The equidistant, or simple, conic projection preserves distances along all meridians and two standard parallels. This projection often serves as a compromise between Lambert conformal conic and Albers equal-area conic projections. It is best suited for land masses extending in an east-to-west orientation at mid-latitudes when area, directions, and angles do not need to be maintained.
The basic projection form was first described by Claudius Ptolemy about A.D. 100 and various improvements were made over time, the biggest by Nicolas de l'Isle in 1745. It is available in ArcGIS Pro 1.0 and later and in ArcGIS Desktop 8.0 and later.
The subsections below describe the equidistant conic projection properties.
Equidistant conic is a conic projection. All the meridians are equally spaced straight lines converging to a common point. The parallels and both poles are represented as circular arcs which are equally spaced and centered on the point of convergence of the meridians. When standard parallels are set on the northern hemisphere, the fan-shape of the graticule is oriented up. When standard parallels are on the southern hemisphere, the fan-shape of the graticule is oriented down. The graticule is symmetric across the central meridian.
The equidistant conic projection preserves distances along all meridians and two standard parallels. Shapes, areas, distances, directions, and angles are all generally distorted. There is no distortion along the standard parallels. Distortion values grow away from the standard parallels. They are the same along any given parallel and symmetric across the central meridian.
This projection is best suited for land masses extending in an east-to-west orientation at mid-latitudes when area, directions, and angles do not need to be maintained. It is best practice to place standard parallels at one-sixth of the latitude range below the top and above the bottom of the area to be mapped. This projection is often used for mapping Russia.
The standard parallels can be set at any latitude, except set at opposite poles.
Equidistant conic parameters are as follows:
- False Easting
- False Northing
- Central Meridian
- Standard Parallel 1
- Standard Parallel 2
- Latitude Of Origin
Particular parameter cases
If both standard parallels are set to the same pole, resulting projection is the azimuthal equidistant projection in polar aspect. When both standard parallels are set at the equator, the outcome is the plate carrée projection. The equidistant cylindrical projection can be obtained by setting the standard parallels symmetrically north and south of the equator.
Snyder, J. P. (1987). Map Projections: A Working Manual. U.S. Geological Survey Professional Paper 1395. Washington, DC: United States Government Printing Office.
Snyder, J. P. (1993). Flattening the Earth. Two Thousand Years of Map Projections. Chicago and London: University of Chicago Press.
Snyder, J. P. and Voxland, P. M. (1989). An Album of Map Projections. U.S. Geological Survey Professional Paper 1453.Washington, DC: United States Government Printing Office.