Chart Symbols

Check the chart's depth unit and datum. A sounding is not a guaranteed minimum: water level can fall below predicted datum, the seabed can change, and surveys have limits.

Add the predicted height of tide and then subtract the drying height to estimate water over the feature, allowing for weather, squat, and uncertainty.

Use contours to understand the shape of shoal water, but inspect individual soundings and survey quality too. A contour is not a guaranteed clearance boundary.

Allow more margin around an approximate contour and inspect the chart's source or survey-quality information before using it for close pilotage.

Silting can reduce the available depth between surveys. Check current harbour information and Notices to Mariners before relying on the maintained figure.

Blank water is not evidence of deep water. Allow a much larger safety margin and avoid shortcutting through the area without reliable local information.

The printed rate is not constant. Use the tidal-stream table or almanac for the time and tidal range of your passage.

Flood and ebb can follow different axes near headlands and entrances. Use time-specific stream data rather than treating the arrow as a constant.

The diamond itself does not show the current. Use the correct reference port, time zone, and spring–neap interpolation.

The rock can be visible at low water and hidden near high water. Plan clearance from chart scale, tide, sea state, and position accuracy.

At most higher states of tide the rock is hidden just below the surface. Do not depend on seeing it before taking avoiding action.

The dotted surround emphasizes a point hazard; it is not a surveyed exclusion radius. Decide clearance from scale, depths, tide, sea state, and position accuracy.

Treat the number as a least depth at chart datum, not a passage recommendation. Apply tide, uncertainty, sea state, and under-keel margin.

Visible structure does not show the full underwater extent of the wreck or its debris. Pass with clearance appropriate to the chart scale.

The visible projections mark the location, not the wreck's full extent. Allow for submerged hull, rigging, and debris.

The surround is symbolic rather than a measured keep-out radius. Determine a safe passing distance from chart scale, depths, survey quality, and conditions.

It is not necessarily deeper than 20 metres and can still endanger anchoring, fishing, diving, or deep-draught craft.

The dotted surround marks the position symbolically rather than defining a safe passing distance. Use the chart scale and all nearby depth information.

Check the surrounding depths before crossing; 'not dangerous to surface navigation' is not a guarantee for every draught or sea condition.

An entrance that is deep enough in calm water can become unsafe with an onshore swell. Check local bar and harbour guidance.

Conditions can become severe at springs or with wind against tide. Small craft often plan these passages near slack water.

Sand can provide good holding, but holding also depends on layer depth, compaction, slope, anchor type, and conditions.

Mud can provide good holding, but very soft or shallow mud behaves differently. Consider the complete bottom description and local conditions.

Rock commonly gives poor or snag-prone anchoring. Use the full chart context and local guidance before choosing the position.

Coral can damage ground tackle and may be protected. Avoid anchoring on coral and use designated moorings where provided.

A shell description alone does not predict holding. Consider whether sand or mud is also charted and assess conditions locally.

Measure from the position mark rather than the flare, and read the complete characteristic instead of inferring range from icon size.

The position circle—not the flare—gives the charted location. Read the adjacent characteristic for colour, rhythm, and nominal range.

White often marks a preferred sector, but it is not a universal promise of safe water. Follow the charted bearings, notes, and pilotage directions.

If the lights open, manoeuvre to close them again while checking the chart and surrounding traffic. Avoid a universal 'steer toward the upper light' mnemonic—it reverses with geometry and direction of travel.

Body shape is only part of the mark. Confirm colour, topmark, light, and the applicable IALA region before deciding the safe side.

Confirm colour, topmark, light, and IALA region. Lateral colours reverse between Regions A and B even though the body shapes do not.

A spherical body alone does not fully identify the mark. Read its colours, topmark, light, and charted purpose.

Pillar form is neutral: interpret the mark from its colour pattern, topmark, light, and charted function.

Spar form is neutral. Use the colour, topmark, light, and charted function to identify how to pass it.

Do not anchor close to moorings: chains, sinkers, and risers occupy more seabed than the surface buoy suggests.

A beacon may stand on the hazard it marks. Identify its complete colour and topmark pattern before choosing the side to pass.

The tower may carry a light, colour pattern, or topmark. Use all charted information rather than the silhouette alone.

Use the charted arrow, not the vessel's heading, to interpret lateral marks. In IALA Region B the lateral colours reverse, while the can/cone shapes do not.

Recommended does not mean guaranteed clear for every vessel. Check depth, draught, traffic, tide, and cross-track effects.

A charted anchorage is not a weather guarantee. Check shelter, depth, bottom, swinging room, local allocation, and current restrictions.

The restriction may protect cables, pipelines, fairways, habitats, or security areas. Read the associated legend and local rules.

The line alone does not say what is forbidden. Read the charted legend, notes, and current local regulations.

COLREG Rule 10 applies. Small craft crossing a lane should do so on a heading as nearly as practicable at right angles and must not impede lane traffic.

Avoid anchoring or dragging gear nearby. If ground tackle fouls a suspected cable, do not attempt to haul the infrastructure aboard.

Avoid anchoring or dragging gear nearby. A pipeline may stand proud of the seabed even where the surrounding depth is ample.

Compare the correct clearance datum with actual water level and the vessel's complete air draught, allowing for waves and vessel motion. High-voltage lines can arc before contact.

The route is a warning, not a grant of right-of-way. Apply the COLREGs, maintain a proper lookout, and never pass between a cable ferry and its landing.

Compare the correct datum with actual water level and full air draught, allowing for waves, vessel motion, uncertainty, and local restrictions.

Confirm whether the figure applies with the bridge closed, plus the opening schedule, signals, height datum, water level, and full air draught.

Use the precise charted position and description rather than matching any vaguely similar skyline object.

Check the abbreviation and charted position before using the structure for a visual bearing.

Aircraft obstruction lights on a chimney are not marine navigation lights. Use the charted position and description.

Use the charted position and any qualifier rather than assuming every visible cylindrical structure is the marked tank.

Red aviation lights are not marine navigation lights. Take bearings from the charted position and confirm the structure description.

The symbol identifies the facility, not berth availability or approach depth. Check the plan, pilot book, and current marina information.

It indicates intended use, not guaranteed availability, suitability, or permission. Check local instructions and the mooring's limits.

The mark does not guarantee a free berth. Contact the harbour or marina and follow berth-allocation instructions.

Tide, surge, local restrictions, and the condition of steps or pontoons still determine whether landing is safe.

The chart does not confirm gradient, surface, tidal access, fees, parking, or trailer suitability. Check current local details.

Availability, hose fittings, opening hours, and whether the water is potable can change—verify locally.

It does not guarantee fuel grade, depth alongside, hours, or current operation. Confirm before depending on it.

Use current VHF, telephone, notices, or posted instructions; the office location alone does not state the reporting procedure.