A navigation chart constructed by means of the gnomonic projection has the characteristic that any great-circle that is plotted will be a straight line. The Lambert-conformal charts are similar, but not as accurate. Both of the above charts are called great-circle charts. The Lambert charts are preferred for aircraft navigation, and the gnomonic charts are preferred for maritime use.- - [ HOME ] - - [ NEXT ] - - [ BACK ] - - firstname.lastname@example.org
The obvious use involves drawing a line from the point of departure to the great-circle destination, and then tabulating the latitude and longitude of points along the path, usually at intervals of five degrees of longitude. The reference points can be transferred to a Mercator chart or Mercator plotting sheet and rhumb line courses can be sailed from point to point.
If you were to sail a rhumb line course from San Francisco to Yokohama Japan you could pick a destination point a few miles outside of Yokohama harbor, but if you were to try and use that point for a great circle course you would run aground many miles from your destination. Using a great-circle chart it would be obvious that you would be approaching Japan from the North-Easterly direction and that there would be land in the way, so it would be better to pick a great-circle destination up the coast to avoid collision.
If you were to sail a great-circle course from Seattle Washington to Yokohama the course would cross the Aleutian islands, so the procedure would be to sail the great-circle to a maximum safe latitude, sail directly West until you meet the great-circle again, and then to continue following the great-circle to the great-circle destination point off the coast of Japan. This procedure is known as composite sailing.
The vertex of a great circle path is that point that is nearest the North or South poles of the earth and where the direction is exactly East and West.A second method of composite sailing involves placing a vertex on the limiting latitude that will give a great-circle path from the point of departure, or to the great-circle destination. This can be done graphically on a great-circle chart by placing a ruler from the point of departure to a tangent on the limiting latitude (drawing in the limiting latitude by hand if necessary). In this case, the composite course is in three sections, the starting great-circle path, the East or West path along the limiting latitude, and a second (different) great-circle path to the destination point.