To read a nautical chart, start by checking the chart scale, datum, depth units, latitude and longitude scales, compass rose, symbols, hazards, navigation marks and tidal information. To determine your position at sea, use GNSS/GPS, visual bearings, depth, transits or a plotted fix, then compare that position with the charted hazards and safe water around you.
A nautical chart is not just a map with water on it. It is a navigation tool showing what a sailor cannot always see: depth, drying areas, rocks, wrecks, channels, buoys, lights, seabed shape and safe-water routes. The UK Hydrographic Office says Admiralty nautical charts depict the depth and nature of the seabed, along with the navigational information mariners need to judge whether a route is safe.
This guide explains how to read a nautical chart and how to use it to determine your position at sea. It is written for beginner and improving sailors, especially those navigating UK coastal waters.
Important: This is an educational introduction. It is not a substitute for RYA training, current official charts, Notices to Mariners, local pilotage information or the judgement of a qualified skipper.
Nautical Chart Features at a Glance
| Chart feature | What it tells you | Why it matters |
|---|---|---|
| Scale | Level of detail on the chart | Determines how accurately you can measure or plot |
| Latitude and longitude | Position coordinates | Lets you read or plot a location |
| Soundings | Charted depth below chart datum | Helps you avoid shallow water |
| Drying heights | Ground that covers and uncovers | Shows areas that may dry at low tide |
| Depth contours | Lines joining equal depths | Shows the shape of the seabed |
| Chart datum | Reference level for depths | Helps calculate actual depth with tide |
| Compass rose | True/magnetic direction information | Helps plot bearings and courses |
| Symbols | Rocks, wrecks, buoys, lights and hazards | Helps you identify dangers and aids |
| Tidal information | Height and stream information | Helps with depth and course planning |
| Notices to Mariners | Chart corrections and updates | Keeps navigation information current |
What Is a Nautical Chart?
A nautical chart is a specialised map for marine navigation. It shows the coastline, seabed, depths, hazards, channels, aids to navigation and other features needed to plan and monitor a passage.
A good chart helps you answer practical questions:
- Where am I?
- How much water is under the boat?
- Where is the safe water?
- What hazards are nearby?
- Which direction should I steer?
- What tide or current may affect me?
- Which buoys, lights or landmarks can I use?
- Is this chart current enough to rely on?
Charts may be paper or electronic. Both can be useful, but both need understanding. A chartplotter can show your position quickly, but it does not remove the need to understand chart symbols, depth, tide, bearings and what is happening outside the boat.
For broader route planning, see our UK coastal sailing safety guide.
Paper Charts and Electronic Charts
Sailors commonly use paper charts, electronic charts, or both.
Paper charts
Paper charts give a fixed view of an area. They are useful for passage planning, backup navigation, training, and seeing the wider context of a route. They need to be kept up to date manually.
Electronic charts
Electronic charts are used on plotters, tablets and electronic navigation systems. Official Electronic Navigational Charts, or ENCs, are vector charts intended for systems such as ECDIS in professional navigation, while leisure sailors may use different electronic chart products. UKHO/Admiralty says chart updates are delivered through Notices to Mariners, with electronic updates applied electronically and paper chart updates applied manually.
Do not assume the electronic screen is always correct just because the boat icon looks precise. The RYA notes that GNSS has made finding position easier, but the focus should move to verifying the position and improving situational awareness, especially in pilotage waters.
The safest habit is to use electronic navigation as one source, then check it against the chart, depth, compass, visual marks and the conditions around you.
Start With the Chart Title, Scale and Datum
Before reading the detail, check the chart information.
Look for:
- chart title
- chart number
- scale
- depth units
- horizontal datum
- chart datum
- edition date
- correction status
- warnings or notes
The scale matters. A large-scale harbour chart shows more detail than a small-scale passage chart. Do not use a small-scale chart for close pilotage if a larger-scale chart is available.
The datum matters because positions must refer to the same reference system. Many modern chart positions use WGS84, but you should always check the chart note and your navigation device settings.
Understanding Latitude and Longitude
Latitude and longitude are the coordinate system used to describe positions on Earth.
