Geocart Projections

What is a projection?

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loximuthal

Parameters: Latitude of origin

Classifications

Geocart menu class: Pseudocylindric
Pseudocylindric
Equally spaced parallels
Neither conformal nor equal area

Graticule

Meridians: Central meridian is a straight line generally over half as long as the Equator, depending on the central latitude. If the central latitude is the Equator, the ratio is 0.5; if it is 40° N. or S., the ratio is 0.65. Other meridians are equally spaced complex curves intersecting at the poles and concave toward the central meridian.
Parallels: Equally spaced straight parallel lines. Perpendicular to the central meridian.
Poles: Points
Symmetry: About the central meridian. Symmetry occurs about the Equator if the specified central latitude is the Equator.

Scale

True along the central meridian
Constant along any given latitude; normally different for the latitude of opposite sign

Distortion

Distortion is moderate to extreme, but the projection is intended for the special property noted below, not as a normal world map. Free of distortion only at the central latitude on the central meridian.

Special feature

Loxodromes (rhumb lines) from the central point (at the intersection of the central meridian and the central parallel) are shown straight, true to scale, and correct in azimuth from the center. Because of angular distortion on the map projection, the azimuths with respect to other points along a rhumb line are not shown correctly.

Origin

Presented by Karl Siemon (?-1937) of Germany in 1935.
Independently presented as “Loximuthal” by Waldo R. Tobler (1930- ) of the University of Michigan (1966).

Similar projections

Loxodromes are shown straight and at the correct azimuth between any two points on the regular Mercator projection, but they are not at true scale.
The Bordone Oval projection of 1520 is similar to the Loximuthal projection having the Equator as the central latitude.

Description adapted from J.P. Snyder and P.M. Voxland, An Album of Map Projections, U.S. Geological Survey Professional Paper 1453. United States Government Printing Office: 1989.