Definition
Projected from the centre of the Earth onto a tangent plane, the gnomonic renders every great circle as a straight line, which is why ocean planning charts and polar charts use it. Angles and shapes are badly distorted away from the tangent point and no more than a hemisphere can be shown. Practical use is to draw the great circle track as a straight line, read off intermediate positions at convenient longitude intervals, and transfer those to a Mercator chart as a series of rhumb legs.
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