(0) The shortest line between any two points on the Earth's surface on a spheroid (ellipsoid).
(1) A line of constant bearing, or azimuth. Straight line is a rhumb line.
(2) The line on a spheroid (ellipsoid) defined by the intersection at the surface by a plane that passes through the center of the spheroid and the start and end points of a segment. This is also known as a great circle when a sphere is used.
(3) Straight line is a normal section.
(4) Keeps the original shape of the geometry or curve.