Charts and Position Basics

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.

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.

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.

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.

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.

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.