Lesson routeWhat a Chart Is For0/6 read
Lesson 1 of 6
What a Chart Is For
What you will learn
Choose a current chart of suitable scale and use it as a decision model for position, hazards, depth and route monitoring.
A nautical chart represents the information a navigator needs to judge a marine route. It is more than a picture of the coast, but it is not live reality and it is not self-validating.
A nautical chart is a special-purpose map for maritime navigation. It can show soundings, depth contours, drying areas, dangers, aids to navigation, coastline, conspicuous features, routeing information, tidal information and notes. Some of those features are below the surface and cannot be checked just by looking over the side, which is why symbols, source quality and uncertainty matter.
Choose a chart that covers the route at a useful scale. A large-scale chart covers a smaller area with more detail and is normally the better view for harbours, anchorages and narrow water. A small-scale chart covers a wider area and is useful for overview planning, but it may omit detail needed for close navigation. Zooming a raster image does not create information that was absent at its source scale.
Check the chart identity, scale, units, horizontal and vertical datums, edition or update status, cautions and source information before relying on it. MCA guidance requires applicable charts and publications to be of sufficient scale and detail, of the latest obtainable edition and kept up to date; a leisure-craft plan should apply the same safety logic proportionately using current authoritative information suitable for the trip.
Use the chart to form expectations before departure and to monitor them underway: where the boat should be, what should be visible, what depth is expected, what danger comes next and what action is available if the evidence disagrees. A route line is a proposal to keep checking, not a corridor that has been declared safe merely because it can be drawn.
| Chart check | Question to answer | Why it matters |
|---|---|---|
| Coverage and scale | Does this view cover the route and show enough detail for the next decision? | Overview charts and harbour charts serve different jobs |
| Status | Is it the applicable current edition or service, with relevant updates applied? | A familiar chart can contain superseded dangers, marks or restrictions |
| Units and datums | What reference is used for position, depth and height? | Numbers cannot be combined safely when their reference systems are misunderstood |
| Notes and sources | What limitations, cautions or survey quality affect the route? | The neatness of the print does not reveal the certainty of the underlying data |
Choose the working chart for a harbour approach
A planning chart shows the whole afternoon route, while a larger-scale harbour chart shows the entrance marks, drying ledges, leading line and local caution in much more detail.
1. Confirm coverage
Use the planning chart for the overview, then confirm that the harbour chart covers the approach, alternative and turning area.
2. Check authority and status
Read the chart identity, scale, units, datums, edition or update information and relevant notes rather than assuming a familiar-looking display is current.
3. Build expectations
Mark the hazards, depth limits, visual or electronic monitoring cues and the point at which the approach should be paused or abandoned.
4. Monitor reality
Compare the charted picture with marks, depth, traffic, timing and position evidence throughout the approach.
Sense check: The chart that shows the entire coast most neatly may still be the wrong scale for deciding which side of the entrance danger to pass.
Common mistake or limitation
- Treating a route line, saved waypoint or printed chart as proof that the underlying information is current and suitable.
- Assuming a smaller scale number or a larger screen zoom automatically means more source detail.
- Reading only the coloured chart area while missing the title, units, datums, source diagram, cautions and local notes.
Recap
- A chart is a scale-dependent decision model, not live reality.
- Choose enough coverage and enough detail for the decision being made.
- Check status, units, datums, notes and sources before relying on the picture.
- Use the chart to predict and monitor; investigate disagreement early.
Optional quick check
Section 1 of 6
Which chart is the stronger working choice for the final harbour approach?
Lesson 2 of 6
Read Depths, Symbols, and Notes
What you will learn
Interpret a charted sounding, contour, drying height, symbol and note without turning colour or memory into unsupported certainty.
Charted numbers and symbols only make sense with their units, datum, legend and surrounding notes. Colour helps organise the picture; it does not replace the exact charted information.
A sounding is a charted depth value referred to the chart's stated vertical datum. A depth contour joins positions of equal charted depth. A drying height describes a feature that rises above chart datum and may cover and uncover. Read the chart's units and datum statement before combining any of these values with tidal information.
