Why Mercator is still used

The job Mercator does well

A constant-bearing route, also called a rhumb line, crosses successive meridians at the same angle. Mercator draws that route as a straight line. This relationship made the projection useful for navigational charts. It is also conformal: on the surface used for the calculation, sufficiently small features retain their angles. Both properties are described in PROJ’s Mercator reference.

A straight constant-bearing route is not generally the shortest route between two places. On a sphere, shortest routes follow great circles; a great-circle route usually curves on a Mercator map. Keeping a heading and minimizing distance are different tasks, so a map can be useful for one without directly solving the other.

Local angle preservation also needs careful wording. It describes a small neighborhood around a point. A continent spans different latitudes and therefore different local scales. Preserving the angles of tiny features does not preserve the complete shape or area of that continent at one uniform scale.

Why a rectangular map is convenient on a screen

Mercator produces a rectangular coordinate plane with simple forward and inverse equations. A finite portion of that plane can be divided into square image tiles, and successive zoom levels can divide each tile into four children. These are useful engineering properties for panning, caching and displaying detail as needed.

That explains a practical attraction of the coordinate system; it is not a claim about why a particular company chose it. A tiled display and a projection are separate decisions. This site uses tiled physical imagery in Equal Earth and Robinson too, by reprojecting the imagery into each map’s coordinate space.

Mercator and Web Mercator are not identical terms

There are several closely related calculations:

Form	Earth model	A distinction that matters
Spherical Mercator	A sphere	Conformal on that sphere; used for this site’s demonstrations.
Ellipsoidal Mercator	An ellipsoid	Includes ellipsoid eccentricity in the north–south equation.
Web Mercator	Spherical equations applied to geographic coordinates	Commonly associated with EPSG:3857; not strictly conformal on the ellipsoid.

PROJ’s Web Mercator documentation describes it as a variant that calculates on a sphere using the ellipsoid’s semimajor axis. The distinctions matter for precision measurements, even where the world maps look similar. Our map’s area calculations use a 6,371 km sphere and should not be substituted for a survey or a distance tool in another coordinate system.

Where Mercator gives a misleading impression

The cost of its local angle property is rapidly increasing area scale toward the poles. A small patch at 60° has four times the map area of an equal geographic patch at the equator in the spherical model. The poles themselves would be infinitely far away on the page.

That makes a whole-world Mercator map a poor basis for comparing country sizes, continental land cover or other questions where colored footprint implies extent. Changing the image resolution does not fix the distortion. Recoloring the map does not fix it either. The coordinate rule must change, or the reader must be given a different way to compare the quantities.

Choose according to the comparison

For a world overview of relative geographic area, try Equal Earth. For a familiar rounded overview without an exact preservation claim, consider Robinson. To teach the difference, place two maps side by side and name their properties explicitly.

The Equal Earth and Mercator guide uses the same geographic data in both views. The map maker can export both projections in one composition. Its frames fit independently, so use the country-area figures or a shared-scale comparison when making a numerical claim.