Latitude measures north or south of the equator.
Longitude measures east or west of the prime meridian at Greenwich.
On a nautical chart:
- latitude is usually read from the side scale
- longitude is usually read from the top or bottom scale
- one minute of latitude is approximately one nautical mile
This is why navigators measure distance from the latitude scale, not the longitude scale. Longitude spacing changes with latitude, so it should not be used for measuring nautical miles.
How to Plot a GPS Position on a Chart
If your GPS or chartplotter gives your position as latitude and longitude, you can plot it on a paper chart.
- Read the latitude from the GPS.
- Find that latitude on the side scale of the chart.
- Lightly draw or imagine a horizontal line across the chart.
- Read the longitude from the GPS.
- Find that longitude on the top or bottom scale.
- Lightly draw or imagine a vertical line.
- Your position is where the two lines cross.
- Check whether the plotted point matches depth, nearby marks, land and your surroundings.
A plotted GPS position is useful, but it should not be treated as magic. Check whether it makes sense.
Soundings: What the Depth Numbers Mean
The numbers printed in the water areas of a nautical chart are called soundings. They show charted depth.
Admiralty describes charted depths as soundings and depth contours as lines joining points of equal depth.
The key point is that charted depths are normally measured from chart datum, not from the actual water surface at the time you are sailing.
If the chart shows a depth of 3 metres and the tide height is 2 metres above chart datum, the approximate depth of water may be 5 metres before allowing for other factors such as atmospheric pressure, surge, waves, squat, transducer offset and safety margin.
For beginners, remember:
charted depth + height of tide = approximate depth of water
Do not forget your boat’s draft. A boat drawing 1.8 metres needs more than 1.8 metres of chart-plus-tide water because you also need a safety margin.
Chart Datum and Drying Heights
Chart datum is the reference level used for soundings and drying heights. The RYA eNavigation glossary describes chart datum as the level used by the chart, with tide tables giving the amount of water above that level.
Soundings
Soundings are depths below chart datum.
Drying heights
Drying heights show areas that cover and uncover with the tide. They are heights above chart datum. These are the places that may be underwater at high tide but dry or become very shallow at low tide.
This matters in UK waters because many harbours, estuaries, creeks and approaches include drying banks, mud, sand, rocks and shallow channels.
If you are entering a drying harbour or crossing shallow water, do not guess. Use the chart, tide table and local guidance.
Depth Contours and Seabed Shape
Depth contours are lines that join places of equal depth. They help you understand the shape of the seabed.
Contours can show:
- channels
- banks
- steep drop-offs
- shallow shelves
- approaches to harbours
- underwater ridges
- areas where waves may steepen
Close contour lines show depth changing quickly. Wide-spaced contours show a gentler change.
Depth contours are not the same as currents. They show seabed shape. Tidal stream arrows, diamonds, tables or tidal atlases show tidal flow.
Symbols and Abbreviations
Nautical charts use many symbols and abbreviations. These may show rocks, wrecks, buoys, lights, seabed type, cables, pipelines, restricted areas, traffic schemes, anchorages and more.
Do not guess a symbol if safety depends on it. Use the correct chart-symbol reference. Admiralty’s NP5011, Symbols and Abbreviations used on Paper Charts, gives details of the symbols and abbreviations used on Admiralty charts and international chart-series paper charts.
Common charted features include:
- rocks
- wrecks
- obstructions
- drying areas
- depth contours
- buoys
- beacons
- lighthouses
- leading lines
- submarine cables
- pipelines
- traffic separation schemes
- anchorages
- restricted areas
- seabed descriptions
A chart is only useful if you understand the symbols. If you do not know a symbol, stop and check it.
Navigation Marks, Buoys and Lights
Navigation marks help you identify channels, hazards and safe water. They include buoys, beacons, leading marks and lights.
Be careful with simple colour rules. Buoyage depends on the buoyage system, the direction of buoyage and the type of mark. Red and green lateral marks do not simply mean “danger” or “safe” in every situation. Read the chart and understand the local buoyage.
