What is difference between Mercator projection and Gnomonic projection?

On a mercator projection chart, lines of latitude are parallel as are lines of longitude. On gnomonic projection charts, meridians converge and lines of latitude are curved.
A rhumb line course of 040° crosses each meridian (lines of longitude) at the same angle. Great circle routes are straight lines and rhumb lines are curved.
A rhumb line course is used in all coastal navigation. If a passage is over 600 miles it may be quicker to sail a great circle route, in practice this not always practical as winds, currents and weather systems all influence the choice of the best route. A great circle is the longest line that can be drawn around the earth. A segment of a great circle is the shortest distance between two points on the earth surface.


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