Guyou Projection



A conformal projection computed with elliptical integrals in the same family as the Peirce projection. Although it could be considered an azimuthal projection, it is listed here as a pseudocylindrical projection because it is often used for "horizontal" display of world regions as are cylindrical projections.


Caution: This is a "one-way" projection in terms of accuracy in that once data is projected into the Guyou coordinate system it cannot be re-projected to other systems without some loss of precision. The definition of the projection in common use does not include formulae for the inverse transformation, so projecting from Guyou to Latitude / Longitude uses iterative approximations and thus can lose precision.




Not true anywhere. Scale is especially false in the corners of the projection, where scale is elongated, and at the center, where scale is compressed.




The projection is conformal.




Useful for showing equatorial regions. In its complete form (the entire Earth), may be tiled infinitely to create a display that allows any point on Earth to be viewed in a direct line from any other point.


Limiting Forms


The Guyou and Peirce projections are transverse cases of each other. The Guyou is the equatorial aspect (hence the classification as a psuedocylindrical projection) and the Peirce is the polar aspect (hence the classification as an azimuthal projection).




Specify the center of the projection by setting the center longitude.




Presented by French mathematician Emile Guyou (1843 - 1915) in 1886.




Emile Guyou (1843 - 1915)


Guyou became a knight of the Légion d'Honneur in 1876 and was elected as a member of the French Academy of Sciences in 1894.  He is memorialized by a street in Toulouse as well as by the Guyou Islands group and Guyou Bay in Antarctica. At the time a Captain in the French navy, Guyou prepared a report on the magnetic results of the Antarctic expedition in 1897-1899 under Gerlache as well as the 1903-1905 Antarctic expedition under Charcot.  He became a Commander of Légion d'Honneur in 1905.