A nautical chart is only as useful as the navigator reading it. Whether piloting a commercial vessel under SOLAS carriage requirements or a coastal cruiser on a weekend passage, knowing how to interpret navigation chart symbols accurately is a fundamental safety skill. Misreading a hazard symbol or confusing a depth contour can turn a routine transit into a grounding event.

The guide below walks through the core categories of maritime symbols and meanings found on both U.S. (NOAA) and international (ADMIRALTY) nautical charts, referencing the two publications that every professional bridge should carry.

The Two References Every Navigator Needs

• U.S. Chart No. 1 is published jointly by NOAA and the National Geospatial-Intelligence Agency (NGA). The 13th edition catalogs every symbol, abbreviation, and term used on NOAA paper charts and the symbology displayed on ECDIS when rendering NOAA Electronic Navigational Charts.
• ADMIRALTY Chart 5011 (NP 5011), published by the UK Hydrographic Office (UKHO), serves the same function for ADMIRALTY and International (INT) chart series. Organized into sections A through X, Chart 5011 follows the IHO Chart Specifications format and covers everything from chart marginal notes (Section A) through small craft facilities (Section U). Vessels operating internationally and carrying ADMIRALTY charts need NP 5011 as their onboard symbology reference.

Both publications are available through American Nautical Services, alongside the full range of ADMIRALTY publications and NOAA charts.

Reading Chart Color Before Symbols

Color is the fastest orientation tool on any chart. Both NOAA and ADMIRALTY charts follow a consistent scheme that experienced navigators process instinctively.

White indicates deep, navigable water. Light blue marks shallow water below a charted safety threshold. Green (on ADMIRALTY charts) or dark blue/green (on NOAA charts) represents intertidal or very shallow zones that cover and uncover with the tide. Tan or yellow denotes dry land. Magenta is reserved exclusively for light-related information, including light characteristics, sector arcs, and lighted buoy flares.

Reading color before individual symbols gives an immediate picture of where safe water lies and where caution is warranted. For navigators transitioning between NOAA and ADMIRALTY chart series, the color logic is nearly identical, making the adjustment straightforward.

Depth Soundings and Contours (NP 5011 Section I)

Depth soundings are the numbers scattered across the water portion of a chart. Each number represents the water depth at that point referenced to the chart datum. On U.S. charts, the standard datum is Mean Lower Low Water (MLLW). On ADMIRALTY charts, the datum is typically Lowest Astronomical Tide (LAT), which generally shows slightly less water than MLLW. The sounding unit (feet, fathoms, or meters) is always stated in the chart's title block, and failing to check it is one of the most common errors made by navigators working with unfamiliar charts.

On metric ADMIRALTY charts, depths less than 21 meters are expressed in meters and decimetres. On fathom charts, depths less than 11 fathoms appear in fathoms and feet. NP 5011 Section I details these conventions along with symbols for doubtful soundings, reported depths, and unsurveyed areas, all of which demand additional caution during passage planning.

Depth contour lines (isobaths) connect points of equal depth. Tightly spaced contours signal a steep underwater slope. Widely spaced contours indicate gradual shoaling. Navigators planning passages in thin water rely on contour interpretation to stay within safe channels, and understanding chart scale is essential to reading contour spacing accurately.

Hazard Symbols: Rocks, Wrecks, and Obstructions (NP 5011 Section K)

Hazard symbols are among the most safety-critical elements on any chart. Section K of NP 5011 devotes four full pages to rocks, wrecks, and obstructions.

• Rocks are classified by their relationship to the chart datum and tidal range. A rock that does not cover (always visible) is shown differently from one that covers and uncovers with the tide, and differently again from an underwater rock of unknown depth considered dangerous to surface navigation. On ADMIRALTY charts, a cross symbol with a dotted danger line surrounding it marks a dangerous underwater rock. Drying heights (rocks that uncover) are shown with underlined figures indicating height above the chart datum.
• Wrecks carry multiple symbol variants depending on survey confidence. A submerged wreck with known depth, a wreck with depth unknown but considered dangerous, and a wreck in deep water not dangerous to surface navigation each have distinct representations. A dotted danger line around any wreck symbol signals that the area requires particular caution.
• Obstructions cover everything from unspecified underwater hazards to submerged piles, fish traps, and marine farms. The abbreviation "Obstn" appears alongside depth information when known.

