A Simplified Guide to Small Marine Craft Navigation.

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Chapter 24 (v.1) - The Construction of Navigational Charts.

Submitted: May 16, 2017

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Submitted: May 16, 2017

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The Construction of Navigational Charts.

 

 

It is quite impossible to draw courses and measure distances and angles accurately on the curved surface of a globe, even if one could be constructed large enough for navigational purposes. Consequently, the planet Earth's spherical surface must somehow be represented on the flat surface of a map.

There are many practical methods of doing this by projection, but unfortunately, each of these methods involves distortion of the planet Earth's true proportions in one way or another.

Most navigational charts, and or maps, are on the Mercator Projection, which is a modified form of the Central Cylindrical Projection.

Imagine a transparent globe surrounded by a cylinder of paper and with a bright light shining at its center. The light will throw shadows of the land, continents, islands, and so on, onto the inner surface of the cylinder, through which their configuration can be traced.

When the cylinder is unrolled, there will be a representation of the planet Earth on a flat surface. If only a small area is required this can be imagined to be peeled – off the cylinder where required to produce a rudimentary chart.

If degrees of Longitude are marked along the top and bottom of this rudimentary chart Eastward, and or Westward of the Greenwich Meridian, and then degrees of Latitude are marked along the sides of the chart Northward, and or Southward of the Equator, there is the makings of a very rough chart unsuitable for precise navigation.

Strictly speaking, the Mercator Projection is not a true Geometrical Projection as described above, but is constructed mathematically so that a constant course steered by a ship or craft may be represented on the chart by a straight line, so that courses are easily drawn, and angles, such as course angles, are not distorted on the chart. However, the graphical conception is approximately correct and easier to understand.

A ship or boat's true course is the angle between the direction of True North and the vessel's fore-and-aft line. When this course is plotted on a Mercator Chart, it cuts all Meridians at the same angle, and in addition cuts Parallels of Latitude at the same angle. A line which does this is called a Rhumb Line.

If drawn on a globe, a Rhumb Line would appear as a curved line spiraling towards the pole.

There are certain projections where the Rhumb Line would not be represented by a straight line, and any chart based on one of these projections would not be suitable for plotting courses.

As already stated, the most commonly used navigational charts are those based on the Mercator Projection. The Mercator Projection, like all projections, is one particular solution to the problem of representing a spherical surface on a two-dimensional piece of paper. However, there are certain peculiarities and distortions on Mercator Charts that navigators must appreciate.

One peculiarity is that on a globe the Meridians converge towards the poles, however on the Mercator Chart they are represented as being parallel to each other. This means that the further North or South of the Equator a land mass is situated, the more it becomes distorted in an East-West direction, and ultimately the Pole, which is a point, is shown as the whole width of the chart. In example, Greenland would appear three times as broad as it should be and almost as large as Africa.

The second peculiarity of the Mercator Chart is that the distance between Parallels of Latitude increases progressively from the Equator towards the Poles, whereas on a globe the Parallels are equally spaced.

Although the Mercator Chart is suitable for most navigational purposes, there are some cases where another projection, the Gnomonic Projection is required. However, Mercator Charts cannot accurately represent the Polar Regions, and so Polar charts are drawn on the Gnomonic Projection

On a Gnomonic Chart, Meridians appear as straight lines converging towards the poles, and Parallels of Latitude appear as curves. However, on very large-scale plans this convergence of the Meridians and curvature of the Parallels is so slight that the chart may be used in the same manner as a Mercator Chart, and in fact, many Harbor Plans are drawn on the Gnomonic Projection.


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