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| :<math>x' = x A_{1 1} + y A_{2 1} + b_{1}\,</math>
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| :<math>y' = x A_{1 2} + y A_{2 2} + b_{2},\,</math>
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| where
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| ::<math>A = \begin{pmatrix} A_{11} & A_{12} \\ A_{21} & A_{22} \end{pmatrix}.</math> <nowiki>[</nowiki>Note the use of row vectors for point coordinates and that the matrix is written on the right.<nowiki>]</nowiki>
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| To be ''orthogonal'', the matrix ''A'' must have [[orthogonal]] rows with same Euclidean length of one, that is,
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| :<math>A_{1 1} A_{2 1} + A_{1 2} A_{2 2} = 0\,</math>
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| and
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| :<math>A_{1 1}^2 + A_{1 2}^2 = A_{2 1}^2 + A_{2 2}^2 = 1.</math>
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| This is equivalent to saying that ''A'' times its [[transpose]] must be the [[identity matrix]]. If these conditions do not hold, the formula describes a more general [[affine transformation]] of the plane provided that the [[determinant]] of ''A'' is not zero.
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| The formula defines a translation [[if and only if]] ''A'' is the [[identity matrix]]. The transformation is a rotation around some point if and only if ''A'' is a [[rotation matrix]], meaning that
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| :<math> A_{1 1} A_{2 2} - A_{2 1} A_{1 2} = 1.\,</math>
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| A reflection or glide reflection is obtained when,
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| :<math> A_{1 1} A_{2 2} - A_{2 1} A_{1 2} = -1.\,</math>
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| Assuming that translation is not used transformations can be combined by simply multiplying the associated transformation matrices.
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| ====Affine transformation====
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| Another way to represent coordinate transformations in Cartesian coordinates is through [[affine transformation]]s. In affine transformations an extra dimension is added and all points are given a value of 1 for this extra dimension. The advantage of doing this is that point translations can be specified in the final column of matrix A. In this way, all of the euclidean transformations become transactable as matrix • point multiplications. The affine transformation is given by:
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| ::<math>\begin{pmatrix} A_{11} & A_{21} & b_{1} \\ A_{12} & A_{22} & b_{2} \\ 0 & 0 & 1 \end{pmatrix} \begin{pmatrix} x \\ y \\ 1 \end{pmatrix} = \begin{pmatrix} x' \\ y' \\ 1 \end{pmatrix}.</math> <nowiki>[</nowiki>Note the A matrix from above was transposed. The matrix is on the left and column vectors for point coordinates are used.<nowiki>]</nowiki>
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| Using affine transformations multiple different euclidean transformations including translation can be combined by simply multiplying the corresponding matrices.
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| ====Scaling====
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| An example of an affine transformation which is not a Euclidean motion is given by scaling. To make a figure larger or smaller is equivalent to multiplying the Cartesian coordinates of every point by the same positive number ''m''. If (''x'',''y'') are the coordinates of a point on the original figure, the corresponding point on the scaled figure has coordinates
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| :<math>(x',y') = (m x, m y).\,</math>
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| If ''m'' is greater than 1, the figure becomes larger; if ''m'' is between 0 and 1, it becomes smaller.
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| ====Shearing====
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| A [[shear mapping|shearing transformation]] will push the top of a square sideways to form a parallelogram. Horizontal shearing is defined by:
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| :<math>(x',y') = (x+y s, y)\,</math>
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| Shearing can also be applied vertically:
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| :<math>(x',y') = (x, x s+y)\,</math>
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| ==Orientation and handedness==
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| {{Main|Orientation (mathematics)}}
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| {{See also|right-hand rule|Axes conventions}}
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| ===In two dimensions===
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| [[File:Rechte-hand-regel.jpg|left|thumb|The [[right hand rule]].]]
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| Fixing or choosing the ''x''-axis determines the ''y''-axis up to direction. Namely, the ''y''-axis is necessarily the [[perpendicular]] to the ''x''-axis through the point marked 0 on the ''x''-axis. But there is a choice of which of the two half lines on the perpendicular to designate as positive and which as negative. Each of these two choices determines a different orientation (also called ''handedness'') of the Cartesian plane.
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| The usual way of orienting the axes, with the positive ''x''-axis pointing right and the positive ''y''-axis pointing up (and the ''x''-axis being the "first" and the ''y''-axis the "second" axis) is considered the ''positive'' or ''standard'' orientation, also called the ''right-handed'' orientation.
