Chart Symbols
Decoding the visual language of nautical charts
Color Conventions and Depth Zones
Nautical charts use a consistent color system to communicate water depth at a glance. White represents navigable water — generally deep enough for most vessels. Light blue indicates shoal water — shallower than the surrounding area and requiring caution. Green or yellow marks areas that dry at low tide. Tan or buff represents land.
These colors are calibrated to the chart datum: blue areas are always covered with water; green/yellow areas are sometimes exposed. The boundary between light blue and white shifts between chart editions and scales — always read the legend rather than assuming a particular shade means a specific depth.
Depth contour lines (like elevation contours on land maps) connect points of equal depth. The 5-fathom contour (30 feet) and 10-fathom contour (60 feet) are the most important for inshore navigation — inside these lines the water is shoaling and care is required.
Print a copy of NOAA Chart No. 1 — the official reference for all US chart symbols. It's free, comprehensive, and belongs aboard every chartplotting or paper-chart vessel.
On a NOAA chart, what does a light blue area indicate?
What do depth contour lines on a chart connect?
Hazard Symbols
Rocks are among the most critical hazards on a chart. A rock awash (submerged at high water but exposed at low water) is shown as a cross with a circle or dot. A dangerous rock (always submerged but shoal enough to threaten vessels) is shown as a plus sign with a dot. A rock with a dot alone indicates a known rock that may or may not be dangerous depending on depth — always check the sounding.
Wrecks are shown with a stylized wreck symbol. If the wreck is considered dangerous to navigation (shallow enough to threaten vessels), it has additional hatching. Shoals and shallow patches are labeled with their least depth. Overfalls and tide rips — turbulent water over uneven bottom or strong tidal flow — are shown with a special stippled symbol.
Obstruction symbols (an anchor with a dot, or simply 'Obstn') mark uncharted or poorly defined hazards. If you see Obstn on a chart, treat it as a solid barrier — the true nature and exact position of the hazard is uncertain.
Any symbol indicating a rock, wreck, or obstruction should be given a generous safety margin — these are charted at their surveyed position, which may have been years or decades ago. Bottom features shift, wrecks settle, and survey accuracy varies.
A chart shows 'Obstn' with a dotted circle near your route. What does this mean?
A rock symbol with a cross and a dot indicates:
Navigation Aids: Lights, Buoys, and Beacons
Buoys on a chart are shown as colored circles or diamonds with their characteristic symbols: color, light pattern, and number. The chart symbol includes the buoy's light characteristic — for example, 'Fl R 4s' means a flashing red light with a 4-second period. Fl = flashing (off longer than on), Iso = isophase (equal on/off), Oc = occulting (on longer than off), Q = quick flashing.
Lighthouses and beacons are shown with a position dot and their light characteristics. The light's nominal range (how far it can be seen in good visibility) and sector (the arc over which it's visible) may be indicated. A lighthouse in a narrow channel may have different colored sectors — red showing the danger zone, white showing the safe channel.
Leading lines (ranges) are charted as two symbols aligned to mark a safe course. When the two marks are in transit (one behind the other), the vessel is on the safe line. These are among the most reliable and simple navigation techniques available.
When planning a night passage or harbor entry, list every light you'll use and memorize its characteristic (color and period). Positively identifying a light by its full characteristic — not just its general location — prevents mistaking one navigation aid for another.
A chart shows 'Fl G 6s' next to a green buoy symbol. What does this mean?
A lighthouse has red and white sectors. What does the red sector indicate?
Seabed Type, Landmarks, and Abbreviations
Charts indicate seabed composition with abbreviations: S = sand, M = mud, R = rock, Sh = shells, G = gravel, Co = coral. This is critical for anchoring — sand and mud hold anchors well; rock and coral are unreliable or damaging.
Landmarks useful for visual navigation are charted with their type and height: Chy = chimney, Tk = tank, Tr = tower, Spire = church spire. Heights are given above Mean High Water Springs (MHWS) — the highest regular tide — so the full height of a landmark above sea level may be slightly less than the charted figure at very high tides.
Chart abbreviations are standardized across NOAA charts and listed fully in Chart No. 1. Common ones to know: PA = position approximate, PD = position doubtful, ED = existence doubtful, Rep = reported (not confirmed). Anything with PD, ED, or Rep demands extra caution — the charted position or existence of the feature is uncertain.
A chart shows 'R' at your intended anchorage. What does this mean?
A chart shows a wreck labeled 'ED.' What does this mean?
Summary
Chart colors: white = deep water, light blue = shoal, green/yellow = drying. Read the legend for depth thresholds.
Key hazard symbols: rock awash (cross + dot), dangerous submerged rock, wreck, obstruction (Obstn). Give all a generous margin.
Light characteristics follow the pattern: type (Fl/Iso/Oc) + color (R/G/W) + period (seconds). Identify fully before using a light for navigation.
Seabed abbreviations (S, M, R, Co) are critical for anchoring decisions — check them before dropping the hook.
Abbreviations PA, PD, ED, Rep indicate uncertain or unconfirmed information — treat these features as real hazards until proven otherwise.
Key Terms
- Light characteristic
- The unique combination of color, rhythm, and period that identifies a navigation light (e.g. Fl R 4s = flashing red, 4-second period)
- Fl (Flashing)
- A light that is off longer than it is on — the single most common buoy light characteristic
- Iso (Isophase)
- A light with equal on and off periods
- Oc (Occulting)
- A light that is on longer than it is off
- Q (Quick)
- A rapidly flashing light — typically 50–60 flashes per minute; used on cardinal marks and isolated danger marks
- Chart No. 1
- NOAA's official reference for all US nautical chart symbols, abbreviations, and conventions
- Sector light
- A lighthouse with different colored arcs of visibility — red for danger, white for safe channel
- Leading line (range)
- Two charted marks that, when in transit, define a safe navigational course
- PA / PD / ED
- Position Approximate / Position Doubtful / Existence Doubtful — warnings that charted information is uncertain
Chart Symbols — Quiz
On a NOAA chart, green or yellow shading indicates:
A light characteristic reads 'Oc W 10s.' What does this describe?
A chart shows 'M' at a potential anchoring spot. This indicates:
A lighthouse has a white sector and a red sector visible from your approach bearing. You are seeing red. What should you do?
A chart shows 'Rep Obstn' along your planned route. What is the correct response?
References & Resources
Related Links
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NOAA Chart No. 1 — Symbols and Abbreviations
The complete official reference for all US nautical chart symbols, free from NOAA.
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US Sailing — Coastal Navigation Course
Formal navigation training covering chart reading, plotting, and coastal piloting.