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cartography Cartography (; from grc, χάρτης , "papyrus, sheet of paper, map"; and , "write") is the study and practice of making and using maps. Combining science, aesthetics and technique, cartography builds on the premise that reality (or an im ...
, a Tissot's indicatrix (Tissot indicatrix, Tissot's ellipse, Tissot ellipse, ellipse of distortion) (plural: "Tissot's indicatrices") is a mathematical contrivance presented by French mathematician
Nicolas Auguste Tissot Nicolas Auguste Tissot (; March 16, 1824 – July 14, 1907) was a French cartographer, who in 1859 and 1881 published an analysis of the distortion that occurs on map projections. He devised Tissot's indicatrix, or distortion circle, which when ...
in 1859 and 1871 in order to characterize local distortions due to
map projection In cartography, map projection is the term used to describe a broad set of transformations employed to represent the two-dimensional curved surface of a globe on a plane. In a map projection, coordinates, often expressed as latitude and longitud ...
. It is the geometry that results from projecting a
circle A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre. Equivalently, it is the curve traced out by a point that moves in a plane so that its distance from a given point is const ...
of
infinitesimal In mathematics, an infinitesimal number is a quantity that is closer to zero than any standard real number, but that is not zero. The word ''infinitesimal'' comes from a 17th-century Modern Latin coinage ''infinitesimus'', which originally referr ...
radius from a curved geometric model, such as a globe, onto a map. Tissot proved that the resulting diagram is an
ellipse In mathematics, an ellipse is a plane curve surrounding two focus (geometry), focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. It generalizes a circle, which is the special ty ...
whose axes indicate the two principal directions along which scale is maximal and minimal at that point on the map. A single indicatrix describes the distortion at a single point. Because distortion varies across a map, generally Tissot's indicatrices are placed across a map to illustrate the spatial change in distortion. A common scheme places them at each intersection of displayed meridians and parallels. These schematics are important in the study of map projections, both to illustrate distortion and to provide the basis for the calculations that represent the magnitude of distortion precisely at each point. There is a one-to-one correspondence between the Tissot indicatrix and the
metric tensor In the mathematical field of differential geometry, a metric tensor (or simply metric) is an additional structure on a manifold (such as a surface) that allows defining distances and angles, just as the inner product on a Euclidean space allows ...
of the map projection coordinate conversion.


Description

Tissot's theory was developed in the context of cartographic analysis. Generally the geometric model represents the Earth, and comes in the form 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 ...
or
ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a surface that may be defined as the ...
. Tissot's indicatrices illustrate linear, angular, and areal distortions of maps: *A map distorts distances (linear distortion) wherever the quotient between the lengths of an infinitesimally short line as projected onto the projection surface, and as it originally is on the Earth model, deviates from 1. The quotient is called the ''scale factor''. Unless the projection is conformal at the point being considered, the scale factor varies by direction around the point. *A map distorts angles wherever the angles measured on the model of the Earth are not conserved in the projection. This is expressed by an ellipse of distortion which is not a circle. *A map distorts areas wherever areas measured in the model of the Earth are not conserved in the projection. This is expressed by ellipses of distortion whose areas vary across the map. In conformal maps, where each point preserves angles projected from the geometric model, the Tissot's indicatrices are all circles of size varying by location, possibly also with varying orientation (given the four circle
quadrants Quadrant may refer to: Companies * Quadrant Cycle Company, 1899 manufacturers in Britain of the Quadrant motorcar * Quadrant (motorcycles), one of the earliest British motorcycle manufacturers, established in Birmingham in 1901 * Quadrant Privat ...
split by
meridian Meridian or a meridian line (from Latin ''meridies'' via Old French ''meridiane'', meaning “midday”) may refer to Science * Meridian (astronomy), imaginary circle in a plane perpendicular to the planes of the celestial equator and horizon * ...
s and parallels). In
equal-area projection In cartography, an equal-area projection is a map projection that preserves area measure, generally distorting shapes in order to do that. Equal-area maps are also called equivalent or authalic. An equal-area map projection cannot be conformal, no ...
s, where area proportions between objects are conserved, the Tissot's indicatrices all have the same area, though their shapes and orientations vary with location. In arbitrary projections, both area and shape vary across the map.


