Introduction to Types of Charts: Gnomonic, Polyconal, and Mercator Projection Basics

A basic understanding of the types of chart projections is helpful to understand the coastal cruising charts you are most likely to encounter for sailing and their limitations. Here’s a brief rundown on the three main projections, with a few pro’s and con’s and when they are most likely to be utilized.

Different projects are more advantageous for different uses. The chart projection type you choose for a long around-the-world voyage will be different from one you use to navigate into a harbor.

Gnomonic Charts:

Used in passage planning to plot great circle routes (shortest distance) as a straight line and use the segment showing the shortest route between end points. They are useful for devising composite rhumb line courses. Rhumb lines: Loxodrome: cross all meridians at the same angle. Rhumb lines: are tracks with a constant track direction between two points on a sphere and therefore must be a longer distance than a Great Circle track: Orthodrome. (Circular view from the poles)

Polyconic Projection

The polyconic projection also is known as American polyconic or ordinary polyconic (many cones) projection. They are created by lining up an infinite number of cones along the central meridian. This affects the shape of the meridians. Unlike other conic projections, the meridians are curved rather than straight. The projection is neither conformal nor equal-area. It is appropriate for regions of predominant north-south extent. (Looks like two big circular areas.) 

Mercator Projection

Mercator projections use parallel longitude and latitude lines to create accurate angles. Continents at the poles appear larger than they should.

Mercator projections show meridians of longitude as parallel. Lines of latitude, while still parallel, are no longer equidistant but wider apart further north. This causes distortion in the shape and size of land and ocean masses as these features are enlarged closer to the poles. “Parallels of latitude and meridians of longitude.”

The advantage of a Mercator chart is that meridians and parallels cross each other at right angles to form a rectangular grid. This means a straight-line course through the water can be plotted as a straight line on the chart – something that’s not possible on other projections.

Only the vertical, latitude scale on a Mercator chart can be used to measure distance, not the horizontal longitude scale. 

This is because each minute of latitude equals one nautical mile, while the distance between minutes of longitude decreases between the equator and the poles.

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