When any symbol appears inside a dotted or pecked danger line, the entire enclosed area should be treated as hazardous, not just the point of the central symbol.

Aids to Navigation: Lights, Buoys, and Beacons (NP 5011 Sections P and Q)

Aids to Navigation (ATONs) guide vessels through channels and around hazards. The IALA Maritime Buoyage System, detailed extensively in NP 5011 Section Q (item 130), defines lateral marks, cardinal marks, isolated danger marks, safe water marks, and special marks used worldwide.

• Lateral marks differ between IALA Region A (Europe, Africa, Asia, Australasia) and Region B (the Americas, Japan, Korea, Philippines). In Region B, red buoys are kept to starboard when returning from sea ("Red Right Returning"). Cardinal marks use the same color and topmark conventions in both regions.
• Light characteristics (Section P) follow a standardized abbreviation system. "Fl" means flashing, "Oc" means occulting, "Q" means quick, and "VQ" means very quick. A full light description like "Fl(3)WRG.15s13m7-5M" encodes the character, group, colors, period, elevation, and range into a single compact string. Section P of NP 5011 provides the complete decoding key along with visual timing diagrams for each light class.
• Magenta is the universal color for all light-related information on both NOAA and ADMIRALTY charts. The small magenta circle (the "flare") adjacent to a buoy or beacon symbol indicates a light. Once a navigator recognizes the magenta convention, picking out lighted ATONs becomes instinctive.

For vessels using ECDIS and electronic navigation, ATON symbology on ENCs follows IHO S-52 presentation library standards, which parallel the paper chart conventions but use vector-based symbols that can be queried for additional attribute data.

Areas, Limits, and Routing (NP 5011 Sections M and N)

Commercial navigators spend significant time reading traffic separation schemes, restricted areas, and routing measures. Section M covers tracks, routes, and IMO-adopted traffic separation schemes (TSS), including separation zones, inshore traffic zones, deep water routes, and precautionary areas. Section N details maritime limits, anchorage areas, restricted zones, and international boundary designations.

Submarine cable and pipeline symbols (Section L) mark zones where anchoring is prohibited. Knowing these symbols prevents costly damage to undersea infrastructure and the legal liability that follows.

Paper vs. Electronic Chart Symbology

Navigators using ECDIS encounter a parallel set of symbols defined by the IHO under the S-52 presentation standard. The foundational symbol logic is the same as paper charts, so a navigator fluent in NP 5011 or Chart No. 1 transitions to electronic displays far more quickly than one starting without that foundation. ANS supports both paper and electronic chart users with ECDIS compliance resources, official ENCs, and the complete ADMIRALTY chart catalog.

FAQs

Q. What is the difference between Chart No. 1 and NP 5011? 

Chart No. 1 covers symbols used on U.S. (NOAA/NGA) charts. NP 5011 (ADMIRALTY Chart 5011) covers symbols used on ADMIRALTY and International (INT) charts published by the UKHO. Vessels carrying both chart series should have both references aboard.

Q. What do the numbers on a nautical chart represent? 

Numbers in water areas are depth soundings representing water depth at chart datum. On U.S. charts, the datum is typically Mean Lower Low Water (MLLW). On ADMIRALTY charts, it is typically Lowest Astronomical Tide (LAT). The unit (feet, fathoms, or meters) is stated in the chart's title block.

Q. What does a dotted danger line around a symbol mean? 

A dotted or pecked line enclosing a hazard symbol signals uncertainty in position or extent, or delimits an area containing dangers through which navigation is unsafe. The entire enclosed area should be treated with caution, not just the central symbol.

Q. How do ECDIS symbols compare to paper chart symbols? 

ECDIS uses IHO S-52 presentation library symbols that parallel paper chart conventions. The foundational logic (colors, hazard indicators, ATON representations) is consistent, though electronic symbols can be queried for additional data attributes not available on paper.

Q. Where can mariners purchase Chart No. 1, NP 5011, and nautical charts? 

American Nautical Services carries both publications alongside NOAA charts, ADMIRALTY charts, and the complete range of maritime reference publications for professional and recreational navigators.