Definition
A map projection in which the curved surface of the Earth is represented on a flat sheet by projecting it onto a cylinder wrapped around the equator. Lines of latitude and longitude appear as straight lines that cross at right angles. A straight line drawn between any two points on a Mercator chart represents a line of constant compass heading (a rhumb line), which makes it especially useful for navigation. The trade-off is that areas near the poles are stretched and appear much larger than they really are.
Plain English
A flat map of the Earth where the lines that run north–south and east–west all appear straight and meet at right angles. It's handy for navigation because you can draw a straight line and fly a single compass heading along it, but it makes regions near the poles look bigger than they actually are.
Context Anchor
Seen in navigation, chart projections, and discussions of how courses are drawn on aeronautical charts.
Derivation
Named after Gerardus Mercator, a Flemish cartographer who introduced this projection in 1569. He designed it specifically so sailors could plot a constant compass course as a straight line, which was a major advance for sea navigation at the time.
Why Pilots Care
Straight lines drawn on the chart represent constant compass headings, allowing simple course plotting without repeated heading changes.
Analogy
Imagine wrapping a sheet of paper around a globe to form a tube touching the equator, then shining a light from the center of the globe outward so the continents project onto the paper. Unroll the paper and you have a Mercator chart — accurate near the equator, increasingly stretched as you move toward the poles.
Intuition Check
Do not read “projection” as a guess about the future. Here it means a method of drawing the curved earth on a flat chart.
Example Sentence 1
The student plotted the cross-country flight on a Mercator projection and noted she could fly a single compass heading the entire way.
Example Sentence 2
Because the Mercator projection stretches distances near the poles, the pilot measured leg lengths carefully before flight planning.