Gnomonic is a perspective projection that projects onto a tangent plane from a position in the center of the Earth. Because of the close perspective, this projection is limited to less than a hemisphere. However, it is the only projection which shows all great circles as straight lines. With a polar aspect, the latitude intervals increase rapidly from the center outwards.
Polar aspect: the meridians are straight lines radiating from the point of tangency.
Oblique and equatorial aspects: the meridians are straight lines.
Polar aspect: the parallels are concentric circles.
Oblique and equatorial aspects: parallels are ellipses, parabolas, or hyperbolas concave toward the poles (except for the Equator, which is straight).
Polar aspect: the meridian spacing is equal and increases away from the pole. The parallel spacing increases rapidly from the pole.
Oblique and equatorial aspects: the graticule spacing increases very rapidly away from the center of the projection.
Linear scale and angular and areal deformation are extreme, rapidly increasing away from the center of the projection.
Seismic work because seismic waves travel in approximately great circles.
With an equatorial or oblique aspect, the Equator is straight. Meridians are straight and parallel, while intervals between parallels increase rapidly from the center and parallels are convex to the Equator.
Because great circles are straight, this projection is useful for air and sea navigation. Rhumb lines are curved, which is the opposite of the Mercator projection.