Colours help distinguish land, intertidal areas and depth bands on standard paper-chart presentations, but colour alone does not state the exact depth or safe side. The relevant sounding, contour, danger symbol, note and vessel requirement still govern the decision. Electronic vector presentations may also hide or simplify detail at some zoom levels or settings.
International chart specifications standardise many symbols and abbreviations, while UKHO NP5011 explains those used on paper charts. Look up an unfamiliar or consequential symbol in the chart key or applicable current reference rather than inferring its meaning from shape, colour or a different app.
Read notes as part of the chart, not as decoration. A note may define a restriction, reporting measure, caution, dredged depth, changing condition, unsurveyed area or source limitation that alters how the visible route should be judged. Where chart information is uncertain or old, increase the margin and seek stronger current evidence.
| Chart element | Meaning to establish | Do not assume |
|---|---|---|
| Sounding or contour | Depth, units and stated vertical datum | That the displayed number is live water depth |
| Drying height | Height above chart datum of a feature that may cover and uncover | That it is a depth beneath the vessel |
| Symbol or abbreviation | Exact meaning and any qualifying text | That colour or resemblance supplies the definition |
| Caution or source note | Limitation, restriction, date or uncertainty | That information outside the plotted line is irrelevant |
Resolve a tempting shortcut
A pale-blue area looks open on a training chart, but the intended line crosses a contour, an unfamiliar obstruction symbol and a caution note near the margin.
1. Read the exact charted data
Identify the soundings, contour labels, units and datum rather than deciding from colour alone.
2. Resolve the symbol
Use the chart key or applicable current symbol reference to identify the obstruction and any qualifiers.
3. Read the note
Check whether the caution changes the reliability, restriction or route choice in that area.
4. Apply the boat and conditions
Combine the chart evidence with tide, draught, margin, weather and a practicable alternative before choosing the route.
Sense check: If the decision can only be defended by saying 'the colour looked deep enough', the governing evidence has not been read.
Common mistake or limitation
- Treating every blue or white area as a universal depth or safe-water instruction.
- Assuming a drying height is an ordinary sounding below chart datum.
- Guessing an unfamiliar symbol from memory without checking its qualifiers, note or publication context.
- Ignoring survey quality, cautions or chart notes because the route line itself looks clear.
Recap
- Read units and datum before using a depth or height.
- Colour organises information; soundings, contours, symbols and notes provide the decision detail.
- Resolve unfamiliar symbols from the applicable current reference.
- Increase margin when the chart itself signals uncertainty.
Optional quick check
Section 2 of 6
A route crosses an unfamiliar chart symbol in apparently deep-coloured water. What is the next sound action?
Lesson 3 of 6
Latitude, Longitude, and Formats
What you will learn
Read, repeat and plot a latitude/longitude position without swapping axes, formats, hemispheres or datums.
Latitude states north or south; longitude states east or west. Preserve the complete format, decimal point, hemisphere and datum so the numbers still describe the intended place.
Latitude is angular position north or south of the equator. Longitude is angular position east or west of the reference meridian. IHO chart specifications normally express geographical positions as degrees, minutes and decimals of a minute, with latitude first and longitude second; degrees, minutes and seconds may also appear when appropriate to the chart.
Keep one notation intact. For example, 50°43.20'N is fifty degrees, forty-three point two zero minutes north; the .20 belongs to minutes. It is not 50.4320 decimal degrees and it is not forty-three minutes twenty seconds. Preserve leading zeros, decimal places and N/S/E/W when copying or speaking the position.
On a paper chart, take latitude from a side border and longitude from a top or bottom border, using the chart's own graduations. Transfer the latitude line across and the longitude line up or down; their intersection is the plotted position. Label the time and source so the mark has operational meaning, and use the plotting convention specified by the course or vessel.
A coordinate also has a horizontal datum. A receiver and chart using different datums can plot the same real position differently. Many modern systems use WGS 84, but do not assume: read the chart and device. Extra decimal places show precision of display, not guaranteed accuracy or integrity.
Worked example
Convert only the minutes portion of 50°43.20'N to degrees as an arithmetic check, without changing the recorded source format.
- 143.20 minutes ÷ 60 = 0.72 degrees.
- 250 degrees + 0.72 degrees = 50.72°N.
