Why Mercator exists.
The Mercator projection, published in 1569, is conformal. At any sufficiently small location, angles and shapes are preserved. A line of constant compass bearing, or rhumb line, is straight on the map. That property is valuable for navigation, although the shortest route between two distant points is generally a great-circle route on a sphere, not a rhumb line.
Mercator is not a mistake. The problem begins when a map designed around one property is used to answer a different question.
Section references[3]
Why the north looks larger.
On the spherical Mercator projection, local linear scale grows as 1 / cos φ, where φ is latitude. Local area scale therefore grows as 1 / cos² φ, relative to the equator. At 60° latitude a tiny area appears four times as large; at 80° it appears about 33.2 times as large. The same happens south of the equator. The poles are infinitely far away and cannot be shown.
These are local factors, not a single correction factor for a whole country. A large region spans many latitudes. Its total map area depends on integrating distortion across the region. Our tool clearly separates the local calculation from the area of a finite patch.
Section references[3]
The little map lab
An area on the move.
Compare the two patches within each panel. Both have equal area on the globe.
The comparison that matters.
| Property | Mercator | Equal Earth |
|---|---|---|
| Relative area | Distorted away from the equator | Preserved |
| Local angles | Preserved | Not preserved everywhere |
| Poles | Cannot be represented | Finite polar lines |
| Good question | What is a constant bearing? | How large are regions relative to each other? |
Do not compare the wrong things.
Web Mercator, familiar from interactive web maps, uses a spherical-style formula with geographic coordinates commonly associated with an ellipsoid. It is not precisely the same as an ellipsoidal Mercator projection. Our educational comparison uses a sphere for both projections and clips Mercator near ±85.05°. No product map is regenerated from this demonstration.
Give students both maps and a globe. Ask what each representation makes easy to see, what it hides and which question would be unfair to ask of it.
Section references[3]
