Nautical Charts & Chart Symbols

Reading Chart Scale

Chart scale describes how much the real world has been reduced to fit on the chart. A large-scale chart such as 1:10,000 shows a small area in great detail and is ideal for harbors, anchorages, and tight coastal passages. A small-scale chart such as 1:500,000 shows a large area with less detail and is better for offshore passage planning.

A common point of confusion: large scale sounds like it should cover a large area, but it means a large fraction. One divided by 10,000 is larger than one divided by 500,000 — so large scale means zoomed in, small scale means zoomed out. Think of it as magnification: large-scale charts magnify the area and reveal more detail.

Always use the largest scale chart available for the waters you are actually navigating. Small-scale passage charts omit hazards that appear in full detail on larger-scale versions. In confined or complex waters — channels, harbors, anchorages — the largest scale chart is not optional.

Publication Date and Correction History

The date of the current chart edition appears below the lower left border. For NOAA charts, the date of the latest U.S. Coast Guard Local Notice to Mariners applied to the chart is shown alongside it. These dates tell you how current the survey data is.

Older surveys are less reliable. Section A item 17 of Chart No. 1 covers the Source Diagram — a zone diagram printed on some charts showing when different areas were surveyed. Waters surveyed decades ago may have shifted bottom topography, new wrecks, or uncharted obstructions not yet incorporated. Treat recently surveyed areas with more confidence than old survey zones.

The Compass Rose and Magnetic Variation

The compass rose (Chart No. 1 item B 70) is printed at one or more locations on the chart. It shows two concentric rings: the outer ring aligned to true north (geographic north) and the inner ring aligned to magnetic north. The angular difference between them is the magnetic variation, or declination, for that location.

Variation is stated in the compass rose along with its annual rate of change. A typical note might read: VAR 14 degrees 30 minutes W (2019), Annual Decrease 8 minutes. To use this, update the variation to the current year by applying the annual change. In this example, 5 years later the variation would be approximately 14 degrees 30 minutes minus 40 minutes equals 13 degrees 50 minutes W.

When plotting a course from the chart, you read a true bearing. To steer it on a magnetic compass, subtract westerly variation or add easterly variation. The mnemonic is: True Virgins Make Dull Company. True, Variation, Magnetic, Deviation, Compass. Work through each step to convert between chart bearings and compass headings.

Calculating Actual Depth

Charted depth is a reference value, not the actual water depth under your keel at any given moment. Actual depth equals charted depth plus the current tide height above datum.

Example: charted depth at a harbor entrance is 8 feet. Current tide height above MLLW is 3.5 feet. Actual depth at this moment is 8 plus 3.5 equals 11.5 feet. Your boat draws 5 feet. With a 2-foot safety margin you need at least 7 feet. At 11.5 feet you can proceed safely. Return at low tide and the same entrance may have only 7.5 feet — a much tighter margin worth checking in advance.

Drying heights are a related concept covered in Section H. Features that cover and uncover with the tide — rocks, sandbanks, tidal flats — are charted with their height above chart datum, shown underlined on the chart. A rock charted with an underlined 2 stands 2 feet above MLLW at its highest exposure.

Tidal Streams and Current Arrows

Section H items 40 and 41 show how tidal streams are depicted on charts. A flood stream (incoming tide, flowing toward land) is shown with an arrow labeled with its mean spring rate in knots. An ebb stream (outgoing tide) is shown with a feathered arrow or different notation pointing seaward, also with its spring rate.

Current arrows on charts give you a starting point for understanding flow, but rates vary significantly with neap and spring tides, weather, and local geography. For critical passages through tidal channels, supplement charted arrows with a current table or tidal atlas for the specific date and time of your transit.

Item H 44 covers overfalls, tide rips, and races — areas of turbulent water caused by strong tidal flow over uneven bottom or opposing currents. These are shown with a stippled pattern and are worth planning around, especially in smaller vessels.

Depth Colors and Contours

Nautical charts use color to communicate depth zones at a glance. White areas represent water believed to be navigable deep water — generally clear of obstructions. Light blue or shaded blue indicates shoal water, shallower than surrounding areas and requiring caution. Green or yellow marks areas that dry at or near chart datum — tidal flats, intertidal rocks, sandbanks that expose at low water. Tan or buff represents land always above water.

Section I items 30 and 31 define depth contours — lines connecting points of equal depth, drawn like elevation contours on a topographic map. The 5-fathom contour (30 feet) and 10-fathom contour (60 feet) are the most critical for inshore navigation; inside these lines the bottom is shoaling rapidly. In ECDIS, depth contours trigger automatic alarms when a vessel approaches the user-set safety depth.

Always read the legend to confirm which depth value corresponds to each color boundary. The threshold between white and light blue varies by chart edition and scale. A chart printed in the 1980s may color zones differently than a modern ENC displaying the same area.