- 3Keep 50°43.20'N as the working chart notation unless a receiving system explicitly requires decimal degrees.
Sense check: The latitude must remain between 50°N and 51°N; 50.72°N fits, while 50.4320°N would not be an equivalent conversion.
Copy and plot a training position
The receiver displays 50°43.20'N 001°16.80'W on WGS 84, and the training chart uses the same horizontal datum.
1. Copy the complete position
Write latitude first, then longitude, including degrees, decimal minutes, N and W: 50°43.20'N 001°16.80'W.
2. Set latitude
Find 50°43.20'N on the side latitude scale and transfer that level across the chart.
3. Set longitude
Find 001°16.80'W on the top or bottom longitude scale and transfer that line to meet the latitude.
4. Label and verify
Mark the intersection with time and source, then check that the plotted place fits the receiver, route and surrounding evidence.
Sense check: Because the coordinate is west, plotting it on the east side of the reference meridian would be a hemisphere error even if every digit were copied correctly.
Common mistake or limitation
- Reading decimal minutes as seconds or copying degrees-minutes as though they were decimal degrees.
- Dropping the N, S, E or W hemisphere, or reversing latitude and longitude.
- Plotting latitude from the longitude border or measuring route distance from longitude graduations.
- Assuming matching digits guarantee matching horizontal datums or accurate position evidence.
Recap
- Latitude is N/S and is normally written first; longitude is E/W and follows.
- Preserve the displayed notation, decimal places, hemisphere and datum.
- Use the chart's side border for latitude and top or bottom border for longitude.
- Displayed precision does not establish accuracy or integrity.
Optional quick check
Section 3 of 6
Which reading preserves the meaning of 50°43.20'N?
Lesson 4 of 6
Measure Distance on the Chart
What you will learn
Measure a straight or multi-leg Mercator-chart route, read the intended nearby scale and turn the distance into a checked time estimate.
Transfer each route span unchanged to the nearby latitude scale, read minutes and tenths as nautical miles for practical Mercator chartwork, then test the result against speed and route geometry.
Open dividers between the endpoints of one safe straight route leg. Lift them without changing the span and transfer that span to the side latitude scale near the route's middle latitude. In practical Mercator chartwork, minutes and decimals of a minute on that nearby scale are read as nautical miles.
Use the nearby latitude because Mercator scale changes with latitude. Do not use the top or bottom longitude border as the nautical-mile ruler: longitude spacing represents a different ground distance at different latitudes. A chart may provide a labelled graphic scale; use it only for the chart and units it explicitly states.
The chartwork convention and the unit definition are related but not identical claims. The international nautical mile is defined exactly as 1,852 metres. Reading a nearby latitude minute as one nautical mile is the practical plotting convention for the Mercator chart, not proof that every geodetic minute everywhere is exactly 1,852 metres.
Measure bent routes as their intended straight legs and retain the intermediate waypoint. Add the legs, then estimate time with a compatible speed: time in hours = distance in nautical miles ÷ speed in knots. The calculation is a forecast, so monitor actual speed, tide, route and conditions underway.
Latitude scale
For this chartwork measure, read 1′ on the nearby latitude scale as 1 M.
Transfer the unchanged route span to the latitude scale near the route; follow any chart-specific scale instructions.
Worked example
Show every step for the 4.1 M route at a planned speed of 5 kn.
- 1Distance = 2.4 M + 1.7 M = 4.1 M.
- 2Time = distance ÷ speed = 4.1 ÷ 5 = 0.82 hours.
- 3Minutes = 0.82 × 60 = 49.2 minutes, so plan on about 49 minutes before applying operational allowances.
- 4Retain the intermediate waypoint because the course, hazards and monitoring evidence change there.
Sense check: The arithmetic cannot prove the route is safe or that 5 kn will be achieved; those remain separate planning and monitoring questions.
Measure and time a two-leg route
A fictional training route has a first leg of 2.4 M and a second leg of 1.7 M. The conservative planned speed over the ground for this simple estimate is 5 kn.
1. Keep the bend
Measure each intended safe leg separately; do not replace the bend with a shorter line across the inside hazard.
2. Read the distances
Transfer the unchanged spans to nearby latitude and record 2.4 M and 1.7 M.