Mathematics

In the adjacent image, ABCD is a circle with unit area defined in a spherical or ellipsoidal model of the Earth, and A′B′C′D′ is the Tissot's indicatrix that results from its projection onto the plane. Segment OA is transformed in OA′, and segment OB is transformed into OB′. Linear scale is not conserved along these two directions, since OA′ is not equal to OA and OB′ is not equal to OB. Angle MOA, in the unit area circle, is transformed in angle M′OA′ in the distortion ellipse. Because M′OA′ ≠ MOA, we know that there is an angular distortion. The area of circle ABCD is, by definition, equal to 1. Because the area of ellipse A′B′ is less than 1, a distortion of area has occurred. In dealing with a Tissot indicatrix, different notions of radius come into play. The first is the infinitesimal radius of the original circle. The resulting ellipse of distortion will also have infinitesimal radius, but by the mathematics of differentials, the ratios of these infinitesimal values are finite. So, for example, if the resulting ellipse of distortion is the same size of infinitesimal as on the sphere, then its radius is considered to be 1. Lastly, the size that the indicatrix gets drawn for human inspection on the map is arbitrary. When an array of indicatrices is drawn on a map, they are all scaled by the same arbitrary amount so that their sizes are proportionally correct. Like ''M'' in the diagram, the axes from ''O'' along the parallel and along the meridian may undergo a change of length and a rotation when projecting. It is common in the literature to represent scale along the meridian as ''h'' and scale along the parallel as ''k'', for a given point. Likewise, the angle between meridian and parallel might have changed from 90° to some other value. Indeed, unless the map is conformal, all angles except the one subtended by the
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the long ...
and
semi-minor axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both focus (geometry), foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major wikt: ...
of the ellipse might have changed. A particular angle will have changed the most, and value of that maximum change is known as the angular deformation, denoted as ''θ′''. Generally which angle that is and how it is oriented do not figure prominently in distortion analysis. It is the value of the change that is significant. The values of ''h'', ''k'', and ''θ′'' can be computed as follows. :\begin h &= \frac\sqrt \\ k &= \frac\sqrt \\ \sin \theta ' &= \frac\left( \right) \\ a' &= \sqrt \\ b' &= \sqrt \\ a &= \frac \\ b &= \frac \\ s &= h k \sin \theta ' \\ \omega &= 2 \arcsin\left(\frac\right) \end where ''φ'' and ''λ'' are latitude and longitude, ''x'' and ''y'' are projected coordinates, and ''R'' is the radius of the globe. As results, ''a'' and ''b'' represent the maximum and minimum scale factors at the point, which is the same thing as the semimajor and semiminor axes of the Tissot ellipse; ''s'' represents the amount of inflation or deflation in area (also given by ''a'' ∙ ''b''); and ''ω'' represents the maximum angular distortion at the point. For the
Mercator projection The Mercator projection () is a cylindrical map projection presented by Flemish geographer and cartographer Gerardus Mercator in 1569. It became the standard map projection for navigation because it is unique in representing north as up and sou ...
, and any other conformal projection, ''h'' = ''k'' and ''θ′'' = 90° so that each ellipse degenerates into a circle with the radius ''h'' = ''k'' being equal to the scale factor in any direction at that point. For the
sinusoidal projection The sinusoidal projection is a pseudocylindrical equal-area map projection, sometimes called the Sanson–Flamsteed or the Mercator equal-area projection. Jean Cossin of Dieppe was one of the first mapmakers to use the sinusoidal, appearing in ...
, and any other equal-area projection, the semi-major axis of the ellipse is the reciprocal of the semi-minor axis so that every ellipse has the same area even though their eccentricities vary. For arbitrary projections, neither the shape nor the area of the ellipses are related to each other in general.More general example of Tissot's indicatrix: the Winkel tripel projection.


An alternative derivation for numerical computation

Another way to understand and derive Tissot's indicatrix is through the differential geometry of surfaces. This approach lends itself well to modern numerical methods, as the parameters of Tissot's indicatrix can be computed using
singular value decomposition In linear algebra, the singular value decomposition (SVD) is a factorization of a real or complex matrix. It generalizes the eigendecomposition of a square normal matrix with an orthonormal eigenbasis to any \ m \times n\ matrix. It is related ...
(SVD) and central difference approximation.


Differential distance on the ellipsoid

Let a 3D point, \hat, on an ellipsoid be parameterized as: : \hat(\lambda, \phi) = \left \begin N\cos\cos\\ -N(1-e^2)\sin\\ N\sin\cos \end \right where (\lambda, \phi) are longitude and latitude, respectively, and N is a function of the equatorial radius, R, and eccentricity, e: :N = \frac The element of distance on the sphere, ds is defined by the
first fundamental form In differential geometry, the first fundamental form is the inner product on the tangent space of a surface in three-dimensional Euclidean space which is induced canonically from the dot product of . It permits the calculation of curvature and me ...
: : ds^2 = \begin d\lambda & d\phi \end \begin E & F\\ F & G \end \begin d\lambda\\ d\phi \end whose coefficients are defined as: : \begin &E = \frac \boldsymbol \frac\\ &F = \frac \boldsymbol \frac\\ &G = \frac \boldsymbol \frac\\ \end Computing the necessary derivatives gives: : \frac = \left \begin -N\sin \cos\\ 0\\ N\cos \cos \end \right \qquad\qquad \frac = \left \begin -M\cos \sin\\ -M\cos\\ M\sin \sin \end \right where M is a function of the equatorial radius, R, and the ellipsoid eccentricity, e: : M = \frac Substituting these values into the first fundamental form gives the formula for elemental distance on the ellipsoid: : ds^2 = \left(N \cos\right)^2 d\lambda^2 + M^2 d\phi^2 This result relates the measure of distance on the ellipsoid surface as a function of the spherical coordinate system.


