Chart Symbols and Abbreviations (Chart No. 1)

TL;DR — US nautical charts use standardized symbols and letter codes to convey depth datum, bottom type, aid-to-navigation identity, light characteristics, and clearance heights; knowing how to read each category is directly tested on the OUPV and Master 100 GT written exams.

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What the Rule Says

A nautical chart is a graphic representation of the navigable waters and adjacent coast. It is the most fundamental tool of marine navigation; modern ECDIS systems use vector ENC files, but paper or raster equivalents remain the back-up. Bowditch Ch. 4 §401 Every symbol, abbreviation, and notation on a US chart follows conventions codified in Chart No. 1 (the joint NOAA/NGA reference for US chart symbols). The sections below address the specific symbol categories that appear on licensing exams.

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Depth Soundings and Datum

US charts show soundings in feet, fathoms, or metres referenced to Mean Lower Low Water (MLLW). The unit is stated in the chart title; always verify before plotting. Bowditch Ch. 4 §403

This single sentence contains two exam-critical facts:

1. Three possible units — feet, fathoms, or metres. A fathom equals six feet. If you misidentify the unit, every depth calculation on that chart is wrong. 2. Datum is MLLW — soundings represent the least water you can statistically expect under average lower-low-tide conditions. Actual water depth at any moment equals the charted sounding plus the height of tide above MLLW (or minus, if tide falls below datum, which is possible in some areas).

The chart title is the authoritative source for the sounding unit. Exam questions sometimes present a scenario where a candidate assumes feet when the chart is in fathoms, producing a dangerously optimistic depth estimate.

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Bottom Type Codes

Charts annotate bottom characteristics with letter codes. The standard codes are:

| Code | Bottom Type | |------|-------------| | M | Mud | | S | Sand | | R | Rock | | Cl | Clay | | Co | Coral | | Sh | Shells | | G | Gravel |

These codes are used to assess holding ground for anchoring. Light List — Bottom Type Codes

In practice, a notation such as "fS" means fine sand; "M,S" means a mixed mud-and-sand bottom. The exam tests whether you can identify what a given code means and whether a particular bottom is suitable for anchoring. Rock (R) and coral (Co) are generally poor holding ground and can damage anchor gear; mud (M) and clay (Cl) typically offer good holding.

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Aid-to-navigation names are shown in italics on US charts; non-aid placenames are shown in upright lettering. This convention helps identify aids quickly under chart-light conditions. Light List — Italicised Aid Names

This is a straightforward but frequently tested convention. If you see a name in italic type on a chart, it identifies an aid to navigation — a buoy, lighthouse, daybeacon, or similar structure maintained for navigational purposes. A city name, a geographic feature, or a shoal name will appear in upright (roman) type. The practical value is speed: a mariner scanning a chart at night under a red chart light can quickly distinguish aids from geography by typeface alone.

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Cartographic symbols indicate the precise charted position of an aid:

Buoys — a small circle; the position is the centre of that circle. Light List — Position Symbol
Lighthouses — a star symbol; the position is the centre of the star. Light List — Position Symbol

This distinction matters for precise plotting. When you take a bearing to a lighthouse and plot it on the chart, you measure from the centre of the star, not from the edge of the lighthouse symbol or the light flare drawn around it. For a buoy, the charted position is the mooring position — the actual buoy may swing on its rode and be offset from that position, which is why buoys are considered less precise position references than fixed aids.

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Light Characteristics: Isophase (Iso)

An isophase light has equal periods of light and darkness. It is charted as "Iso \<colour\> \<period\>s". Light List — light-characteristic (Iso)

For example, "Iso R 6s" describes a red isophase light with a total period of six seconds — three seconds of light followed by three seconds of darkness, repeating continuously. The defining characteristic of Iso is the 1:1 ratio of light to dark within each period. Exam questions may present a light description and ask you to identify the characteristic, or present the characteristic name and ask what the light/dark ratio is.

Common light characteristics tested alongside Iso include:

Fl (Flashing) — light period shorter than dark period
Oc (Occulting) — light period longer than dark period
Iso (Isophase) — equal light and dark periods

Knowing these three in relation to each other is the fastest way to answer characteristic questions without memorizing every variant.

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Overhead Clearance

Bridge or power-line clearance heights are shown above the symbol in feet (US) or metres, referenced to either Mean High Water or High Water Spring depending on the chart. Light List — Overhead Clearance

Two exam points here:

1. The datum for clearance is the high-water reference (MHW or HWS), not MLLW. This is the opposite of soundings. Soundings use the lowest expected water to give you the least depth; clearances use the highest expected water to give you the least vertical clearance. Both conventions are conservative — they represent the worst case for the mariner. 2. Units may be feet or metres — verify on the chart. A bridge showing "65" in feet is very different from "65" in metres.