3. Add and time
Total distance = 2.4 + 1.7 = 4.1 M. Time = 4.1 ÷ 5 = 0.82 hours = 49.2 minutes.
4. Monitor the assumption
Use about 49 minutes as an estimate and compare actual progress with the planned 5 kn speed and the waypoint evidence.
Sense check: At 5 kn, 5 M would take one hour, so a 4.1 M route should take a little under an hour; about 49 minutes fits.
Common mistake or limitation
- Changing the divider span while transferring it from the route to the scale.
- Using longitude graduations as nautical miles or using a distant latitude scale without considering projection change.
- Measuring a bent safe route as one straight shortcut and understating both distance and hazard exposure.
- Treating a calculated ETA as fixed after tide, speed or route changes.
Recap
- Measure one safe straight leg at a time and preserve each divider span.
- Read distance on nearby latitude for practical Mercator chartwork, unless the chart supplies another suitable labelled scale.
- One nautical mile is exactly 1,852 metres; keep that unit fact distinct from the plotting convention.
- Turn distance into a checked time estimate and monitor the assumption underway.
Optional quick check
Section 4 of 6
A two-leg route measures 2.4 M and 1.7 M. What total should be carried into the time calculation?
Lesson 5 of 6
Position Awareness
What you will learn
Maintain a believable navigation picture by comparing the last trusted position, expected progress and independent observed evidence.
Position awareness is not a glowing boat icon. It is a continuously tested account of where the vessel probably is, what should happen next and how uncertainty will be controlled.
Start from the last position you had good reason to trust and record its time and source. Carry the boat's expected progress forward using course, speed, elapsed time and relevant effects. The longer the interval and the less certain the inputs, the wider the uncertainty becomes.
Compare independent evidence where practical: visual bearings or transits, buoyage, depth trend, coastline or harbour features, timing, GNSS or plotter information and the vessel's movement. Agreement between repeated displays fed by the same GNSS source is not independent confirmation.
Modern GNSS can display a highly precise coordinate, but precision does not establish integrity. RYA digital-first guidance emphasises verifying the offered position and using situational awareness. In pilotage, knowing the boat is on the intended transit between dangers can be more decision-useful than merely reading latitude and longitude.
A mismatch is useful information. Slow down or stop in safe water if appropriate, preserve sea room, establish the last trusted evidence, check chart and device settings, seek another observation and avoid continuing towards danger while hoping one source will correct itself. Depth can warn that the picture is wrong, but one sounding rarely provides a unique fix.
| Evidence | Useful question | Important limitation |
|---|---|---|
| Last trusted position | Where and when was the picture last supported? | Uncertainty grows with movement and time |
| GNSS or plotter | Does the position and route fit other evidence? | Precision and repeated displays do not guarantee integrity |
| Visual mark or transit | Is it correctly identified and appearing as expected? | Visibility, identification and charting can be wrong |
| Depth trend | Is depth changing in the expected direction and range? | Tide, offset, seabed shape and measurement uncertainty affect the comparison |
Respond when the approach picture disagrees
The plotter icon sits on the planned line, but the expected leading marks are not aligned, the next buoy is not where it should appear and the depth is shoaling earlier than planned.
1. Protect control
Reduce speed, preserve sea room and avoid advancing towards the charted danger while the discrepancy is unresolved.
2. Return to trusted evidence
Identify the last position, time and source that were genuinely supported, then estimate what movement has occurred since.
3. Cross-check independently
Check the visual marks, depth trend, chart scale and datum, device source and settings, and another position method where available.
4. Choose a controlled next action
Continue, hold, return or divert only when the combined evidence and available margin support that choice.
Sense check: Three disagreements with the plan are stronger evidence of uncertainty than one precise screen icon is evidence of safety.
Common mistake or limitation
- Treating multiple screens connected to the same receiver as independent position checks.
- Waiting for certainty while continuing at the same speed towards a hazard.
- Using one depth value as a unique position without considering tide, sounder reference and similar contours elsewhere.
- Discarding the last trusted position because the current display contains more decimal places.
Recap
- Anchor the picture to a time-labelled last trusted position.
- Predict what should happen, then compare independent evidence.
