Navigation and Chart work - Nautical Chart Symbols

Warnings

Below the tidal levels table and at different places around the chart are warnings that are specific to the area covered by the chart. It is always a good idea to read these, most will not be relevant to small craft, but some definitely will.

One of the warnings concerns satellite derived positions. Positions on Global Positioning Systems (GPS) are referred to WGS 84 (World Geodetic Survey 1984) datum. This chart and many others are referred to OS36, so there is a correction to be applied to GPS positions before using them on this chart. As you can see it is so small in this case that it can be ignored for most practical use in a small craft.

On Practice Chart 1, there is a warning about the Race of Alderney, can you find it? It is certainly one you would need to read before sailing to the Channel Islands.

Source Data Diagram

In the Southeast corner of Practice Chart 1 there is a diagram, this diagram tells us the dates of the surveys used to create the chart. Sometimes they were a surprisingly long time ago. In an area of rocks, little will have changed, but where there is sand or mud to move about, there may be considerable differences. There is not much that you can do about this, but a prudent skipper would behave cautiously in an area that was surveyed over a hundred years ago, with a lead line and sextant.

What is the date of the earliest survey on Chart 1? Answer.

Compass Roses

You will notice that Compass Roses are positioned about the charts. On Practice Chart 1, these are in two parts. The outer ring is related to magnetic north and the inner to true north. We will look at these in more detail later, but they were essential for drawing directions on charts, before we used chart plotters which have a compass rose of their own.

Chart Symbols

There is a standard system of chart symbols used internationally, and although there are different conventions from one country to another, it is fairly easy to read any chart once you have learnt the symbols.

The Admiralty publishes a book called Symbols and Abbreviations; this is given the number 5011, just like a chart.

It is probably impossible to know every symbol in use, but we should be striving to learn them all. Most can be grouped in to certain classes such as buildings or dangers and the general classification of the symbol is often obvious.

Inside the back cover of 5011, there is a key, which covers many of the most common symbols. If you find a symbol you do not know it is a good idea to find the most similar one from this key then turn to the appropriate section of 5011. You will notice that symbols are grouped by types, Hazards, Buoys, Navigation Marks and so on.

You will notice that many of the features used on charts are composite, which is they are made up of several pieces of information. An example is:

This symbol is used for many features; next to it will be a legend, which describes what it is, e.g.

• Fs Flagstaff
• Bn Beacon
• Tr Tower

On Chart 2, find Start Point, just inland from the point are some radio masts, the position of which is indicated by this symbol.

It is worth having a look around the chart now. See if you can identify some of the symbols by looking them up in 5011.

• In the South East part of Tor Bay, just north of Berry Head, there is a diamond shape in a circle with the legend Brixham and Torquay written alongside.

If you open the back cover of Symbols and Abbreviations, you will find this symbol in section IT Services. Find this section on page 58. Symbol 1.2 is the definition to access to identify this symbol. This is a pilot boarding point for Brixham and Torquay Pilots.

• In Horsesands Bay on the north west corner of North Coombe Island there is a 6 pointed star symbol.

On the back cover of 5011, this symbol is in section IK Rocks, Wrecks, Obstructions. Section IK covers pages 30-32, see if you can identify what this symbol indicates.

• Just above the legend TOR BAY in the middle of Tor Bay you will find fS.M written.

From the contents table this is in section IJ Nature of the Seabed. The bottom of the seabed is indicated in places as this can help when you are looking for somewhere to anchor. This is a composite symbol. The "f" indicates "fine", page 28, symbol 30. "S" indicates sand, page 27, symbol 1. "M" indicates mud, page 27 symbol 2. So the bottom in this area is fine sand and mud, not a bad choice of material to anchor in.

Spend some time finding other chart symbols and identifying them from Symbols and Abbreviations.

 

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