Common mark types include:
- lateral marks
- cardinal marks
- isolated danger marks
- safe water marks
- special marks
- emergency wreck marking buoys
Lights are identified by characteristics such as colour, pattern, period and range. A light might be flashing, quick flashing, occulting or grouped in a pattern. The chart gives the information needed to identify it.
Do not use a lighthouse or buoy simply because you can see “a light.” Confirm that the light characteristic matches the chart.
Compass Rose, True North and Magnetic North
The compass rose on a nautical chart helps you measure directions and bearings.
It may show:
- true north
- magnetic north
- magnetic variation
- annual change in variation
True north is direction toward the geographic North Pole.
Magnetic north is direction toward magnetic north, which changes over time and differs from true north.
The difference between them is called variation. Your steering compass may also have deviation, caused by magnetic effects on the boat.
For basic chartwork:
- courses plotted on the chart are usually true
- compass steering uses magnetic or compass headings
- you must apply variation and deviation correctly when converting between them
This is where formal navigation training helps. The important beginner point is simple: the compass rose is not decoration. It is how you connect chart direction to compass direction.
Measuring Distance on a Nautical Chart
Distance is measured using the latitude scale at the side of the chart.
One minute of latitude is approximately one nautical mile.
To measure distance:
- Open dividers between two points on the chart.
- Move the dividers to the latitude scale.
- Read the number of minutes of latitude.
- Treat those minutes as nautical miles.
Do not measure distance from the longitude scale. Longitude spacing changes depending on where you are on Earth.
If you are planning a route, measure each leg separately and allow extra time for tide, leeway, tacking angles, traffic, visibility and harbour approaches.
Plotting a Course
A course is the direction you intend to travel over the ground or steer through the water, depending on the calculation being made.
A simple beginner route plan might include:
- departure point
- waypoint 1
- waypoint 2
- destination
- course for each leg
- distance for each leg
- estimated speed
- estimated time
- hazards near the track
- tide and weather notes
Do not draw a route over shallow water, rocks, restricted areas or traffic lanes simply because it is the shortest line. The safe route is often slightly longer.
For a broader navigation overview, read sail navigation explained.
Position Fixing: The Main Methods
There are several ways to determine your position at sea. The best sailors use more than one and compare them.
GNSS/GPS position
Your chartplotter, phone, tablet or handheld GPS can give a latitude and longitude position. This is useful, but should be verified against the chart, depth and surroundings.
Visual bearings
A bearing to a known object gives a line of position. Two or three bearings to charted objects can produce a fix.
Transits
A transit occurs when two charted objects line up. If you are on that line, you are somewhere along the transit. Transits are useful for pilotage and checking safe-water routes.
Depth contour matching
If your depth sounder reading, adjusted with tide and draft awareness, matches a charted depth or contour, it can help confirm your position.
Dead reckoning
Dead reckoning estimates position from course steered, speed and time. It does not fully account for tide, leeway or steering errors.
Estimated position
An estimated position improves a dead-reckoned position by allowing for known effects such as tidal stream or leeway.
How to Plot a Three-Point Fix
A three-point fix uses bearings to three charted objects. It is a classic way to check position when you can see reliable landmarks or navigation marks.
You need:
- a chart
- compass or hand-bearing compass
- pencil
- plotter or parallel rules
- three identifiable charted objects
Steps:
- Choose three objects you can identify on the chart and see in the real world.
- Take a bearing to the first object.
- Convert the bearing if needed so it can be plotted correctly.
- Draw the line of position on the chart.
- Repeat for the second object.
- Repeat for the third object.
- Your position is where the lines meet or form a small triangle.
- Check whether the fix makes sense with depth, course, speed and surroundings.
If the lines form a large triangle, something may be wrong. Possible causes include a misidentified object, steering compass error, poor bearing technique, plotting error or time delay between bearings.
Using Depth to Check Your Position
Depth is a useful cross-check because it gives independent information.
If the charted depth around your plotted position is 8 metres but your depth sounder is reading 2 metres, something needs explaining.
