Equidistant projection
Definition
An equidistant projection is a type of map projection that preserves true scale along one or more specific lines or from one or two central points to all other points on the map, allowing distances measured along these designated paths or radii to be proportionally correct relative to the Earth's surface; however, this property of maintaining accurate linear scale in particular directions or from specific centers comes at the cost of distorting area, shape, and scale in all other parts of the map, making this projection particularly useful for applications like radio broadcasting range maps, airline distance charts, or seismic analysis where measuring exact distances from a key location is more critical than maintaining overall geometric fidelity across the entire mapped area.
Application
A key practical application of an equidistant projection is in airline route planning and distance cartography. For instance, an azimuthal equidistant projection centered on a major hub airport like London or Dubai accurately represents true linear distances along the straight lines (great circle routes) radiating from that center to any other city on the map. This allows airline planners and the public to quickly and correctly gauge the flight distance to any destination, assess fuel requirements, and visualize a hub's global connectivity radius without the distance distortion present in other world projections, making it ideal for creating intuitive reference maps for navigation and logistics centered on a specific point of interest.
FAQ
What does an "equidistant" projection actually preserve?
An equidistant projection is designed to preserve true linear scale along one or more specific lines, or from one or two central points to everywhere else on the map. Crucially, it does not preserve scale across the entire map. For example, an Azimuthal Equidistant projection centered on the North Pole shows accurate distances from the Pole to any other point, but distances between two cities in Asia on that same map would be distorted.
How is this different from a map with a constant scale bar?
A standard scale bar on most maps is only valid for a limited area due to projection-induced scale distortion. In an equidistant projection, the property of true distance is built into the mathematical transformation itself along specific paths. You can measure a correct distance with a ruler directly from the central point or along designated lines (like meridians in a cylindrical equidistant projection), without needing to consult a variable scale.
What is the most recognizable example of an equidistant projection?
The most iconic example is the Azimuthal Equidistant projection, famously used for the flag of the United Nations and many logos. It often shows the globe as a circle with the North Pole at the center, radiating lines of accurate distance. Another common type is the Simple Cylindrical (Plate Carr?e) projection, where scale is true along all meridians and the equator.
When should I use one, and what are its major drawbacks?
Use an equidistant projection when your map's primary purpose is to visualize or measure correct distances from a specific location (e.g., mapping earthquake epicenters from a seismic station, showing flight ranges from an airport, illustrating radio broadcast reach). Its major drawback is that, by preserving scale in specific ways, it severely distorts shapes, areas, and scales in all other parts of the map. It is a specialized tool, not suitable for general-purpose world maps where area or shape comparison is needed.
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