Sounding Units: Feet, Meters, and Fathoms

Soundings on charts are expressed in feet, meters, or fathoms, and the unit varies by region and era. Understanding which unit a chart uses is critical. The chart legend and title block state the unit clearly — always verify before interpreting any depth.

Feet are common on older U.S. paper charts, particularly those published before the 1980s. Meters are the standard on modern U.S. charts and nearly all international charts. One fathom equals 6 feet; older charts, especially from the 19th and early 20th centuries, often show depths in fathoms, sometimes with feet in smaller type.

Chart No. 1 notes that on NOAA paper charts, fathom-and-feet soundings use vertical numbers while meter soundings use slanting numbers — a built-in visual cue. Modern ENCs standardize to meters and allow automatic conversion. Confusing feet and meters could lead you to underestimate depth by a factor of three, turning a safe passage into a grounding.

Rock Symbols (Section K, Items 10-17)

Chart No. 1 distinguishes rocks by whether they are always above water, sometimes above water, awash, or always submerged. Each state has a distinct symbol.

K 10: A rock (islet) that never covers — always above water. Shown as a small land symbol.

K 11: A rock that covers and uncovers with the tide — charted with its height above chart datum shown in parentheses and underlined. This is one of the most dangerous features because its exposure varies hour by hour.

K 12: A rock awash at chart datum — barely at the surface at the lowest normal tides. Almost always submerged but periodically exposed.

K 13: An underwater rock of unknown depth, dangerous to surface navigation. Shown as an asterisk or cross with a dot on NOAA charts.

K 14: An underwater rock of known depth — shown with its sounding when inside the corresponding depth contour (not dangerous) or outside it (dangerous).

K 15: An underwater rock of known depth that is not dangerous to surface navigation — safely below the draft of normal vessels at all tides.

Wreck Symbols (Section K, Items 20-31)

Chart No. 1 items K 20 through K 31 show how wrecks are charted depending on their depth and visibility. A wreck whose hull never covers at high water is shown as a stylized hull symbol. A wreck that covers and uncovers is shown with its height above chart datum. A submerged wreck with known depth is shown with the depth. A dangerous wreck of unknown depth carries a distinctive warning symbol.

Wrecks marked K 28 (dangerous wreck, depth unknown) are among the most hazardous features a sailor can encounter. The wreck exists, it is dangerous, and nobody knows how close to the surface it is. Give these a wide margin regardless of how much water appears to be around them.

K 29: A sunken wreck not dangerous to surface navigation — safely below normal draft. K 31: Foul ground, not dangerous to surface but hazardous to anchors or trawl gear — relevant for choosing anchorages.

Obstruction Symbols and Swept Depths

Obstruction (Obstn) marks an uncharted or poorly defined hazard. The true nature and position are uncertain. If you see Obstn on a chart near your route, treat it as a solid barrier — go around it with a margin.

Chart No. 1 item K 2 covers wire-drag sweeping and diver confirmation. When an area has been swept by wire drag to a specific depth, the swept depth is shown. This is a minimum safe depth confirmed by physically dragging a wire at that depth through the area. It does not mean the depth is exactly that — it means nothing shallower than that depth was found during the sweep.

PA (Position Approximate) and Rep (Reported) apply to hazards as well as to features. Rep Obstn means an obstruction has been reported by a mariner but not confirmed by survey. Treat reported hazards as real.

Restricted and Prohibited Areas

Chart No. 1 items N 2.1 and N 2.2 define the difference between a restricted area (limited entry or activity) and a prohibited area (no entry). On the chart, restricted areas are outlined with a distinctive line pattern and labeled RESTRICTED AREA. Prohibited areas are labeled PROHIBITED AREA. Both may also carry additional labels describing what is prohibited — FISH PROHIB, ANCH PROHIB, DIVING PROHIB.

Military practice areas (N 30 through N 34) include firing practice zones, submarine exercise areas, and minefields. These appear on charts with specific labels and boundary lines. Minefields are rarely encountered in peacetime but appear on some charts in specific regions — treat any minefield boundary as absolute.

Item N 22 covers environmentally sensitive sea areas and nature reserves. These are increasingly common on modern charts, particularly in coral reef zones, marine protected areas, and wildlife sanctuaries. Speed limits, anchoring restrictions, and no-wake zones may apply.

Traffic Separation Schemes

Traffic separation schemes (TSS) appear in Section M (Tracks, Routes) but are closely related to the area limits in Section N. A TSS divides opposing streams of vessel traffic into one-way lanes separated by a buffer zone. On the chart they appear as parallel lines with arrows showing traffic direction and a central separation zone.

Small craft are not required to use a TSS, but if you do transit one, you must follow the traffic direction, cross at right angles or as close to right angles as possible, and give way to vessels in the lanes. Anchoring in a TSS separation zone is prohibited.

