Hammer projection

Hammer projection of the world

The Hammer projection is an equal-area map projection described by Ernst Hammer in 1892. Using the same 2:1 elliptical outer shape as the Mollweide projection, Hammer intended to reduce distortion in the regions of the outer meridians, where it is extreme in the Mollweide.

Development

Directly inspired by the Aitoff projection, Hammer suggested the use of the equatorial form of the Lambert azimuthal equal-area projection instead of Aitoff's use of the azimuthal equidistant projection:

{\begin{aligned}x&=\operatorname {laea} _{x}\left({\frac {\lambda }{2}},\varphi \right)\\y&={\tfrac {1}{2}}\operatorname {laea} _{y}\left({\frac {\lambda }{2}},\varphi \right)\end{aligned}}

where laea_x and laea_y are the x and y components of the equatorial Lambert azimuthal equal-area projection. Written out explicitly:

{\begin{aligned}x&={\frac {2{\sqrt {2}}\cos \varphi \sin {\frac {\lambda }{2}}}{\sqrt {1+\cos \varphi \cos {\frac {\lambda }{2}}}}}\\y&={\frac {{\sqrt {2}}\sin \varphi }{\sqrt {1+\cos \varphi \cos {\frac {\lambda }{2}}}}}\end{aligned}}

The inverse is calculated with the intermediate variable

z\equiv {\sqrt {1-\left({\tfrac {1}{4}}x\right)^{2}-\left({\tfrac {1}{2}}y\right)^{2}}}

The longitude and latitudes can then be calculated by

{\begin{aligned}\lambda &=2\arctan {\frac {zx}{2\left(2z^{2}-1\right)}}\\\varphi &=\arcsin zy\end{aligned}}

where λ is the longitude from the central meridian and φ is the latitude.^[1]^[2]

Visually, the Aitoff and Hammer projections are very similar. The Hammer has seen more use because of its equal-area property. The Mollweide projection is another equal-area projection of similar aspect, though with straight parallels of latitude, unlike the Hammer's curved parallels.

Briesemeister

William A. Briesemeister presented a variant of the Hammer in 1953. In this version, the central meridian is set to 10°E, the coordinate system is rotated to bring the 45°N parallel to the center, and the resulting map is squashed horizontally and reciprocally stretched vertically to achieve a 7:4 aspect ratio instead of the 2:1 of the Hammer. The purpose is to present the land masses more centrally and with lower distortion.^[3]

Nordic

Before projecting to Hammer, John Bartholomew rotated the coordinate system to bring the 45° north parallel to the center, leaving the prime meridian as the central meridian. He called this variant the "Nordic" projection.^[3]

References

↑ Flattening the Earth: Two Thousand Years of Map Projections, John P. Snyder, 1993, pp. 130–133, ISBN 0-226-76747-7.
↑ Weisstein, Eric W. "Hammer–Aitoff Equal-Area Projection." From MathWorld—A Wolfram Web Resource
1 2 Snyder, John P. (1989). An Album of Map Projections (PDF). p. 162.

External links

Wikimedia Commons has media related to Hammer projection.

Map projection

History
List
Portal

By surface

Cylindrical

Mercator-conformal	Gauss–Krüger Transverse Mercator

Equal-area	Balthasart Behrmann Gall–Peters Hobo–Dyer Lambert Smyth equal-surface Trystan Edwards

Cassini Central Equirectangular Gall stereographic Miller Space-oblique Mercator Web Mercator

General perspective	Gnomonic Orthographic Stereographic

Equidistant Lambert equal-area

Bonne	Sinusoidal Werner

Bottomley	Sinusoidal Werner

Cylindrical	Balthasart Behrmann Gall–Peters Hobo–Dyer Lambert cylindrical equal-area Smyth equal-surface Trystan Edwards

Tobler hyperelliptical	Collignon Mollweide

Albers Briesemeister Eckert II Eckert IV Eckert VI Hammer Lambert azimuthal equal-area Quadrilateralized spherical cube

Equidistant in
some aspect

Gnomonic

Gnomonic

Loxodromic

Retroazimuthal
(Mecca or Qibla)

Planar	Gnomonic Orthographic Stereographic

Central cylindrical

Polyhedral

See also

Latitude Longitude Tissot's indicatrix

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