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Chart Projections
(Bowditch, The American Practical Navigator, 2002)

Because a cartographer cannot transfer a sphere to a flat surface without distortion, he must project the surface of a sphere onto a developable surface. A developable surface is one that can be flattened to form a plane. This process is known as chart projection. If points on the surface of the sphere are projected from a single point, the projection is said to be perspective or geometric.

As the use of electronic charts becomes increasingly widespread, it is important to remember that the same cartographic principles that apply to paper charts apply to their depiction on video screens.

The majority of NOAA charts use a Mercator projection, classified as a cylindrical projection upon a plane, the cylinder tangent along the equator. The Mercator is the most common projection used in maritime navigation, primarily because rhumb lines plot as straight lines.

Some characteristics of a Mercator projection:
-Both meridians and parallels are expanded at the same ratio with increased latitude
-Expansion is the same in all directions and angles are shown correctly (conformal)
-Rhumb lines appear as straight lines, the directions of which can be measured on chart
-Distances can be measured directly to practical accuracy


Graphic shows a map of the world in a Mercator Projection.  Longitude lines are parallel and latitude lines are parallel.
MERCATOR MAP OF THE WORLD: Bowditch Practical Navigator (2002)
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