Information Found on Charts
A sensible person would never think of going hill walking or set off touring a foreign country without a map to guide them. A chart is a map of the sea, however unlike most conventional land maps, a nautical chart primarily gives us lots of information about what lurks below sea level. There is also lots of additional and supplementary information on a chart that is not relevant or presented on their land-map equivalents.
You may recognise some of the names on the RYA training charts, and some bits of coastline may be familiar, however, it is important to note that they are fictitious places developed for training purposes.
When you are working through this module have the training charts and almanac beside you on a desk so you can consult the training material. There are also exercises in this module and an opportunity to develop your plotting skills.
The Chart Title
All charts have a ‘Chart Title’ which tells the reader what general location the chart is representing (Fig 2.1 & 2.2). The chart title on RYA Training Chart 3 and RYA Training Chart 4, is positioned above a block of chart information. The chart title information block and other textual information is usually located on the land areas of marine charts to limit the loss of nautical detail where it is needed.
In magenta ink below the chart title we are reminded that the RYA training materials, charts and almanac, are fictitious representations and not to be used for real navigation! This warning may seem silly but has been known for people, using previous editions, to go navigating with the training charts and practice almanac!
Admiralty charts normally have a catalogue chart number, in the chart title, on the margins of the chart and on the unprinted reverse side (Fig 2.3). These numbers make for easy chart identification, filing and storage.
We can see that we have two separate charts, RYA Training Chart 3 (RYA TC3) and RYA Training Chart 4 (RYA TC4).
Scale on Passage and Pilotage Charts
If you lay both RYA Training Charts out on a table you will immediately notice that they are different scales as indicated in the chart title information. RYA Training Chart 3 (Fig 2.5) is a smaller scale and what we use to plan longer coastal passages . RYA Training Chart 4 (Fig 2.6) is made up of six larger scale chartlets that correspond to places on RYA Chart 3.
The larger scale charts on RYA 4 are ideal for pilotage around harbours and ports where we need additional detail. If you examine the chartlets on RYA 4 you will notice the more detailed information available on the chart and how this can help with our navigation as we gets close to land.
You can see that each larger scale chartlet has it’s own title, scale and relevant information for that area. Whenever you work between charts you must examine this information and appreciate changes in scale as it will be relevant to your pilotage around the area.
Magenta boxes on RYA Training Chart 3
If you inspect RYA TC3 you will notice magenta coloured boxes surrounding some of the ‘busier’ harbours . These boxes on the passage chart, called ‘cells’, correspond and outline the areas that are covered by a larger scale RYA TC4 ‘pilotage chartlets’.
Fig 2.7 – Arrow A – shows an area on RYA 3 ‘passage chart’ near Port Fraser. It is bounded by a magenta cell and corresponds to the larger scale pilotage chartlet ‘D’ on RYA Training Chart 4 (RYA TC4D).
We can use the smaller scale RYA TC3 as a passage planning chart where we get a general overview of the area and when approaching Port Fraser Harbour we can use RYA TC4D to get more detailed information.
Positions and Datum
Positions are mentioned on the title block information and tells us that positions on both our charts refer to WGS84 Datum (World Geodetic Survey 1984) Fig 2.17 and then goes on to tell us that we can plot satellite derived positions directly onto the charts Fig 2.18.
The earth is certainly not flat, but contrary to popular belief it is not a perfect sphere either, but actually what we call an oblate spheroid (Fig 2.19) and as such does not have an exact centre. This gives us a problem when we are determining our position as different parts and countries in the world have different reference points which are also called ‘Datums’. We can use these reference points or ‘Datums’ to help build a grid system that we can help fix our position.
European charts used to have a European Datum 1950 (abbreviated to ED50) and British Admiralty Charts would use Ordinance Survey of Great Britain 1936 Datum (abbreviated to OSGB36). These datums have different reference points, and therefore have different calculated centres of the earth. The Datum we use on our RYA Training Charts is World Geodetic Survey 1984 (abbreviated WGS84).
As the Global Positioning System (GPS) has become more widespread the WGS84 datum has become more popular and there has been a gradual shift for this datum to be used on modern charts. It is important, when you are working with a chart to note the Datum that it is being used.
There are calculations that can be used to account for the ‘datum shift’ between different charts, and examine how to calculate this in the Yactmaster level navigation course. For now, it is suffice to say, that when you are using a chart, check which datum it is and make sure the GPS equipment on the vessel is set to the same datum!
Around the Ionian Islands in Greece, for example, the data was gathered using celestial navigation and trigonometry and in some places there is a huge datum shift between these early calculations and what the GPS tells you. The GPS is correct, but does not marry up to the information found on the chart and can be anything as much as half a mile out in places.
Navigational marks are mentioned in the Chart Title information block (Fig 2.20). The information tells us that we are using the IALA Maritime Buoyage System Region A and B on our charts. IALA stand for the International Association of Lighthouse Authorities whose members have agreed systems of buoyage in their national waters.
The system we use in UK is Region A where we would find Red Port hand marks and Green Starboard Hand marks (Fig 2.21). This is the system we find on most of the areas of the RYA Training charts.
Region B which is more commonly found in, or countries more influenced by, the USA (Fig 2.22). To reflect this international difference the RYA Training Charts have adopted IALA System B in the Neptune Islands. Here we will find that we have Red Starboard hand markers and Green Port hand markers.
Don’t worry if this sounds confusing as will be looking at the differences more closely in a later module.
Notices to Mariners
Towards the bottom left hand corner of admiralty charts you will see a customer information box and a ‘Notice to Mariners’ (Fig 2.39). The information box indicates which edition the chart is and when it was produced. The ‘Notice to Mariners’ tells us the chart was printed in 2006. There are printed serial numbers next which correspond to changes that have been issued since the chart was first issued. These numbers on the chart, 8195 – 8812 – 9040, are corrections that have been incorporated into the chart at the time of printing.
There are always things changing in the area the chart covers. Therefore corrections and alterations need to be made to charts to keep them up to date. Corrections are issued weekly in the “Admiralty Notice to Mariners” on the Hydrographic Office website. If any of these updates apply to your charts, you must make the necessary correction which is summarised at the Notice to Mariners position on the chart, Marking the year of the correction and the serial reference number for the correction. Most are small corrections, other larger corrections, like changes to buoyage are usually issued in the yachting press or local news papers but cannot be relied upon and Hydrographic Office should be consulted for the proper update notices.
Knowing what the seabed looks like has preoccupied man since he started heading out on the water and it has taken a long time to survey. Over the years many techniques have been used to gather this information. The Source Data box tells us about this survey information and when and how the information was gathered (Fig 2.41 & Fig 2.42).
The seabed is a highly dynamic environment and constantly changing. Sand bars move with currents and storms, the earth’s crust is in perpetual motion, wrecks appear etc. Before the advent of sonar techniques older methods of estimating depth were prone to error. The more up to date the hydrographic information, the more accurate it should be. Places are still being surveyed with information incorporated into up to date charts and given in the Notice to Mariners at the Hydrographic Office
If you look at the chart you will notice other warnings that must be heeded. It tells us things like:
Submarine activity in a specific area
To stay away from Historic Wrecks
Indicates when the seabed was last surveyed
Where the information for these charts was originally sourced
Information about firing and military practice areas