Transforming the element of distance

Recall that the purpose of Tissot's indicatrix is to relate how distances on the sphere change when mapped to a planar surface. Specifically, the desired relation is the transform \mathcal that relates differential distance along the bases of the spherical coordinate system to differential distance along the bases of the Cartesian coordinate system on the planar map. This can be expressed by the relation: : \begin dx \\ dy \end = \mathcal \begin ds(\lambda, 0)\\ ds(0, \phi) \end where ds(\lambda, 0) and ds(0, \phi) represent the computation of ds along the longitudinal and latitudinal axes, respectively. Computation of ds(\lambda, 0) and ds(0, \phi) can be performed directly from the equation above, yielding: : \begin &ds(\lambda, 0) = N \cos(\phi) d\lambda \\ &ds(0, \phi) = M d\phi \end For the purposes of this computation, it is useful to express this relationship as a matrix operation: : \begin d\lambda\\ d\phi \end = K \begin ds(\lambda, 0)\\ ds(0, \phi) \end , \qquad K = \begin \frac & 0\\ 0 & \frac \end Now, in order to relate the distances on the ellipsoid surface to those on the plane, we need to relate the coordinate systems. From the chain rule, we can write: : \begin dx \\ dy \end = J \begin d\lambda \\ d\phi \end where J is the
Jacobian matrix In vector calculus, the Jacobian matrix (, ) of a vector-valued function of several variables is the matrix of all its first-order partial derivatives. When this matrix is square, that is, when the function takes the same number of variables as ...
: : J = \begin \frac & \frac \\ \frac & \frac \end Plugging in the matrix expression for d\lambda and d\phi yields the definition of the transform \mathcal represented by the indicatrix: : \begin dx \\ dy \end = JK \begin ds(\lambda, 0)\\ ds(0, \phi) \end : \mathcal = JK This transform \mathcal encapsulates the mapping from the ellipsoid surface to the plane. Expressed in this form,
SVD ''Svenska Dagbladet'' (, "The Swedish Daily News"), abbreviated SvD, is a daily newspaper published in Stockholm, Sweden. History and profile The first issue of ''Svenska Dagbladet'' appeared on 18 December 1884. During the beginning of the ...
can be used to parcel out the important components of the local transformation.


Numerical computation and SVD

In order to extract the desired distortion information, at any given location in the spherical coordinate system, the values of K can be computed directly. The Jacobian, J, can be computed analytically from the mapping function itself, but it is often simpler to numerically approximate the values at any location on the map using central differences. Once these values are computed, SVD can be applied to each transformation matrix to extract the local distortion information. Remember that, because distortion is local, every location on the map will have its own transformation. Recall the definition of SVD: : \mathrm(\mathcal) = U\Lambda V^T It is the decomposition of the transformation, \mathcal, into a rotation in the source domain (i.e. the ellipsoid surface), V^T, a scaling along the basis, \Lambda, and a subsequent second rotation, U. For understanding distortion, the first rotation is irrelevant, as it rotates the axes of the circle but has no bearing on the final orientation of the ellipse. The next operation, represented by the diagonal singular value matrix, scales the circle along its axes, deforming it to an ellipse. Thus, the singular values represent the scale factors along axes of the ellipse. The first singular value provides the semi-major axis, a, and the second provides the semi-minor axis, b, which are the directional scaling factors of distortion. Scale distortion can be computed as the area of the ellipse, ab, or equivalently by the determinant of \mathcal. Finally, the orientation of the ellipse, \theta, can be extracted from the first column of U as: : \theta = \arctan \left( \frac \right)


Gallery


See also

*
MacAdam ellipse In the study of color vision, a MacAdam ellipse is a region on a chromaticity diagram which contains all colors which are indistinguishable, to the average human eye, from the color at the center of the ellipse. The contour of the ellipse therefore ...


References


External links


Java applet with interactive projections showing Tissot's indicatrix
{{Map projection Cartography