# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Great Circle Charts**

**From:**Lu Abel

**Date:**2001 Sep 05, 10:26 PM

At 04:26 PM 8/29/2001 +0200, Russel Sher wrote: I think it is a Gnomonic projection (conical projection ?) The great circle chart is indeed a gnomic projection (which is not the same as a conical). One of the properties of the projection is that all straight lines are great circles. The most common gnomic projections one sees are in the classic classroom map which has a Mercator of most of the world and then the two polar areas shown with gnomics. Quick lesson on projections. The problem is to show the globe (a sphere) on a flat piece of paper. Basically, possible only with distortion (hence the fact that Greenland appears bigger than Mexico in the classic Mercator map) While this is a technical oversimplification, one can think of a projection as mapping from the surface of the earth onto the sheet of paper by shining a light at the center of the earth through the earth's surface and thence tracing things onto the surface of the paper. You can make three geometrical shapes with a sheet of paper -- leave it flat, roll it into a cone, or roll it into a cylinder. Which shape the paper has while doing the projection gives us the three types of projections. Flat sheet - gnomic projection The sheet touches the earth's surface at a single point, called the point of tangency. On polar maps, it's the pole. On great circle charts, the point of tangency is indicated on the chart. Distorts both sizes and directions, therefore not a very useful projection *except* for determining great circle courses. Cone - conic projection Not used a lot in navigation because parallels of latitude form arcs and meridians of longitude fan out from the nearer pole, so it's hard to determine courses. But used a lot in mid-latitude land maps because it distorts sizes of features the least. For example, with a Lambert projection where the cone isn't tangent to the earth's surface but actually briefly goes beneath it, the 48 contiguous states of the US can be mapped with less than 2% distortion in size. Cylinder - cylindrical projection The Mercator chart is the archetype of a cylindrical projection. Biggest advantage is that the parallels and meridians are parallel and perpendicular. True direction is the same all across the chart. Also while land masses grow in size as one gets away from the equator, their shape is shown correctly. There's a good summary of projections in Dutton's. Lu Abel