are depicted as conic sections. As with all azimuthal projections, angles from the tangent point are preserved. The map distance from that point is a function r(d) of the true distance d, given by where R is the radius of the Earth. The radial scale is and the transverse scale so the transverse scale increases outwardly, and the radial scale even more. Use Gnomonic projections are used in seismic work because seismic waves tend to travel along great circles. They are also used by navies in plotting direction finding bearings, since radio signals travel along great circles. Meteors also travel along great circles, with the Gnomonic Atlas Brno 2000.0 being the IMO's recommended set of star charts for visual meteor observations. Aircraft and ship pilots use the projection to find the shortest route between start and destination. The gnomonic projection is used extensively in photography, where it is called rectilinear projection. Because they are equivalent, the same viewer used for photographic panoramas can be used to render gnomonic maps . The gnomonic projection is used in astronomy where the tangent point is centered on the object of interest. The sphere being projected in this case is the celestial sphere, R = 1, and not the surface of the Earth. See also Local tangent plane List of map projections Beltrami–Klein model, the analogous mapping of the hyperbolic plane References External links Gnomonic Projection Map projections Navigation Projective geometry