- Precision is not the same as accuracy or integrity.
- Treat mismatch as a prompt to control the boat and investigate early.
Optional quick check
Section 5 of 6
The plotter agrees with the route line, but the transit, buoy and depth trend do not. What is the strongest response?
Lesson 6 of 6
Depth, Draught, and Raw Clearance
What you will learn
Calculate raw under-keel clearance from compatible chart, tide and vessel data, then keep safety allowance and uncertainty visible as separate decisions.
Water depth and clearance are not the same number. First combine charted depth with height of tide on the same vertical reference; then subtract draught; then judge the margin and uncertainty.
For the simple training case where both values use chart datum, estimated water depth = charted depth + predicted height of tide. Raw under-keel clearance = estimated water depth − vessel draught. Show the units and references beside the working so an incompatible datum or sign is easier to catch.
Raw clearance is not a safety margin. The decision may also need allowance for forecast and tidal uncertainty, pressure and wind effects, waves, heel, squat, vessel loading, seabed and survey uncertainty, and the consequence of contact. The appropriate policy depends on the vessel, area, conditions and competent operating guidance.
Know what draught represents for the actual loading and appendages. Know what the sounder displays: depth below the transducer, below the keel or below the surface may be selected through an offset. A sounder reading should not be combined with chart datum as if both numbers shared the same reference without checking.
A conservative beginner route avoids using a small calculated remainder as proof of safety. If the plan depends on precise tide, draught or seabed assumptions, seek competent instruction, current local information and a route with more margin. This screen lesson and fictional arithmetic do not establish practical chartwork competence or safe clearance for a real vessel.
Worked example
Calculate the simple raw clearance and state what the answer does not prove.
- 1Estimated water depth = 1.8 m charted depth + 2.1 m predicted height of tide = 3.9 m.
- 2Raw under-keel clearance = 3.9 m water depth − 1.4 m draught = 2.5 m.
- 3Keep the 2.5 m as raw clearance before the chosen allowance for uncertainty and vessel operation.
- 4Recheck the route if any datum, tide, loading, offset, seabed or condition assumption changes.
Sense check: Subtracting tide or adding draught would move the arithmetic in the wrong physical direction for this stated case.
| Quantity | Training value | Reference check |
|---|---|---|
| Charted depth | 1.8 m | Depth below the chart's stated vertical datum |
| Height of tide | +2.1 m | Predicted height above the same datum for the relevant place and time |
| Estimated water depth | 3.9 m | 1.8 + 2.1 for this simple compatible case |
| Loaded draught | −1.4 m | Actual vessel requirement for the stated loading |
| Raw clearance | 2.5 m | Before the chosen safety allowance and uncertainties |
Separate arithmetic from the safety decision
On a fictional training chart, the route crosses a 1.8 m sounding. Predicted height of tide is 2.1 m above the same chart datum, and loaded vessel draught is 1.4 m.
1. Check references
Confirm that the 1.8 m sounding and 2.1 m tidal height use the same chart datum and that draught is for the loaded vessel.
2. Find estimated water depth
1.8 m + 2.1 m = 3.9 m estimated water depth for the simple case.
3. Find raw clearance
3.9 m − 1.4 m = 2.5 m raw under-keel clearance.
4. Judge the margin separately
Apply the vessel's competent clearance policy and current uncertainty; do not label the route safe from 2.5 m alone.
Sense check: Water depth must exceed draught for positive raw clearance. Here 3.9 m − 1.4 m leaves 2.5 m, which is arithmetically consistent but not yet an operational verdict.
Common mistake or limitation
- Calling charted depth the current water depth without adding the compatible height of tide.
- Forgetting to subtract loaded draught, or treating the remainder as an automatic safety margin.
- Mixing a depth-sounder offset with chart-datum values without reconciling their reference points.
- Using predicted tide and a single sounding as exact despite weather, survey, vessel and seabed uncertainty.
Recap
- Check units and vertical references before combining chart and tide values.
- Water depth = charted depth + compatible height of tide for the simple case.
- Raw clearance = water depth − loaded draught.
- Apply allowance and uncertainty separately; arithmetic alone does not declare a route safe.
Optional quick check
Section 6 of 6