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| A commonly used mnemonic for defining the positive orientation is the ''[[right hand rule]]''. Placing a somewhat closed right hand on the plane with the thumb pointing up, the fingers point from the ''x''-axis to the ''y''-axis, in a positively oriented coordinate system.
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| The other way of orienting the axes is following the ''left hand rule'', placing the left hand on the plane with the thumb pointing up.
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| [[File:3D Cartesian Coodinate Handedness.jpg|thumb|3D Cartesian Coodinate Handedness]]
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| When pointing the thumb away from the origin along an axis towards positive, the curvature of the fingers indicates a positive rotation along that axis.
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| Regardless of the rule used to orient the axes, rotating the coordinate system will preserve the orientation. Switching any two axes will reverse the orientation, but switching both will leave the orientation unchanged.
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| ===In three dimensions===
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| [[File:Cartesian coordinate system handedness.svg|left|250px|thumb|Fig. 7 – The left-handed orientation is shown on the left, and the right-handed on the right.]]
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| [[File:Right hand cartesian.svg|right|thumb|250px|Fig. 8 – The right-handed Cartesian coordinate system indicating the coordinate planes.]]
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| Once the ''x''- and ''y''-axes are specified, they determine the [[line (geometry)|line]] along which the ''z''-axis should lie, but there are two possible directions on this line. The two possible coordinate systems which result are called 'right-handed' and 'left-handed'. The standard orientation, where the ''xy''-plane is horizontal and the ''z''-axis points up (and the ''x''- and the ''y''-axis form a positively oriented two-dimensional coordinate system in the ''xy''-plane if observed from ''above'' the ''xy''-plane) is called '''right-handed''' or '''positive'''.
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| The name derives from the [[right-hand rule]]. If the [[index finger]] of the right hand is pointed forward, the [[middle finger]] bent inward at a right angle to it, and the [[thumb]] placed at a right angle to both, the three fingers indicate the relative directions of the ''x''-, ''y''-, and ''z''-axes in a ''right-handed'' system. The thumb indicates the ''x''-axis, the index finger the ''y''-axis and the middle finger the ''z''-axis. Conversely, if the same is done with the left hand, a left-handed system results.
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| Figure 7 depicts a left and a right-handed coordinate system. Because a three-dimensional object is represented on the two-dimensional screen, distortion and ambiguity result. The axis pointing downward (and to the right) is also meant to point ''towards'' the observer, whereas the "middle" axis is meant to point ''away'' from the observer. The red circle is ''parallel'' to the horizontal ''xy''-plane and indicates rotation from the ''x''-axis to the ''y''-axis (in both cases). Hence the red arrow passes ''in front of'' the ''z''-axis.
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| Figure 8 is another attempt at depicting a right-handed coordinate system. Again, there is an ambiguity caused by projecting the three-dimensional coordinate system into the plane. Many observers see Figure 8 as "flipping in and out" between a [[wikt:convex|convex]] cube and a [[wikt:concave|concave]] "corner". This corresponds to the two possible orientations of the coordinate system. Seeing the figure as convex gives a left-handed coordinate system. Thus the "correct" way to view Figure 8 is to imagine the ''x''-axis as pointing ''towards'' the observer and thus seeing a concave corner.
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| {{Clear}}
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| ==Representing a vector in the standard basis==
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| A point in space in a Cartesian coordinate system may also be represented by a position [[Euclidean vector|vector]], which can be thought of as an arrow pointing from the origin of the coordinate system to the point.<ref>{{harvnb|Brannan|Esplen|Gray|1998|loc=Appendix 2, pp. 377–382}}</ref> If the coordinates represent spatial positions (displacements), it is common to represent the vector from the origin to the point of interest as <math>\mathbf{r}</math>. In two dimensions, the vector from the origin to the point with Cartesian coordinates (x, y) can be written as:
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| :<math> \mathbf{r} = x \mathbf{i} + y \mathbf{j} </math>
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| where <math>\mathbf{i} = \begin{pmatrix} 1 \\ 0 \end{pmatrix}</math>, and <math>\mathbf{j} = \begin{pmatrix} 0 \\ 1 \end{pmatrix}</math> are [[unit vectors]] in the direction of the ''x''-axis and ''y''-axis respectively, generally referred to as the ''standard basis'' (in some application areas these may also be referred to as [[versor]]s). Similarly, in three dimensions, the vector from the origin to the point with Cartesian coordinates <math>(x,y,z)</math> can be written as:<ref>{{Cite book| author = David J. Griffiths | title = Introduction to Electrodynamics | publisher=Prentice Hall|year=1999|isbn= 0-13-805326-X}}</ref>
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| :<math> \mathbf{r} = x \mathbf{i} + y \mathbf{j} + z \mathbf{k} </math>
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| where <math>\mathbf{k} = \begin{pmatrix} 0 \\ 0 \\ 1 \end{pmatrix} </math> is the unit vector in the direction of the z-axis.
