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Planning and Constructing Great-Circle Tracks
Considerations
The navigator (NAV) and assistant navigator (ANAV) must lay out the
ship’s complete intended track on the proper chart format. This task is
undertaken after the planning stage is complete but several days or
weeks before getting under way, depending on the length of cruise.
If your track will be less than 300 nautical miles, a small-scale Mercator
chart will be adequate. However, for those tracks exceeding 300
nautical miles, you will probably use the gnomonic or great-circle chart.
There may be some cruises longer than 300 nautical miles where a
Mercator or other type of chart is more appropriate than the great-circle
chart.
You will recall from chapter 1 the shortest distance between two points
is a straight line. A straight line is perfect for navigational track
planning using a great-circle chart (gnomonic projection).
The Defense Mapping Agency (DMA) publishes a number of charts, at
various scales, using the gnomonic projection and covering the usually
navigated portions of Earth. These are listed in the DMA Catalog of
Maps, Charts, and Related Products, part 2, volume X. The point of
tangency is chosen for each chart to give the least distortion for the area
to be covered. On this type of chart, a grreat circle appears as a straight
line. Because of this property, the chart is useful in great-circle sailing.
The following table shows the different stages of constructing a
great-circle track:
Stage
Description
1.
Select a great-circle chart that has a point of tangency nearest
your ship’s predicted track.
2.
Draw the track and check for dangers (consult sailing
directions).
3.
Transfer to open ocean Mercator charts (plotting sheets).
4.
Label all departure points.
5.
Determine SOA and lay out PIM.
12-2
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