Browse Sub-Categories. Gnomonic Charts are used in passage planning to plot great circle routes as straight lines and for devising composite rhumb line courses. Show More. International Admiralty Chart Agent Delivering navigation safety since Adding item to basket.
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Mercator projection. A chart which is very useful in great circle sailing based on the gnomonic projection. This is a perspective projection in which part of a spherical surface is projected from the centre of the sphere onto a plane surface tangential to the sphere's surface. The principal property of this projection is that great circle arcs are projected as straight lines. In order to draw a great circle on a Mercator chart—the projection being a relatively complex curve always concave to the equator—the route is first drawn on a gnomonic chart by connecting the plotted positions of the places of departure and destination with a straight line.
Positions of a series of points on this line are taken from the gnomonic chart and marked on the Mercator chart. A fair curve is then drawn through these points, which is the required projection of the great circle route on the Mercator chart.
The gnomonic chart became popular with the publication by Hugh Godfray in of two polar gnomonic charts covering the greater part of the world, one for the northern and the other for the southern hemisphere. Although it was generally believed that Godfray was the original inventor of this method of great circle sailing, it is interesting to note that a complete explanation of the construction of a polar gnomonic chart, with a detailed example of a great circle route from the Lizard to the Bermudas, appeared in Samuel Sturmey's Mariners' Mirror, of All Rights Reserved.
Password Please enter your Password. Forgot password? Don't have an account? Sign in via your Institution. You could not be signed in, please check and try again. Sign in with your library card Please enter your library card number. Related Content Related Overviews Mercator projection. Show Summary Details Overview gnomonic chart. Projection of a gnomonic chart. Subjects: History. All rights reserved.
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A gnomonic map projection displays all great circles as straight lines, resulting in any straight line segment on a gnomonic map showing a geodesic , the shortest route between the segment's two endpoints. This is achieved by casting surface points of the sphere onto a tangent plane, each landing where a ray from the center of the sphere passes through the point on the surface and then on to the plane. No distortion occurs at the tangent point, but distortion increases rapidly away from it. Less than half of the sphere can be projected onto a finite map. The gnomonic projection is said to be the oldest map projection, developed by Thales in the 6th century BC  : The path of the shadow-tip or light-spot in a nodus-based sundial traces out the same hyperbolae formed by parallels on a gnomonic map. The gnonmonic projection is from the centre of a sphere to a plane tangential to the sphere Fig 1 below.
Mercator projection. A chart which is very useful in great circle sailing based on the gnomonic projection. This is a perspective projection in which part of a spherical surface is projected from the centre of the sphere onto a plane surface tangential to the sphere's surface. The principal property of this projection is that great circle arcs are projected as straight lines.