Nautical chart symbols

A plain number on the water is a depth reading at that spot, taken at the chart's low-water reference (the datum), usually mean lower low water in the US.

Underway: It is close to the least water you can expect there. The real depth is this number plus the current height of tide.

An underlined number is a height that dries, meaning it stands above the water at low tide by that amount. On some charts it is shown in parentheses.

Underway: This ground is exposed at low water and covered at high water. Treat it as a hazard until the tide clearly covers it with room to spare.

A line joining points of equal depth, often labelled with its value. Shallow water inside a contour is usually tinted blue.

Underway: Following a contour is a classic way to skirt a shoal. Set your own safety contour and stay outside it.

A sounding shown with a line over it, or marked No bm, means the sounder reached that depth without finding the bottom.

Underway: The water is at least that deep. It is a minimum, not a measured seabed depth.

A rock that is roughly at the level of the chart datum, shown as an asterisk-like star or a cross with dots. It is awash near low water.

Underway: A serious hazard at low tide and often invisible at high tide. Give it a wide berth.

A rock that stays underwater, drawn as a plus sign, often ringed by a dotted danger line. A number beside it is its depth.

Underway: Dangerous because you cannot see it. If it is inside a dotted circle, stay well clear.

A rock that shows above water at low tide, drawn with its drying height, sometimes as a small islet outline.

Underway: Visible at low water, hidden at high water. Judge your clearance against the state of tide.

A small hazard, such as a rock or wreck, with navigable water all around it, often circled by a dotted danger line.

Underway: You can pass on any side, but not over it. It is frequently marked by an isolated danger buoy.

A wreck with a shallow or unknown depth over it, drawn as a hull outline with cross-hatching or a dotted danger line, sometimes with a depth or the letters Wk.

Underway: The one to avoid. Assume little clearance and keep clear unless a safe charted depth says otherwise.

A wreck with enough charted water over it to be safe for surface navigation, usually a simpler hull symbol with a depth figure.

Underway: Deep enough that you clear it, but still a fishing snag and a good landmark. Confirm the charted depth against your draft.

A wreck whose hull or masts stand above the water at the chart datum, drawn showing the structure.

Underway: Visible, but the wreck often spreads underwater beyond what shows. Do not shave it close.

An area, often marked Foul or with a dotted boundary, cluttered with debris, old moorings, or rocky ground.

Underway: Usually safe to pass over on the surface but bad for anchoring, as your anchor can snag or foul. Anchor elsewhere.

Anything on the bottom that obstructs navigation but is not clearly a wreck or rock, marked Obstn, often with a depth.

Underway: Treat like a submerged hazard. If no depth is given, assume it could be shallow.

In US waters (IALA Region B), a red buoy, usually a flat-topped can with an even number, marks the edge of the channel to keep on your right when returning from sea.

Underway: Red, right, returning: keep red marks to starboard when heading upstream or in from sea.

A green buoy, usually a pointed-top nun or spar with an odd number, marks the other edge of the channel in US waters.

Underway: Keep green marks to port when returning from sea. Between the red and green is the channel.

A red-and-white vertically striped buoy marking mid-channel or a safe landfall, with navigable water all around.

Underway: Often the sea buoy at a harbor entrance. You can pass either side; it is not a danger.

A black buoy with a broad red band and two black spheres on top, moored on or near an isolated hazard.

Underway: Stay clear all around: there is a danger right here with safe water surrounding it.

A fixed mark on a pile rather than a floating buoy, shown as a small triangle (green, keep to port) or square (red, keep to starboard).

Underway: Same channel rules as buoys, but fixed and reliable. Green square-point triangle vs red square tells you the side.

A lighted mark is drawn with a magenta teardrop or flare over its position. The text beside it spells out the light's character.

Underway: Match the flashing pattern, color and timing you see to the chart label to confirm which light you are looking at.

Fl means a flashing light: the light is off more than it is on, showing single flashes. Example: Fl G 4s is a green flash every 4 seconds.

Underway: Read it as pattern, color, period, height, range. Fl R 6s means a red flash every 6 seconds.

Oc means occulting: the light is on more than it is off, with brief eclipses. The opposite balance to a flashing light.

Underway: A steady light that briefly winks out. Useful for telling two nearby lights apart.

Iso means isophase: equal times on and off. Iso 6s is 3 seconds lit, 3 seconds dark.

Underway: A regular, even blink. The period is the full on-plus-off cycle.

Some lights show different colors over different arcs of bearing, drawn as sectors around the light, often with a red sector over a danger.

Underway: The color you see tells you which sector, and therefore which side of a hazard, you are on. A red sector usually means stand off.

An anchor symbol, often within a dashed magenta boundary, marks a designated or recommended anchorage.

Underway: Good holding is likely and it is a sanctioned place to anchor, but check for cables and depth. A crossed-out anchor means anchoring is prohibited.

A magenta dashed boundary, often with a note, around water where navigation is limited or prohibited, such as a naval or security zone.

Underway: Read the note. Entry may be forbidden, seasonal, or require permission. Do not assume you can transit.

A magenta line down a waterway marks a recommended route, most famously the Intracoastal Waterway.

Underway: A planning aid, not a guarantee of depth. Shoaling moves; the magenta line does not, so trust current soundings and local knowledge.

A wavy line, often magenta, marks a submarine cable or pipeline on the seabed, sometimes within a cable area box.

Underway: Do not anchor or trawl here. A dragged anchor can part a cable and hook you to it.

PA next to a feature means its charted position is only approximate.

Underway: The object is roughly, not exactly, where it is drawn. Give it extra room.

PD means the position is doubtful: it may not be where charted at all.

Underway: Treat the location as unreliable and keep well clear.

ED means existence doubtful: the feature was reported but may not actually be there.

Underway: It might not exist, but you cannot assume that. Navigate as if it does.

SD means a sounding is doubtful: the depth shown may be unreliable.

Underway: Do not trust that depth. Assume it could be less and leave margin.

Rep, sometimes with a year, means a feature or depth was reported by a mariner but not confirmed by survey.

Underway: Credible enough to chart, but not verified. Treat with caution.

S beside a sounding means the bottom is sand.

Underway: Generally good anchor holding, especially firm sand. One of the better bottoms to drop the hook.

M means a mud bottom.

Underway: Usually good holding once the anchor digs in, though soft mud can be poor. Common in harbors and estuaries.

Rk (or R) means a rocky bottom.

Underway: Poor and unreliable holding, and hard on ground tackle. Your anchor can jam or drag over rock. Avoid anchoring if you can.

Wd (weed) or Grs (grass) means a weedy or grassy bottom.

Underway: Often poor holding: the anchor grabs weed instead of the seabed. Set carefully and check it is dug in.

Co means a coral bottom.

Underway: Poor holding and environmentally sensitive. Use a mooring where provided rather than anchoring in coral.