There is no pre-reading before this section of the course.
Why is this important?
At sea there are no sign posts, so the chart is your only way of finding your position and pointing you in the right direction. It is important to know the basics of how to read a chart.
Why buy charts?
You can purchase many different types of maps of different areas of the world. However, only marine charts will give you the details you need to navigate on water safely.
For example, Ordinance Survey maps are great on land but will not show any detail of a rock that is just below the water.
Colours on the chart
Land – yellow
Tidal zone – green
Shallow water – blue
Deep water – white
Imray Charts: The most common alternative to Admiralty Chart are charts published by Imray. In most cases they use the same raw data but have a different colours are used.
Land – green
Tidal zone – yellow
Shallow water – white
Deep water – blue
Charts from around the world broadly share the same symbols. The Admiralty publishes the book Symbols and Abbreviations (often referred to as chart 5011) which lists all the symbols you will need to know.
You may wish to buy a copy of Chart 5011 or borrow a copy during your course with us.
You can view a copy of the symbols used on Imray charts here.
Latitude and Longitude
Every single spot on the earth can be identified by a unique latitude and longitude.
Latitude is measured in degrees, north and south, of the Equator. For example the south of the UK is approximately 50 degrees North.
Notice that each latitude is parallel to all others. The actual distance between latitudes is always the same.
The scale goes from 90 degrees south to 90 degrees north.
Longitude is the angle east or west around the earth from Greenwich (London). For example, West Cornwall is approximately 5 degrees West.
Longitude lines are called meridians.
The scale goes from 180 degrees west to 180 degrees east.
How we write latitude and longitude
Each degree of latitude and longitude is divided into 60 “minutes” (not 100). Each minute is then divided into its decimal parts. It will normally be written in this format:
46º 27’.800N 006º 15’.300W
This translates to:
Latitude: 46 degrees and 27.800 minutes north of the Equator.
Longitude: 6 degrees and 15.300 minutes west of Greenwich (London).
Examples of reading the scales
A: 40º 30’.530N
B: 40º 29’.750N
C: 40º 28’.850N
D: 074º 02’.670W
E: 074º 00’.520W
F: 073º 59’.300W
Video: additional information
As we learnt when reading about Latitude and Longitude, a minute of latitude (scale on the left and right of your chart) is constant* around the world and corresponds to one nautical mile this allows us to measure distance on a chart. So in simple terms 1 minute (on the vertical scale) = 1 Nm.
Please note that a minute of longitude (scale on the bottom and top of your chart) is not constant so cannot be used to measure distance.
*A minute of latitude is not quite constant due to the earth not being a perfect sphere however for most practical applications we can assume it is.
We measure bearings clockwise in degrees from north.
The most common instrument for measuring angle on a chart is the Portland Plotter.
Measuring the bearing between two points
Marking a bearing from a point
Keeping charts up-to-date
Details that are recorded on your chart change all of the time so it is important to keep your chart up to date. For example a few years ago a light was added to The Gear Isolated danger in Mounts Bay, if you not local to the area you would not know about this change.
To find out about changes to Charts both Admiralty and Imray have websites listing these “Notices to Mariners”.
The most commonly used charts.
Check your understanding
Check your knowledge of the basics using this quick self-marking test.
If you understand the topics on this page, this knowledge will form the basis of much of the rest of this course. Start reading about Chart Datum next.
The content of these pages is put together in good faith and is constantly evolving. It is possible that errors exist within this content. If you spot an error or would like to add anything to these pages please contact use via email.
Reading the content of these pages is not a substitute for completing a RYA Shorebased course or similar.