Check:
- tide height
- chart datum
- transducer offset
- boat draft
- whether the sounder shows depth below transducer or below keel
- whether you are near a steep contour
- whether you plotted the position correctly
Depth alone rarely gives an exact position, but it can confirm or challenge a fix.
In pilotage waters, depth can be a warning that you are not where you think you are.
Using Transits and Leading Lines
A transit is when two visible objects line up. A leading line is a charted alignment used to guide vessels through safe water.
Examples include:
- two posts lining up
- leading lights lining up
- a church tower lining up with a beacon
- harbour entrance marks in line
Transits are useful because they connect the chart to the real world. They can help you confirm you are in the right channel or clear of a hazard.
When using a transit, make sure both objects are correctly identified. Do not use an accidental alignment that is not the intended or charted one.
Tides, Streams, Set and Drift
Tide affects both depth and direction of movement.
Tidal height
Tidal height affects how much water is available above chart datum. This matters when crossing shallow areas, entering drying harbours or anchoring.
Tidal stream
Tidal stream is the horizontal movement of water. It can push the boat sideways or speed it up or slow it down over the ground.
Set and drift
Set is the direction the tide pushes you.
Drift is the distance or speed of that movement.
If you steer a course without allowing for tidal stream, the boat may not follow the line you drew on the chart. The heading of the boat and the course over the ground may be different.
This is one reason you should monitor your position during the passage, not just plot a route before leaving.
Weather, Visibility and Sea State
A chart does not show today’s weather. It shows fixed and charted information. Before using it for a passage, combine it with the current marine forecast.
For UK waters, the Met Office Inshore Waters Forecast includes wind direction and force, weather, visibility and sea state, with strong coastal winds displayed for the first 24 hours.
Weather matters because it changes how you use the chart:
- poor visibility makes visual bearings harder
- wind against tide can make seas rougher
- rain can make cockpit chartwork harder
- strong wind may make a lee shore dangerous
- fog may require slower speed and extra lookout
- night navigation increases reliance on lights and pilotage marks
Navigation is not just plotting. It is continuously comparing chart, instruments, forecast and what you can actually see.
Keeping Charts Up to Date
A chart can be beautifully printed and still be unsafe if it is out of date.
Harbour works, buoy moves, wrecks, dredging, new restrictions and changes to lights can all affect navigation. Admiralty chart corrections are supplied through Notices to Mariners; paper chart users must apply relevant corrections manually, while electronic users apply electronic updates through their systems.
For UK and Admiralty chart users, check relevant Notices to Mariners and chart correction status. Admiralty also publishes NP5011 for chart symbols and abbreviations.
Before relying on a chart, ask:
- Is it the right chart for this area?
- Is the scale suitable?
- Is it current?
- Have corrections been applied?
- Are there temporary or preliminary notices?
- Do local harbour notices affect the route?
Do not use an old chart for close navigation unless you have verified the information you need.
Common Nautical Chart Mistakes
Using the wrong scale
A small-scale passage chart may not show the detail needed for harbour entry or close pilotage.
Measuring distance on the longitude scale
Use the latitude scale for nautical miles.
Forgetting chart datum
Soundings are not the same as actual water depth at that moment. Add tide height and keep a safety margin.
Trusting GPS without checking
A precise-looking position is not the same as a verified safe position.
Misreading drying heights
Drying heights are above chart datum and indicate areas that may dry.
Ignoring Notices to Mariners
Out-of-date charts may miss important changes.
Guessing chart symbols
If you do not know a symbol, check NP5011 or the relevant chart-symbol reference.
Confusing heading with course over ground
Your heading is where the bow points. Your course over ground is the direction you actually move over the seabed. Tide and wind can make them different.
A Simple Chart-Reading Routine
Use this order when approaching a new chart or planning a short coastal leg.
- Check chart title, scale, datum and depth units.
- Identify departure point and destination.
- Look for hazards between them.
- Check soundings, contours and drying areas.
- Identify buoys, lights, leading lines and landmarks.
- Check tidal height and stream information.
- Plot a safe route with suitable waypoints.
- Measure course and distance for each leg.
- Check the weather and visibility forecast.
- Decide bailout options and turn-back points.
- Monitor your position during the passage.