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| There is no ''natural'' interpretation of multiplying vectors to obtain another vector that works in all dimensions, however there is a way to use [[complex number]]s to provide such a multiplication. In a two dimensional cartesian plane, identify the point with coordinates (''x'', ''y'') with the complex number ''z'' = ''x'' + '''i'''''y''. Here, '''i''' is the complex number whose square is the real number −1 and is identified with the point with coordinates (0,1), so it is '''not''' the unit vector in the direction of the ''x''-axis (this confusion is just an unfortunate historical accident). Since the complex numbers can be multiplied giving another complex number, this identification provides a means to "multiply" vectors. In a three dimensional cartesian space a similar identification can be made with a subset of the [[quaternion]]s.
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| ==Applications==
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| Cartesian coordinates are an abstraction that have a multitude of possible applications in the real world. However, three constructive steps are involved in superimposing coordinates on a problem application. 1) Units of distance must be decided defining the spatial size represented by the numbers used as coordinates. 2) An origin must be assigned to a specific spatial location or landmark, and 3) the orientation of the axes must be defined using available directional cues for (n-1) of the n axes.
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| Consider as an example superimposing 3D Cartesian coordinates over all points on the Earth (i.e. geospatial 3D). What units make sense? Kilometers are a good choice, since the original definition of the kilometer was geospatial...10,000 km equalling the surface distance from Equator to North Pole. Where to place the origin? Based on symmetry, the gravitational center of the Earth suggests a natural landmark (which can be sensed via satellite orbits). Finally, how to orient X, Y and Z axis directions? The axis of Earth's spin provides a natural direction strongly associated with "up vs. down", so positive Z can adopt the direction from geocenter to North Pole. A location on the Equator is needed to define the X-axis, and the [[Prime Meridian]] stands out as a reference direction, so the X-axis takes the direction from geocenter out to [ 0 degrees longitude, 0 degrees latitude ]. Note that with 3 dimensions, and two perpendicular axes directions pinned down for X and Z, the Y-axis is determined by the first two choices. In order to obey the right hand rule, the Y-axis must point out from the geocenter to [ 90 degrees longitude, 0 degrees latitude ]. So what are the geocentric coordinates of the Empire State Building in New York City? Using [ longitude = -73.985656, latitude = 40.748433 ], Earth radius = 40,000 km / 2 pi, and transforming from spherical --> Cartesian coordinates, you can estimate the geocentric coordinates of the Empire State Building, [ x, y, z ] = [ 1330.53 km, -4635.75 km, 4155.46 km ]. GPS navigation relies on such geocentric coordinates.
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| In engineering projects, agreement on the definition of coordinates is a crucial foundation. One cannot assume that coordinates come predefined for a novel application, so knowledge of how to erect a coordinate system where there is none is essential to applying René Descartes' ingenious thinking.
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| While spatial apps employ identical units along all axes, in business and scientific apps, each axis may have different [[units of measurement]] associated with it (such as kilograms, seconds, pounds, etc.). Although four- and higher-dimensional spaces are difficult to visualize, the algebra of Cartesian coordinates can be extended relatively easily to four or more variables, so that certain calculations involving many variables can be done. (This sort of algebraic extension is what is used to define the geometry of higher-dimensional spaces.) Conversely, it is often helpful to use the geometry of Cartesian coordinates in two or three dimensions to visualize algebraic relationships between two or three of many non-spatial variables.
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| The [[graph of a function]] or [[relation (mathematics)|relation]] is the set of all points satisfying that function or relation. For a function of one variable, ''f'', the set of all points (''x'',''y'') where ''y'' = ''f''(''x'') is the graph of the function ''f''. For a function of two variables, ''g'', the set of all points (''x'',''y'',''z'') where ''z'' = ''g''(''x'',''y'') is the graph of the function ''g''. A sketch of the graph of such a function or relation would consist of all the salient parts of the function or relation which would include its relative extrema, its concavity and points of inflection, any points of discontinuity and its end behavior. All of these terms are more fully defined in calculus. Such graphs are useful in calculus to understand the nature and behavior of a function or relation.