A common exam trap: a candidate knows the charted clearance but fails to account for the fact that at low tide the vessel's air draft (mast height, antenna, etc.) is measured from the actual water surface, which may be lower than MHW, giving more clearance than the charted figure. The charted clearance is the minimum you will have under normal tidal conditions.

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Why It Matters on the Exam

The OUPV and Master 100 GT written exams include a dedicated Navigation General section. Chart symbol and abbreviation questions appear consistently and tend to be straightforward if you have memorized the conventions — they reward preparation over seamanship intuition.

Specific question types you should anticipate:

Sounding unit identification: "A chart title reads 'Soundings in Fathoms.' A sounding of 3 is shown. What is the depth in feet?" Answer: 18 feet. Bowditch Ch. 4 §403
Bottom type: "The chart shows the notation 'Co' at your intended anchorage. What does this indicate, and is it suitable holding ground?" Answer: Coral; generally poor holding. Light List — Bottom Type Codes
Italic vs. upright type: "A name on the chart appears in italic type. What does this indicate?" Answer: The name identifies an aid to navigation. Light List — Italicised Aid Names
Position symbol: "Where is the precise charted position of a lighthouse?" Answer: The centre of the star symbol. Light List — Position Symbol
Light characteristic: "A light is described as Iso W 4s. How long is each dark period?" Answer: 2 seconds (equal to the light period within a 4-second total period). Light List — light-characteristic (Iso)
Overhead clearance datum: "Bridge clearance on a US chart is referenced to which datum?" Answer: Mean High Water or High Water Spring. Light List — Overhead Clearance

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Common Pitfalls

1. Confusing sounding datum with clearance datum. Soundings reference MLLW (lowest water, least depth). Clearances reference MHW or HWS (highest water, least clearance). Mixing these up produces the wrong answer every time. Bowditch Ch. 4 §403 Light List — Overhead Clearance

2. Failing to check the chart title for sounding units. Never assume feet. US charts may be in feet, fathoms, or metres. A 3-fathom sounding is 18 feet — a vessel drawing 15 feet has only 3 feet of underkeel clearance, not 3 feet minus 15 feet. Bowditch Ch. 4 §403

3. Misidentifying Iso as Fl or Oc. Isophase means equal light and dark. If the light period is shorter than the dark, it is Flashing. If longer, it is Occulting. The word "isophase" itself contains the clue: "iso" means equal. Light List — light-characteristic (Iso)

4. Plotting from the wrong point on a lighthouse symbol. The charted position is the centre of the star, not the outer edge of any light-flare graphic. Off-centre plotting introduces bearing error. Light List — Position Symbol

5. Treating all bottom types as equivalent for anchoring. Rock (R) and coral (Co) are poor holding and can foul or damage ground tackle. Mud (M) and clay (Cl) are generally good holding. Sand (S) and gravel (G) are moderate. The exam may present a scenario requiring you to select the best anchorage based on bottom type codes. Light List — Bottom Type Codes

6. Assuming italic type means emphasis, not aid identity. On a nautical chart, italic type is a functional convention, not a typographic emphasis. Every italicized name is an aid to navigation. Light List — Italicised Aid Names

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Quick Check

Q1 — A chart title states "Soundings in Fathoms." A sounding of 4 is shown near your intended track. What is that depth in feet, and what datum does it reference?

24 feet, referenced to Mean Lower Low Water (MLLW). Multiply fathoms by 6 to convert to feet. Always verify the unit in the chart title before plotting. Bowditch Ch. 4 §403

Q2 — You see the notation "Cl" on the chart at a potential anchorage. What bottom type does this indicate, and is it generally considered good holding ground?

"Cl" indicates clay. Clay is generally considered good holding ground for anchoring. Light List — Bottom Type Codes

Q3 — A name on a US nautical chart appears in italic type. What does this tell you about the feature it labels?

The feature is an aid to navigation. Aid names are shown in italics; non-aid geographic names appear in upright (roman) type. Light List — Italicised Aid Names

Q4 — A light is charted as "Iso G 6s." Describe the light and dark periods.

The light is green, isophase, with a total period of 6 seconds. Because isophase means equal light and dark periods, each light period is 3 seconds and each dark period is 3 seconds. Light List — light-characteristic (Iso)

Q5 — Where is the precise charted position of a buoy, and where is the precise charted position of a lighthouse?

For a buoy, the precise charted position is the centre of the small circle symbol. For a lighthouse, it is the centre of the star symbol. Light List — Position Symbol

Q6 — A bridge is charted with a clearance of 55 feet. To which tidal datum is that clearance referenced, and does that represent the maximum or minimum clearance you can expect?

Bridge clearance is referenced to Mean High Water (or High Water Spring). It represents the minimum clearance — the least vertical space available under normal high-water conditions. At lower stages of tide, actual clearance will be greater than the charted figure. Light List — Overhead Clearance