The stronger line marks the equator.
Why this map exists
A map made for a course at sea.
Make a course of constant compass bearing easy to draw on a chart. Such a route is called a rhumb line; it is not generally the shortest route between two places.
How the idea developed
1569
A world map for navigation
Gerardus Mercator published the world map associated with this projection in 1569. Its expanded spacing toward the poles makes the chart look different from a regular latitude–longitude grid.
Royal Museums Greenwich · Mercator’s 1569 world mapThe navigational idea
A straight line can hold a bearing
Mercator’s importance lies in the relationship between angles on Earth and angles on the chart. The classic form turns constant-bearing courses into straight lines, while sacrificing comparable areas across latitudes.
Snyder · USGS, Map projections: A working manualDigital mapping
A related projection reaches the web
Web Mercator is widely used by online mapping services. It is a related spherical calculation applied to geographic coordinates, rather than the ellipsoidal Mercator formulation. MapStudy’s explorer uses a spherical Earth model.
PROJ · Web Mercator / Pseudo Mercator
What you gain. What changes.
What stays true
Local angles and the shapes of very small regions. In the classic orientation, rhumb lines are straight.
What changes
Scale increases toward the poles. Regions at high latitudes occupy much more map area relative to regions near the equator. Neither pole can fit on a finite classic Mercator map.
Projection properties: PROJ · Mercator
A common misunderstanding
“Preserves shape” does not mean an entire large country keeps its exact shape or size. The property is local, and area distortion can be very large.
What changes in a country-centered view?
These historical descriptions refer to the classic orientation. MapStudy can rotate the projection’s reference frame so the place facing you becomes the center. The projection still preserves its defining property, but references to the equator, poles, and navigation do not transfer unchanged to the rotated view. Geographic latitude and longitude lines may curve.
The unfolding animation illustrates a mathematical transition. The intermediate shapes are not separate named projections, and Earth is not physically peeled into this map.
See the idea for yourself
What happens to the same small area as it moves north?
- Open the classic Mercator view and unfold Earth.
- Move the orange sample from 0° to 60° latitude. Its local area scale rises from 1.0× to 4.0×.
- Compare Mercator with Mollweide. Look at proportions within each map, rather than pixel sizes across panels.
Opens in another tab so you can keep the steps nearby.
Sources & further reading
The history and explanations are original MapStudy writing based on the references linked above. Uncertain origins are identified as such. We credit the projection’s creators and later advocates; MapStudy’s contribution is this interactive learning experience.
Our diagrams use the same spherical equations as the explorer. They show geographic grids, not historical source material.