DEFINITION

What is an Equidistant Projection?

An equidistant projection is a map projection that accurately preserves distances from one or two specific points or along particular lines, depending on the type of projection used. However, not all distances on the map are preserved – only those defined by the projection's constraints. For example:

In an azimuthal equidistant projection, distances from the center point to any other point on the map are accurate.
In a conic or cylindrical equidistant projection, distances are preserved along specific meridians or parallels.

Understanding Distance Accuracy

While equidistant projections do not preserve shape or area, they are useful for applications that require accurate distances from a specific location, such as in radio communication, seismic mapping, or airline route planning.

On a globe, the shortest distance between two points is along a great-circle path. On an equidistant map centered at one of those points, this great-circle distance is represented correctly as a straight line from the center to the other point. However, distances between two non-central points may not be accurate unless the projection specifically maintains those.

Example

In the map, different types of distance are shown to demonstrate a key property of the projection: it preserves distance and direction from the center, but not between other locations.

The black straight lines all start at the center of the projection at (0°, 0°). In an Azimuthal Equidistant Projection, these lines represent true great-circle distances from the center. The length and label of each black line correctly reflect the real-world distance from the center to each endpoint.
The red curved line represents a great-circle path–the shortest routes on a globe between two points that do not include the center. While the line correctly illustrate the spherical path direction, the distance is distorted in this projection and should not be measured directly.
The red straight line across the Atlantic does not represent an accurate distance in this projection either, because neither endpoint is at the center of the projection (0°, 0°). Despite appearing straight, the distance and direction are distorted due to projection limitations. Only straight lines from the center to other points (like the black lines) are valid representations of true great-circle distances.

GIS Software

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DEFINITION

What is an Equidistant Projection?

Understanding Distance Accuracy

Example

GIS Software

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