Projection Options

There are several options for the type of geographic projection to use for georeferenced design layers. For larger maps, in particular, the proper projection can be very important.

On the Document Georeferencing and the Georeferencing dialog boxes, the projections at the top of the Projection list are variations based upon the basic types, which display at the bottom of the list. For example, the Plate Carrée option is based upon the Equirectangular projection. The following table describes the basic projection types.

Projection Type	Description
Equirectangular	Simplest projection, good for maps of the entire world. Latitude and longitude lines are straight and equally spaced. The Lat/Lon Document Units projection is Equirectangular, centered around the equator and prime meridian, and scaled such that one document unit represents one degree of latitude and longitude. The Plate Carrée projection is Equirectangular, centered around the equator and prime meridian, and scaled in real-world units.
Azimuthal Equidistant	Good for areas centered around a given point, roughly equal in height and width. Straight lines drawn from the center point represent the accurate geographic distance.
Transverse Mercator	Excellent for mapping narrow (north-south) areas around a chosen longitude. Very great distortion farther east and west. Center longitude defines the chosen meridian; center latitude should be close to the area being mapped. Scale should be 1 or close to it (for example .9996). The Universal Transverse Mercator projection is Transverse Mercator defined by 6° longitude zones. In the US State Plane Coordinate System (SPCS) projections, most US state zones are either Transverse Mercator or Lambert Conformal Conic, depending on the shape of the state and its zones. There are two systems, based on two different North American Datums: NAD27 (now obsolete) and NAD83 (the current standard).
Cassini-Soldner	Good for mapping narrow (north-south) areas around the central longitude. Distorts shapes farther east and west.
Gnomonic	Good for navigational maps and small areas around a chosen point. All straight lines represent great circles (shortest geographical distance).
Lambert Conformal Conic	Good for mapping wide (east-west) areas between chosen parallels. Distortion increases farther from the chosen parallels. In the US State Plane Coordinate System (SPCS) projections, most US state zones are either Transverse Mercator or Lambert Conformal Conic, depending on the shape of the state and its zones. There are two systems, based on two different North American Datums: NAD27 (now obsolete) and NAD83 (the current standard).
Stereographic	Good for mapping polar areas. Parameters define the center point, which is 90 latitude for the North Pole or -90 latitude for the South Pole. Other points can be used. Scale should be 1 or very close to it (for example .9996).