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Day Skipper - Part 4 - Flattening
the earth
The shape of the earth is projected onto charts in two main ways, called
projections. Either way, they are a compromise because they project a
curved surface on to a flat format.
Mercator's projection
(most yachting charts)
Lines of longitude (vertical) are parallel. To stabilise the projection,
the lines of latitude are also parallel, but they get further apart the
further from the equator they are. So it is important to measure distances
off the latitude scale at the appropriate latitude because one nautical
mile (one minute of latitude) closer to the equator will not be exactly
the same as one nautical mile further away. Mercator projection is not
ideal at high latitudes because land masses are greatly distorted.
Gnomonic projection
(pronounced no-monic) 
This is used on large-scale harbour charts and on small-scale polar
charts. On a gnomonic chart all great circles appear as straight lines.
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