To determine what charts you need for your cruising area, the hydrographic agencies produce free chart catalogues, both paper and digital.
FUN FACT
The first navigation charts (called Portolan maps at the time) were made at the end of the 13th century and depicted the main trading
ports in the Mediterranean basin. 100 years earlier the magnetic compass came into use and this is considered to have been the catalyst for the development of charts. Prior to
this sailors relied on documented landmarks known as “Sailing Directions” and, of course, celestial navigation.
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Projections
As the earth is not flat (contrary to the belief of some), it presents distortion issues when producing a 2D map or chart. To represent a curved surface on a flat one there are
many different ways, these are known as projections, each with its own characteristic set of distortions. The projection used depends on what distortion is most acceptable on the
finished chart.
One projection, known as Mercator's projection, is particularly good at representing direction. This is a purely mathematical concept, but is easily visualised by imagining the
image cast on a sheet of paper wrapped around a globe with a bright light in the centre. This distorts the meridians to appear as vertical parallel lines, intersecting with the
lines of latitude at right angles. Distances are therefore distorted, particularly towards the poles, where the meridians on the world converge and eventually meet. On a Mercator
chart, the meridians never meet, so east-west distances near the poles are exaggerated.
The common solution to the problem of representing polar areas is to use an alternative projection, known as gnomonic projection. Similarly, to Mercator's projection, it is a
mathematical concept that is easily visualised as the image that would be produced by a translucent globe, with a bright light at its centre, standing on a flat sheet of paper.
The globe could touch the paper at any point, but for polar charts it is best to touch at the pole itself. This point of contact is known as the tangent point.
On any gnomonic chart, the meridians appear as straight lines, while parallels of latitude appear as curves. The overall pattern varies considerably, depending on where the
tangent point is located. That is, if the tangent point is at the Equator, the meridians appear as straight parallel lines, but if the tangent point is the Pole, the pattern shows
the parallels of latitude as a series of concentric circles. The gnomonic projection is the only one on which the shortest distance between two points (known as the great circle
route) appears as a straight line. For planning ocean passages, particularly those with a large east-west movement. the distance saved can be significant.
Scale
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Scale is the relationship between distance on a chart and the distance it represents. So, if 1cm represents 2km, distances on the chart/map are 1/200,000, and the chart/map
could be described as being at a scale of 1:200,000. If 10cm on a chart/map represents 1 km, it shows considerably more detail, but the paper size would need to increase to show
even a small area and its scale would be 1:10,000.
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It is essential to choose the right scale chart for the intended voyage. Scales are typically between 1:5,000 and 1:25,000, and are used for harbour plans and port approaches.
Conversely, small scale charts are used for planning purposes, at scales down to about 1:3,500,000. It is useful to note that coastal navigation charts are usually at scales of
between 1:75,000 and 1:150,000.
TAKE NOTE
A good way to understand the scale on a chart, is to imagine an apple cut into slices. The more slices you cut, the larger the
quantity of slices, but the less apple you get with every slice. So, to relate this to a chart... the larger the scale, the less detail (or apple) you get.
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Symbols and Abbreviations
Symbols and abbreviations are used on charts to save space but also to highlight the important/useful navigational features. For example, a 200m tall chimney that stands out
against the skyline and is illuminated with red lights at night makes an excellent landmark… its base, however, may only be 25m in diameter, so even at the scale 1:25,000 it would
be represented by an inconspicuous dot with a 1mm diameter. To overcome these problems, symbols are used.
Each hydrographic agency issues a definitive list of symbols, and publishes these for clear reference, for example, “Symbols and Abbreviations Used on ADMIRALTY charts”
(NP5011). These publications give details on symbols and abbreviations used on charts, and also contain information on hydrography, topography, navigational aids and services.
Most hydrographic agencies use similar symbols and abbreviations, as well as providing the publication in both digital and paper formats.
Electronic Charts
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The digital era is upon us, thus the idea of paper anything these days is almost considered a crime. Thus, the digitising of nautical charts has become common practice in the
form of electronic chart systems (or chart plotters) and makes them an excellent practical tool on small vessels with no room for large chart tables. Though it should be noted
that the starting point for the production of any electronic chart is likely to be its paper counterpart.
The big advantage of these systems is that when linked to an electronic position fixer such as GPS, they show the vessel's continuously updated position. Their biggest drawbacks
are that they are potentially less reliable and less intuitive than traditional paper and pencil, as well as being a cybersecurity risk.
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Type of Chart – Raster vs Vector
The International Maritime Organization (IMO) published a comparison of raster and vector charting systems in 1999, the following categories were defined:
| ECDIS |
= Electronic Chart Display and Information System (vector) |
| RCDS |
= Raster Chart Display System (raster) |
| ENC |
= Electronic Navigational Chart (vector) |
| RNC |
= Raster Navigational Chart (raster) |
| GNSS |
= Global Navigational Satellite System |
Raster charts are digitised by scanning a paper chart. This means that each tiny segment of each line on a chart is converted to a raster picture element or pixel. So, much like a
television picture, the greater the zoom the more pixelated the image.
ENCs are vector or "smart” charts and are coded with additional information not available on paper or raster charts. ENCs carry a wealth of geo-spatial intelligence through a
database of information associated with them. On an ENC the user can click on different features, such as a light or buoy, and retrieve additional information about the
feature.
Chart Updates
Whether paper or digital, all charts require regular updating and should not be used for navigation if they are not. Rather than publishing new charts, corrections to the
existing published charts are provided by hydrographic offices. These changes are published by way of weekly “Notices to Mariners” (NTM) and should be confirmed as applied prior
to the chart being used.
Publications
While charts are the main source of information for navigation, there are many other useful reference books, or “publications”.
These include:
- Tide Tables;
- Tidal Stream Atlases;
- List of Lights;
- List of Radio Signals;
- Nautical Almanacs; and,
- Pilot and Sailing Directions.
All of these publications are important and should be referred to, in the planning of any nautical passage.
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