Equal-area projection

"Area-preserving maps" redirects here. For the mathematical concept, seeMeasure-preserving dynamical system.

The equal-areaMollweide projection

Incartography,anequivalent,authalic,orequal-area projectionis amap projectionthat preserves relativeareameasure between any and all map regions. Equivalent projections are widely used for thematic maps showing scenario distribution such as population, farmland distribution, forested areas, and so forth, because an equal-area map does not change apparent density of the phenomenon being mapped.

ByGauss'sTheorema Egregium,an equal-area projection cannot beconformal.This implies that an equal-area projection inevitably distorts shapes. Even though a point or points or a path or paths on a map might have no distortion, the greater the area of the region being mapped, the greater and more obvious the distortion of shapes inevitably becomes.

Lambert azimuthal equal-area projection of the world centered on 0° N 0° E.

Description

In order for a map projection of the sphere to be equal-area, its generating formulae must meet thisCauchy-Riemann-like condition:

${\frac {\partial y}{\partial \varphi }}\cdot {\frac {\partial x}{\partial \lambda }}-{\frac {\partial y}{\partial \lambda }}\cdot {\frac {\partial x}{\partial \varphi }}=s\cdot \cos \varphi$

where$s$is constant throughout the map. Here,$\varphi$represents latitude;$\lambda$represents longitude; and$x$and$y$are the projected (planar) coordinates for a given$(\varphi,\lambda )$coordinate pair.

For example, thesinusoidal projectionis a very simple equal-area projection. Its generating formulae are:

${\begin{aligned}x&=R\cdot \lambda \cos \varphi \\y&=R\cdot \varphi \end{aligned}}$

where$R$is the radius of the globe. Computing the partial derivatives,

${\frac {\partial x}{\partial \varphi }}=-R\cdot \lambda \cdot \sin \varphi,\quad R\cdot {\frac {\partial x}{\partial \lambda }}=R\cdot \cos \varphi,\quad {\frac {\partial y}{\partial \varphi }}=R,\quad {\frac {\partial y}{\partial \lambda }}=0$

and so

${\frac {\partial y}{\partial \varphi }}\cdot {\frac {\partial x}{\partial \lambda }}-{\frac {\partial y}{\partial \lambda }}\cdot {\frac {\partial x}{\partial \varphi }}=R\cdot R\cdot \cos \varphi -0\cdot (-R\cdot \lambda \cdot \sin \varphi )=R^{2}\cdot \cos \varphi =s\cdot \cos \varphi$

with$s$taking the value of the constant$R^{2}$.

For an equal-area map of theellipsoid,the corresponding differential condition that must be met is:

${\frac {\partial y}{\partial \varphi }}\cdot {\frac {\partial x}{\partial \lambda }}-{\frac {\partial y}{\partial \lambda }}\cdot {\frac {\partial x}{\partial \varphi }}=s\cdot \cos \varphi \cdot {\frac {(1-e^{2})}{(1-e^{2}\sin ^{2}\varphi )^{2}}}$

where$e$is theeccentricityof the ellipsoid of revolution.

Statistical grid

The term "statistical grid" refers to adiscrete grid(global or local) of an equal-area surface representation, used fordata visualization,geocodeandstatistical spatial analysis.

List of equal-area projections

These are some projections that preserve area:

• Azimuthal
  • Lambert azimuthal equal-area
  • Wiechel(pseudoazimuthal)

Albers projection of the world with standard parallels 20° N and 50° N.

• Conic
  • Albers
  • Lambert equal-area conic projection

Bottomley projection of the world with standard parallel at 30° N.

• Pseudoconical
  • Bonne
  • Bottomley
  • Werner

Lambert cylindrical equal-area projection of the world

• Cylindrical(with latitude of no distortion)
  • Lambert cylindrical equal-area(0°)
  • Behrmann(30°)
  • Hobo–Dyer(37°30′)
  • Gall–Peters(45°)

Equal Earth projection, an equal-area pseudocylindrical projection

• Pseudocylindrical
  • Boggs eumorphic
  • Collignon
  • Eckert II,IVandVI
  • Equal Earth
  • Goode's homolosine
  • Mollweide
  • Sinusoidal
  • Tobler hyperelliptical
• Other
  • Eckert-Greifendorff
  • McBryde-Thomas Flat-Polar Quartic Projection
  • Hammer
  • Strebe 1995
  • Snyder equal-area projection,used forgeodesic grids.

See also

• Authalic latitude
• Authalic radius
• Equiareal map (mathematics)
• Measure-preserving dynamical system
• Geodesic polygon area