Mercator Projection

Mercator Projection Explained

The Mercator projection is constructed by projecting the Earth's surface onto a cylinder tangent at the equator. When the cylinder is unwrapped, meridians appear as evenly spaced, parallel vertical lines, and parallels of latitude are straight, horizontal lines. This creates a convenient rectangular grid, making it easy to plot courses and bearings.

Key Properties

• Conformal Projection: The Mercator projection preserves angles, so shapes are locally accurate. This is crucial for navigation, as compass bearings (rhumb lines) plot as straight lines.
• Scale Variation: The scale is true only along the equator. As latitude increases, the scale expands according to the secant of the latitude, making landmasses appear much larger near the poles. For example, Greenland appears enormous compared to its actual size.
• Distortion: Distance and area become increasingly exaggerated away from the equator. The projection cannot represent the poles; distortion becomes infinite as you approach them.
• Rhumb Lines vs. Great Circles: On a Mercator chart, rhumb lines (constant compass direction) are straight, but great circles (shortest path between two points) appear as curves, except at the equator and along meridians.

Mercator vs. Lambert

Unlike the Mercator, the Lambert Conformal Conic projection is better suited for mid-latitude aviation charts because it manages scale and area distortion more effectively over large east-west distances. The Mercator is rarely used in aviation today except for some meteorological charts.

Practical Implications

• Distance Measurement: A chart distance measured at one latitude does not represent the same ground distance at another latitude due to scale changes.
• Scale Calculation: To find the scale at a different latitude, multiply the equatorial scale by the secant of the new latitude.

Mercator Projection Causes and Symptoms

The main cause of distortion is the mathematical stretching required to keep meridians parallel and angles correct. The 'symptoms' are exaggerated landmass sizes at high latitudes and the inability to chart the poles.