Chart Symbols

Always check the chart title block for the unit (meters, feet, or fathoms) before trusting any sounding — the same '3' can mean 3 m or 3 ft. Chart datum is a low-water level, so the real depth is usually more, never rely on less.

Drying areas are usually tinted green. At high water there may be plenty of depth here; near low water it is dry land. The underline is the giveaway that the number means height, not depth.

Contours give the shape of the bottom at a glance. Many skippers pick a 'safety contour' (say 5 m) and treat everything inshore of it as out of bounds — a simple, powerful passage-planning habit.

Treat dashed contours with extra margin — the bottom between old survey lines can hold surprises. The source diagram on the chart shows how well each area was surveyed.

If the legend shows a survey year rather than 'maintained', silting may have reduced the depth since. Big ships need the whole dredged depth — small craft should stay clear of their track anyway.

Visible at low water, invisible and just awash near high water — the most treacherous state. Give charted asterisks a wide berth whenever the tide is above the rock's drying height.

At almost any state of tide this rock is hidden just below the surface. It rarely shows enough to be seen, which is exactly why it gets its own symbol.

'Depth unknown' means nobody has measured how little water is over it — assume the worst. The dotted danger circle is the chart's way of saying: keep outside this line.

Unlike an ordinary sounding, this depth belongs to a pinnacle — the bottom around it may be far deeper, so it will not show on your depth sounder until you are on top of it.

Visible wrecks are easy to avoid by eye in daylight — but remember debris often extends underwater well beyond what shows.

The visible masts mark the spot but not the extent — the hull, rigging, and debris spread around them. Pass well clear rather than steering close to 'have a look'.

This is the wreck symbol to fear: nobody knows how close it comes to the surface. It is often screened in blue as an extra warning. Never anchor near it either — ground tackle loves old wrecks.

'Non-dangerous' applies to surface navigation only — it still matters to anyone anchoring, trawling, or diving. The presence or absence of the dotted circle is the whole difference from K 28.

An obstruction could be anything — a lost container, pilings, machinery. With no depth given, treat it exactly like an unknown rock and stay outside the dotted line.

Foul ground is the classic anchor-eater. There is enough water to sail across it — just never put gear on the bottom there.

Breakers visible from seaward are nature's own danger buoyage. Charted breakers over a bar can make an otherwise navigable entrance impassable in onshore swell — time such entrances carefully.

Kelp needs rock to anchor to, so charted kelp is a strong hint of foul, rocky bottom close below. It also fouls propellers and drags anchors — treat kelp patches as hazards, not scenery.

The water here can be violent in an otherwise calm sea, and worst at spring tides with wind against tide. Small craft plan to pass races at slack water or give them miles of clearance.

The 'authorized' depth is a permit condition, not a survey — material may stand higher. Avoid anchoring or passing closely over fish havens in shallow water.

Sand is usually good anchor holding once the hook digs in. Bottom-nature letters cluster around designated and popular anchorages for exactly this reason.

Mud gives excellent holding for most anchor types, though very soft mud may let a light anchor drag. Expect a dirty foredeck when you weigh.

On rock an anchor either skates or jams — neither is good. If you must anchor, use a trip line so you can pull the anchor out backwards when it snags.

Coral is razor-sharp (it chafes through nylon rode fast), snags chain, and is a protected living habitat in many places — anchoring on it is often illegal as well as unwise. Find sand nearby instead.

Loose shell layers can prevent an anchor digging into the firmer material beneath. Combinations tell the real story: 'M.Sh' holds far better than plain 'Sh'.

Magenta is the chart's color for lit aids: wherever you see the flare, the mark works at night. Decode the characteristic label with the Light Characteristics topic — it is the light's fingerprint.

The circle, not the flare, is the light's true position — measure from the circle. Minor lights have shorter ranges, so do not expect to raise them far offshore.

Red sectors usually cover dangers: if a sector light turns from white to red, you have crossed into hazardous water — alter course until it shows white again. The sector limits are bearings toward the light, as seen from seaward.

When the two lights appear one above the other you are exactly on the safe track — if the top light slides left, steer left (and vice versa). Ranges are the most precise visual guidance a channel can give.