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| ==See also==
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| * [[Horizontal and vertical]]
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| * [[Jones diagram]], which plots four variables rather than two.
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| * [[Orthogonal coordinates]]
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| * [[Spherical coordinate system]]
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| * [[Polar coordinate system]]
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| ==Notes==
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| {{Reflist}}
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| ==References==
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| * {{citation|last1=Brennan|first1=David A.|last2=Esplen|first2=Matthew F.|last3=Gray|first3=Jeremy J.|title=Geometry|publisher=Cambridge University Press|place=Cambridge|year=1998|isbn=0-521-59787-0}}
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| * {{citation|last=Burton|first=David M.|title=The History of Mathematics/An Introduction|year=2011|edition=7th|publisher=McGraw-Hill|place=New York|isbn=978-0-07-338315-6}}
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| * {{citation|last=Smart|first=James R.|title=Modern Geometries |edition=5th |publisher=Brooks/Cole|place=Pacific Grove|year=1998|isbn=0-534-35188-3}}
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| == Further reading ==
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| * {{Cite book| author = Descartes, René |authorlink=René Descartes |others=Trans. by Paul J. Oscamp | year = 2001 | title = Discourse on Method, Optics, Geometry, and Meteorology |edition=Revised |publisher=Hackett Publishing |location=Indianapolis, IN |isbn=0-87220-567-3 |oclc=488633510 |url=http://books.google.com/books?id=XKVvclclrnwC}}
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| * {{Cite book| author = Korn GA, Korn TM |year = 1961 | title = Mathematical Handbook for Scientists and Engineers | publisher = McGraw-Hill | location = New York |edition=1st | lccn = 59-14456 | oclc= 19959906 | pages = 55–79}}
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| * {{Cite book| author = [[Henry Margenau|Margenau H]], Murphy GM | year = 1956 | title = The Mathematics of Physics and Chemistry | publisher = D. van Nostrand | location = New York | lccn = 55-10911}}
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| * {{Cite book| author = Moon P, Spencer DE | year = 1988 | chapter = Rectangular Coordinates (x, y, z) | title = Field Theory Handbook, Including Coordinate Systems, Differential Equations, and Their Solutions | edition = corrected 2nd, 3rd print | publisher = Springer-Verlag | location = New York | pages = 9–11 (Table 1.01) | isbn = 978-0-387-18430-2}}
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| * {{Cite book| author = [[Philip M. Morse|Morse PM]], [[Herman Feshbach|Feshbach H]] | year = 1953 | title = Methods of Theoretical Physics, Part I | publisher = McGraw-Hill | location = New York | isbn = 0-07-043316-X | lccn = 52-11515}}
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| * {{Cite book| author = Sauer R, Szabó I | year = 1967 | title = Mathematische Hilfsmittel des Ingenieurs | publisher = Springer Verlag | location = New York | lccn = 67-25285}}
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| ==External links==
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| *[http://www.cut-the-knot.org/Curriculum/Calculus/Coordinates.shtml Cartesian Coordinate System]
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| *[http://www.printfreegraphpaper.com/ Printable Cartesian Coordinates]
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| *{{planetmath reference|id=6016|title=Cartesian coordinates}}
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| *[http://mathworld.wolfram.com/CartesianCoordinates.html MathWorld description of Cartesian coordinates]
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| *[http://www.random-science-tools.com/maths/coordinate-converter.htm Coordinate Converter – converts between polar, Cartesian and spherical coordinates]
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| *[http://www.mathopenref.com/coordpoint.html Coordinates of a point] Interactive tool to explore coordinates of a point
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| *[https://github.com/DanIsraelMalta/CoordSysJS open source JavaScript class for 2D/3D Cartesian coordinate system manipulation]
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| {{Orthogonal coordinate systems}}
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| {{DEFAULTSORT:Cartesian Coordinate System}}
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| [[Category:Coordinate systems]]
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| [[Category:Elementary mathematics]]
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| [[Category:Dimension]]
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| [[Category:René Descartes]]
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| [[Category:Analytic geometry]]
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| [[fi:Koordinaatisto#Suorakulmainen koordinaatisto]]
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