Light Characters (Section P, Item 10)

Chart No. 1 item P 10 defines all light characters — the rhythmic pattern of a light's flashes and eclipses. Each character is unique enough to distinguish one light from another at sea.

F (Fixed): a steady, continuous light with no interruption. Unusual on its own at sea — most navigational lights flash.

Fl (Flashing): total time off is longer than total time on. The most common buoy light character. Fl 4s means one flash every 4 seconds.

Oc (Occulting): total time on is longer than total time off. The light is briefly interrupted rather than briefly shown.

Iso (Isophase): equal time on and off. The light and eclipse are the same duration.

LFl (Long Flashing): a single flash lasting 2 seconds or more, giving a distinctive long burst.

Q (Quick): 50 to 60 flashes per minute — a rapid-fire flash. Used on cardinal marks and isolated danger marks.

VQ (Very Quick): 80 to 159 flashes per minute.

Mo (Morse): flashes in a specific Morse code letter pattern, shown as Mo(K) for the letter K.

Al (Alternating): the light alternates between two colors, such as AlWR (alternating white and red).

Sector Lights and Leading Lights

A sector light (Chart No. 1 item P 40) shows different colors in different arcs of visibility. A lighthouse with a white sector and a red sector uses color to communicate position relative to a hazard: white marks the safe channel, red marks the danger zone. When you see red, alter course until the light shows white.

Leading lights (also called ranges) are covered in Chart No. 1 items P 20.1 through P 20.6. Two lights placed in line define a precise bearing — the leading line. When both lights are in vertical alignment, one above the other, your vessel is on the safe leading course. If the front light appears to move left of the rear light, you have drifted right of the line; if it moves right, you have drifted left.

The full light description is plotted on the chart beside the light symbol (Section P item P 16 gives a worked example). Reading it correctly means you can positively identify any light before you commit to using it for navigation.

The IALA Maritime Buoyage System (Section Q, Item 130)

Chart No. 1 item Q 130 introduces the IALA Maritime Buoyage System — the international framework that defines what every buoy's color, shape, topmark, and light mean. IALA stands for International Association of Marine Aids to Navigation and Lighthouse Authorities.

There are two IALA regions: Region A covers Europe, Africa, Australia, Asia (except Japan, Korea, Taiwan, and the Philippines), and Greenland. Region B covers the Americas, Japan, Korea, Taiwan, and the Philippines. The system is identical in both regions except that lateral mark colors are reversed. U.S. waters are Region B.

Lateral marks (Section Q item 130.1) indicate the sides of a channel. In Region B (U.S. waters): red marks are on the right when returning from sea, green marks are on the left. The memory aid is 'Red Right Returning.' In Region A, the colors are reversed. If you sail internationally, confirm which region you are in before trusting buoy colors.

Types of IALA Marks

Lateral marks define channel edges. In Region B, red lateral marks (often can-shaped, Fl R) are kept to starboard when entering a harbor or going upstream. Green lateral marks (often conical or nun-shaped, Fl G) are kept to port. A junction buoy with three horizontal color bands marks a channel split; the preferred channel is indicated by the topmost color band.

Cardinal marks indicate safe water relative to a hazard using the four cardinal directions — north, south, east, west. A north cardinal mark means pass to the north of the hazard (the mark is north of the danger). Cardinal marks use black-and-yellow horizontal bands with distinctive double-cone topmarks pointing in patterns that indicate direction. Cardinal marks always carry a quick or very quick white light.

Isolated danger marks are placed directly on or over a hazard. They have two black spheres as a topmark, are colored black with one or more red horizontal bands, and carry a group flashing white light (Fl(2)). Their presence means the hazard is right there — do not pass between the mark and the nearest hazard.

Safe water marks — red-and-white vertical stripes — indicate open navigable water in all directions. They are placed at the center of a channel, at a landfall point, or to mark the beginning of a marked channel.

Special marks are yellow with an X topmark. They indicate areas or features whose nature is explained in chart notes or sailing directions: racing course buoys, spoil ground limits, ODAS data buoys, and similar markers that are not strictly navigational.

Buoy Shapes

Chart No. 1 items Q 20 through Q 26 describe standard buoy shapes. In the IALA system, shape indicates the mark type when color cannot be seen clearly. Conical buoys (pointed top) are starboard-hand marks in Region A; can buoys (flat top, cylindrical) are port-hand. In Region B the colors reverse but shapes may not always follow — U.S. buoys often use any shape with the correct color and light.

Pillar buoys (Q 23) and spar buoys (Q 24) are used for cardinal marks, isolated danger marks, and safe water marks where shape alone cannot convey information — those marks rely on topmark and color instead.

Mooring buoys (Q 40 through Q 45) are shown on charts to indicate where vessels can pick up a mooring. They are typically white or yellow with a blue horizontal band and have no navigational significance — they do not mark channels or hazards.