Shape is a navigation signal in itself: cones/nuns are 'pointed' marks (red, starboard-hand in US IALA B waters; green starboard-hand in Region A). Color letters beside the glyph (R, G, Y) confirm it.

In US (IALA B) waters cans are green port-hand marks ('leave green cans to port returning'); in Region A cans are red port-hand marks. The charted shape plus the color letter tells you which side to pass.

Spheres never mark a side of the channel: the safe-water (fairway) mark is spherical or carries a red sphere topmark, meaning navigable water all around.

Because a pillar's shape is neutral, its meaning comes entirely from color, topmark, and light — read those, not the outline.

Spars are cheap and ice-resistant, so they are everywhere in small-craft channels. Like pillars, their meaning is carried by color and topmark, not shape.

Heavy ground tackle — chains and sinkers — radiates from a mooring buoy on the bottom, so never anchor close to one. Picking one up uninvited may also earn you a large, fast neighbor.

Beacons do not swing or drag like buoys, so their position is exact — but they stand on the very shoal they mark. Never pass close on the 'wrong' side.

Beacon towers are excellent daytime fixing marks — solid, charted precisely, and visible far off. Many carry lights and appear with a magenta flare at night.

Dayboards do the work of buoys in shallow US channels like the ICW. Same memory aid applies: red triangles one side, green squares the other — and 'red right returning' in IALA B.

Racons make key marks unmistakable on radar in fog or at night — the flash of Morse on the screen begins at (or just beyond) the mark's position. Magenta again signals an electronic/lit aid.

Time the run both ways and average, to cancel the tidal stream. It is one of the few chart features meant for boat testing rather than hazard avoidance.

Charted anchorages exist because the holding, depth, and shelter are proven — check the nearby bottom-nature letters and soundings before letting go. In busy ports, some numbered areas are reserved for ships.

These areas usually protect submarine cables, pipelines, or fairways. Anchoring there risks your gear, the infrastructure, and a hefty fine — check what the zone protects before even a 'quick stop'.

Restrictions range from 'no anchoring' to 'no entry at any time' (security zones, firing ranges). The T-ticks point into the restricted side — read the legend before assuming you may cross.

COLREGs Rule 10 applies: join and leave at lane ends or shallow angles, cross on a heading at right angles to the flow, and do not impede through traffic. Vessels under 20 m and sailing vessels must not impede power-driven vessels using the lane.

Fouling a power cable can be lethal as well as expensive — if your anchor snags near a charted cable, slip and buoy the rode rather than winching up. Anchoring in cable areas is often prohibited outright.

Pipelines may stand proud of the bottom, so they can snag gear even in 'deep enough' water. Gas under pressure plus a dragged anchor is a genuinely dangerous combination — give charted pipelines a wide berth when anchoring.

Compare the clearance with your air draft honestly — and remember high-voltage lines can arc to a mast before contact. Sailboats have been lost to 'plenty of room' guesses; if in doubt, don't pass under.

Ferries hold schedule, not course-and-speed courtesy — expect them to appear quickly and maneuver late. Never pass between a cable ferry and either shore: the cable rises near the ferry and can catch your keel or prop.

The bottom here changes with every dumping run — a spot charted at 10 m may now be 4 m. Avoid crossing spoil grounds in a deep-draft vessel and never anchor in one.

Platforms show a white Morse 'U' light (··—, 'you are running into danger') and foghorns. Passing inside the safety zone without business there is an offense in most jurisdictions.

Spires were the GPS of the age of sail: tall, fixed, and charted to the meter. A bearing on a charted church is still one of the quickest position lines you can take.

Any charted tower is a candidate for a compass bearing. Check the abbreviation — a water tower's tank shape looks very different from a lighthouse at distance.

On flat industrial coasts chimneys out-mark every natural feature — many are lit with red air-obstruction lights at night, but do not confuse those with navigation lights.

Tank farms sit beside the very harbors you are entering, and their white or silver paint shows up for miles — a practical daytime steering mark.

Masts several hundred meters tall are visible far beyond the coastline itself. Their red flashing obstruction lights are for aircraft — never plot them as navigation lights.

On a rough day this little sailboat is the most welcome symbol on the chart. Check approach depths and the marina's own entry notes — the glyph promises berths, not depth.