- Verify GNSS with depth, bearings or visual marks.
Worked Example: Plotting Your Position From GPS
Imagine your GPS shows:
- Latitude: 50° 42.60’ N
- Longitude: 001° 18.40’ W
To plot this position:
- Find 50° 42.60’ N on the side latitude scale.
- Make a light pencil mark.
- Find 001° 18.40’ W on the top or bottom longitude scale.
- Make a light pencil mark.
- Draw or project a line from each mark.
- Where they cross is your plotted position.
- Check the charted depth, nearby hazards and navigation marks.
- Compare with your depth sounder and what you see around you.
If the position appears to put you on land, in shallow water, or somewhere that does not match your surroundings, stop and check your data, datum, plot and equipment.
Worked Example: Checking a Position With Bearings
Imagine you can see:
- a lighthouse
- a church tower
- a harbour beacon
You identify all three on the chart and take bearings to each one.
- Take the first bearing and plot its line of position.
- Take the second bearing and plot its line.
- Take the third bearing and plot its line.
- The lines should meet or form a small triangle.
- Your position is inside or very close to that triangle.
- Check depth and visual surroundings to confirm.
This is useful because it does not depend on a GPS position alone.
Can You Use GPS Instead of Reading a Chart?
GPS is extremely useful, but it should not replace chart understanding.
A GPS can tell you where it thinks you are. A chart helps you understand what that position means.
The key questions are:
- Are we in safe water?
- What hazards are nearby?
- What is the depth?
- Where is the tide pushing us?
- What is our next safe mark?
- What happens if visibility drops?
- What is the bailout option?
RYA guidance on electronic navigation says there will always be a need for pilotage techniques such as transits and buoyage, and that at least one other way of plotting or confirming position is essential.
Use GPS. Use the chart. Use your eyes. Use depth. Use seamanship.
Frequently Asked Questions
What is a nautical chart?
A nautical chart is a specialised map for marine navigation. It shows coastlines, depths, seabed features, hazards, navigation marks, lights, channels and other information sailors use to plan and monitor a route.
What do the numbers on a nautical chart mean?
The numbers in the water are soundings. They show charted depth, usually below chart datum. To estimate actual water depth, add the height of tide and allow for your boat’s draft and safety margin.
What is chart datum?
Chart datum is the reference level used for charted depths and drying heights. Tide tables show how much water is above chart datum at different times.
What is a drying height?
A drying height is the height of seabed or a feature above chart datum. It marks an area that may be covered at high tide but dry or become very shallow at low tide.
How do you plot latitude and longitude on a nautical chart?
Read the latitude from the side scale and the longitude from the top or bottom scale. Draw or project lines from both values; the intersection is the plotted position.
What is a compass rose?
A compass rose is the circular direction guide printed on a chart. It helps navigators plot true or magnetic courses and bearings.
What is the difference between true north and magnetic north?
True north points to the geographic North Pole. Magnetic north points toward the Earth’s magnetic field direction. The difference is called variation.
What is a line of position?
A line of position is a line on which the vessel is believed to be located. A bearing to a known object can create a line of position.
What is a three-point fix?
A three-point fix uses bearings to three known charted objects. Where the lines meet, or form a small triangle, gives your estimated position.
Can you use GPS instead of a paper chart?
GPS is useful, but it should not be your only navigation method. You should understand the chart, verify position and have another way to confirm where you are.
How do you measure distance on a nautical chart?
Use dividers and the latitude scale at the side of the chart. One minute of latitude is approximately one nautical mile.
How often should nautical charts be updated?
Charts should be kept current using the relevant official correction system, such as Admiralty Notices to Mariners for Admiralty charts.
Editorial Source Notes
This guide should be reviewed against current navigation and safety references before publication. Useful UK sources include Admiralty/UKHO nautical charts, NP5011 chart symbols and abbreviations, Admiralty Notices to Mariners, RYA navigation guidance, the Met Office Inshore Waters Forecast and local harbour information.
For safety-critical navigation, use current official charts and publications, check relevant corrections, and seek qualified instruction if you are unsure.
Further Reading
