Distance

An equidistant map preserves the property of distance. Another way to say this is that the map has true scale. But wait, this has to be qualified.

Consider a map with a stated, or nominal, scale of 1:100,000. You might naturally assume this means than a measurement of 1 unit on the map is equal to 100,000 units on the earth. In fact, that is what it means—but on most maps it's only true of a few special lines, such as standard parallels.

Even on an equidistant map, true scale is not possible for all lines. Back in Module 2, you learned that an equidistant map preserves true scale for all lines passing through (or radiating from) a single, specific point.

An Azimuthal Equidistant projection centered on New Delhi, India. Any straight line radiating from New Delhi has true scale and follows the arc of a great circle.

True scale for more than one point

The Two-Point Equidistant projection maintains true scale for all lines that pass through either of two points, such as New Delhi and Rio de Janeiro or Hong Kong and Honolulu. The Chamberlin Trimetric projection maintains almost true scale for any three points. This projection is supported only in Workstation ArcInfo™.

The term “equidistant” is also applied to other projections that maintain true scale along all meridians.

Top: An Equidistant Cylindrical projection. Bottom: An Equidistant Conic projection. Both correctly measure the distance from pole to pole (about 20,000 kilometers) along each meridian.

There are also projections that maintain true scale the other way—along all parallels. The Sinusoidal is one example and the Polyconic is another. But these projections are not considered equidistant.