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Chapter 26
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Charts

There are three basic types of chart normally used by the cruising yachtsman.

   The passage chart is a small scale chart that covers the whole area of a planned passage from point of departure to point of arrival.

   Coastal charts are larger scale charts that give details of the coastline, navigation marks, channels and approaches to harbours.

   Harbour charts are drawn to a very large scale in order to show full details of harbours.  They may be given as inserts on coastal charts or included in pilots and guide books.

Chart Projection

   There are two main types of projection used in the preparation of charts.

   Most charts are drawn to Mercator's projection, in which meridians (lines of longitude) are shown as equally spaced parallel lines running from top to bottom of the chart, and not converging towards the poles. The parallels of latitude, which in reality are equally spaced, are drawn on the chart as being further and further apart from the equator to the poles; (this compensates for the non-convergence of the meridians).

   On a Mercator's chart, angles on the earth's surface are equal to corresponding angles on the chart, and the scale of distance as given on the latitude scale at the latitude concerned is correct. The disadvantage of a Mercator's projection is that the shortest distance between two points, that of a great circle, would appear as a curved line on the chart, but this has no material effect except for very long direct passages.

   To overcome this difficulty, Gnomic projection is used for great circle charts, and also for large scale harbour charts.  Gnomic projection is made from the centre of the earth to a tangential plane on the earth's surface, so that meridians converge to the poles, lines of latitude are drawn as curves and great circles appear as straight lines. Large scale Gnomic charts used as harbour charts give a truer representation of the area, and may be treated as Mercator's charts. The slight distortion involved has no effect at such a large scale.

Chart Scale

   The amount of detail shown on a chart depends on the scale to which it is drawn. Coastal charts are usually at a scale of about 1 : 50 000, while passage charts will be about

   1 : 1 000 000.  The scale The scale of harbour charts normally depends on the actual size of the harbour.

   It is important that the largest scale chart practicable is used, because much inshore detail is deliberately omitted from small scale charts.

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