- What is a shipping chart?
- Chart colouring
- Chart symbols
- Chart features
- Variation and Deviation
- Tides and Tidal Streams
- Tidal atlases
What is a shipping chart?
A shipping chart (or British Admiralty (BA) chart in the UK ) is effectively a nautical equivalent of an OS map. It is made up of numerous colours, has lots of symbols, and is covered in lines and numbers.
Once you understand these basic elements it is possible to do some simple chart work. For more advanced chart work and navigation you also need indepth knowledge of other resources such as tide tables, tidal streams, variation and deviation.
A general chart layout is shown below:
The YELLOW areas are DRY LAND
The GREEN areas are Parts which Dry at low water
The DARK BLUE and LIGHT BLUE areas are Always Covered by Water
The WHITE areas are CLEAR (navigable) WATER
Just like an OS map, a shipping chart has a whole host of symbols attached to it. By understanding these, one gains a greater understanding of what it is the shipping chart is trying to portray.
Since there are too many symbols to remember, the admiralty chart of symbols 5011 lists them in booklet form.
Some of the common symbols are shown below:
|Example of an Easterly cardinal buoy, showing the essential features of a buoyage symbol.|
|Port Hand Marker. Can be cones or posts.|
|Starboard hand marker. Can be cones or posts.|
|Example of a marker post. In this case a special mark.|
|A rock which is awash at low water|
|A drying rock|
|A major light. Includes information on light characteristic, height and range. (E.g. Q. 8m 17M )|
|Church spire or tower|
As mentioned previously, shipping charts are covered in lines and numbers. These are common to all charts and mean different things.
The chart scale is basically how detailed the map is:
Large scales are used for harbours and approaches
Medium scales are used for coastlines
Small scales are used for sections of open sea
The scale can be seen around the edge of the chart where the distance is given in nautical miles
(1 nautical mile = 1.15 miles)
True and magnetic compass roses can be found at various places on a chart. These are used in conjunction with a parallel rule to obtain directions in degrees true or degrees magnetic.
They also allow conversion between the two different standards.
Lattitude and Longitude
The horizontal and vertical lines marked on charts are lines of latitude and longitude respectively. They provide a type of grid reference system for measuring a vessels position.
They can also be used as the reference point from which directions are measured.
Latitude and Longitude is measured in degrees and minutes (e.g. 056o 40'N).
1' = 1 minute of latitude = 1 nautical mile
1o = 60 minutes of latitude = 60 nautical miles (NM)
Latitude is read off from the sides of the chart and longitude from the top and bottom.
Information on tidal streams can be found in a panel at the side of the chart. This information relates to 'tidal diamonds' drawn on the chart.
The tidal diamond gives the speed and direction of a tidal stream at that point.
Chart datum is the level from which all water depths and drying heights are measured. This height is determined by the hydrographical office as the lowest predicted astronomical tide.
Contour lines are drawn which join areas of the same depth (like an OS map) at 2, 5 and 10m. These are shaded blue.
All heights on the yellow parts are measured from mean high water springs (highest predicted tide)!
Variation and Deviation
As mentioned earlier, the bearings measured in true and magnetic vary. This difference is known as variation and differs according to location. Hence the compass roses give the correct amount of variation for the year of publication.
Variation is measured in degrees easterly or westerly.
Any bearing measured from a vessel will be in degrees magnetic. They must be converted to degrees true before continuing.
On a small boat you can work in degrees magnetic provided you are consistent.
When the variation is WESTERLY you SUBTRACT the variation
When the variation is EASTERLY you ADD the variation
Three bearings are taken- 336o M, 101o M and 044o M. [M = degrees magnetic]
The chart defines the variation to be 11o easterly.
The true bearings are therefore:
336o + 11 o = 347o T
[T = degrees true]
A mariners compass should point to magnetic north but in reality is affected by the boats own magnetic field. This magnetic field exists due to the presence of metal used in the boats construction.
The amount of deviation varies on different headings
[It may also be easterly or westerly]
Before using a compass, the amount of deviation must be known. For a small boat this can be achieved by sailing along a transit (whose bearing is known) on 8 different headings.
The result is a list of recorded and known bearings. The difference between them can be used to draw a deviation card.
Tides and tidal streams
The effects of tides and tidal streams must be considered when planning a route and plotting a course.
Tides affect the height of the water at a given point at a given time and can prevent entry to a port or harbour if the low water level is less than the draught of your vessel.
- Tidal streams are the movement of water around the UK and can aid or impede ones progress. A strong tidal current can easily set a small boat off course.
Therefore, when planning a route it is important to consult tide tables and tidal atlases.
As mentioned previously the rate and direction of a tidal stream is given on the chart. This information relates the speed and direction of the tide at various points on the chart to the time of high water at a particular given port.
I.e. All tidal stream information is related to a reference port (known as a standard port) at the time of high water.
To find out the speed and direction of the tide at any time , at any point , you first need to determine the time of high water for the standard port on that day. This is achieved through the use of tide tables.
Tide tables give the height of the water above chart datum at a particular time of day for that day of the year. It is essentially a curved plot which indicates the rate of the tide with horizontal graduations for the time and vertical gradients for the height.
As well as these, the plot includes mean low water and mean high water marks.
The time of high tide occurs at the peak of the plot (approximately mid-way along the x-axis). Reading off the height at this point gives the depth at high tide.
It is important to remember to compensate any times when working under BST as all official times are taken using GMT.
To convert GMT to BST simply add 1 hour
(BST runs from 31 st March to 27 th October)
Twelfth's rule of thumb
If you are in a situation where only a rough estimate of the tide is needed you can use a method known as the twelfth's rule of thumb.
It assumes the tides rise and fall proportionally with time and that the time between low water and high water is 6 hours. It also assumes that in the first hour after high or low water the tide rises or falls by 1/12 th its range; in the second hour 2/12 th its range; in the third hour 3/12 th ; in the fourth hour 3/12 th ; in the fifth hour 2/12 th and in the sixth hour 1/12 th .
The tide table for Liverpool Alfred Dock (Page 5.2) shows that low water occurs at 1800. If we want the height of the tide at 2000 then we calculate it thus,
The range of the tide is 7.4 m:
The time we require is 2 hours after LW so the tide will be at approximately 2/12 th its range.
This gives the height above LW as 2/12 x 7.4 = 1.23
The total height of water is LW + 1.23 = 1.6 + 1.23
So the approximate height of water is 3m which is not far off the actual value.
Now that the time of high water for the standard port has been determined, the speed and direction of a tidal stream at a particular time is found by turning to the page with the closest time. This usually covers a larger area.
The above diagram represents a much simplified tidal stream map for the Irish Sea.
More specific information can be determined by using the same time to look up the reference to the required tidal diamond on the chart.