Chart Types & Scales
NOAA classifies charts by scale. A larger scale number means a smaller geographic area shown with more detail. “Large scale” = more detail, smaller area. “Small scale” = less detail, larger area. This is counterintuitive and frequently tested.
| Chart Type | Scale | Primary Use | Detail Level |
|---|---|---|---|
| Sailing Chart | 1:600,000 and smaller | Offshore ocean passage planning | Low |
| General Chart | 1:150,000 – 1:600,000 | Coastal passage, approaching from sea | Moderate |
| Coastal Chart | 1:50,000 – 1:150,000 | Inshore piloting, inside bays and sounds | Good |
| Harbor ChartExam Favorite | 1:50,000 or larger | Port entry, anchorage, narrow channels | High |
Scale Memory Rule
Think of it like a fraction: 1/80,000 is a larger number than 1/1,200,000. A large-scale chart has a scale ratio closer to 1:1 — meaning each inch on the chart represents fewer real-world miles. More detail = larger scale.
When to Use Which Chart
- ▸Planning a transatlantic crossing → Sailing chart
- ▸Coastal hop, 50–200 miles → General or Coastal chart
- ▸Entering a marina or inlet → Harbor chart
- ▸Anchoring in a new cove → Harbor or largest-scale available
Depth Soundings & Datums
Every depth figure on a NOAA chart is referenced to a tidal datum — the baseline from which tide heights and water depths are measured.
Key Tidal Datums
MLLW — Mean Lower Low Water
Chart datum for soundings. All depths measured from this level. This is the zero reference.
MLW — Mean Low Water
Average of all low tides. Slightly higher than MLLW in most areas.
MHW — Mean High Water
Legal shoreline on charts. Heights of lights, bridges, and overhead clearances are measured from MHW.
MHHW — Mean Higher High Water
Average of the higher of the two daily high tides. Used for bridge vertical clearance in some publications.
Calculating Actual Depth
Actual Depth = Charted Depth + Tide Height
If the chart shows 8 feet and the tide is currently +3 feet above MLLW, the actual depth is 11 feet.
At low tide, actual depth approaches the charted depth. At minus tides (below MLLW), actual depth is less than charted.
Sounding Units
- ▸U.S. charts: feet (inshore) or fathoms (offshore)
- ▸1 fathom = 6 feet
- ▸Modern NOAA charts increasingly use meters
- ▸Unit is printed in the chart title block — always verify
- ▸Underlined soundings = heights above water (drying heights)
Critical Exam Point: Bridge Clearances
Bridge vertical clearances on NOAA charts are measured from Mean High Water (MHW), not from MLLW. This is the minimum clearance you can expect — at low tide, you will have more room than charted. If your mast height is close to the charted clearance, calculate carefully or wait for a lower tide stage.
Lateral Buoy System — IALA-B (United States)
The United States uses the IALA Region B lateral buoy system. The fundamental rule: “Red Right Returning” — keep red marks on your starboard side when returning from sea (heading toward port, upriver, or in the conventional direction of buoyage).
When heading seaward (departing), red marks are on your port side and green on your starboard.
Nun (conical top)
Numbered: EvenSide
Right side returning from sea (starboard hand)
Light Color
Red
Can (cylindrical flat top)
Numbered: OddSide
Left side returning from sea (port hand)
Light Color
Green
Nun or Can (top band = preferred channel)
Side
Junction / bifurcation — preferred channel is top band color
Light Color
Red or Green (matches top band)
Spherical or octagonal
Side
Safe water / mid-channel — navigable on either side
Light Color
White Morse A (· –)
Various
Side
Special purpose — boundaries, military, scientific, cable area
Light Color
Yellow
Conventional Direction of Buoyage
On rivers and intracoastal waterways, “returning from sea” means traveling upstream or in the conventional direction published in the chart. On the U.S. Atlantic ICW, the conventional direction is generally southward. On the Gulf ICW, it is generally westward. NOAA charts show an arrow and note to indicate conventional direction where it may be unclear.
Light Characteristics
Every lighted aid to navigation has a unique combination of color, rhythm, and period that allows it to be identified on a chart. The characteristic is always listed in the order: Type · Color · Period · Height · Range. Example: Fl R 4s 42ft 5M = flashing red, 4-second period, 42 feet elevation, 5 nautical mile range.
Continuous, steady light. Does not flash.
F R — fixed red light
Single flash; light is off longer than on. Most common.
Fl 4s — one flash every 4 seconds
Group of 2 flashes repeated; light off longer than on.
Fl(2) 10s — two flashes every 10 seconds
Light is on longer than off; darkness (eclipse) interrupts steady light.
Oc 6s — one eclipse every 6 seconds
Equal periods of light and darkness.
Iso 2s — 1s on, 1s off
Approximately 60 flashes per minute, continuous.
Q — quick, continuous
Approximately 120 flashes per minute, continuous.
VQ — very quick, continuous
Flashes in Morse code letter groups.
Mo(A) — dot-dash (Morse A); used at mid-channel buoys
Light Sectors
Many lighthouses project different colored light sectors over different arcs of bearing — providing navigational guidance and hazard warnings. On a chart, sectors appear as magenta lines radiating from the light with arc labels.
- White sector: Safe water — you are in the main channel
- Red sector: Danger zone — shoals, rocks, or restricted area to the right
- Green sector: Caution zone — hazards to the left, or port sector of channel
Hazard Symbols
NOAA chart hazards use standardized symbology from Chart No. 1 — the official U.S. reference for all nautical chart symbols. Knowing these symbols is essential for safe passage planning and is tested on the USCG exam.
| Symbol | Name | Description | Danger |
|---|---|---|---|
| + | Submerged Rock | Rock below chart datum, depth given. Depth value printed alongside. | High |
| * | Rock Awash | Rock at or near the surface at chart datum (MLLW). Covers and uncovers with tide. | Extreme |
| Wk | Wreck (non-dangerous) | Sunken wreck with adequate clearance above it. Depth sounding shown. | Low |
| Wkd | Dangerous Wreck | Wreck with minimal or unknown clearance. Often marked with buoy. Avoid. | Extreme |
| Shoal | Shoal / Shallow Area | Blue-shaded area or dotted contour line indicating shallow water. Check soundings. | High |
| Obstr | Obstruction | Submerged object of unknown nature — wreck, debris, or gear. Unknown depth. | High |
| Rk | Rock (above water) | Rock visible above MLLW. Marked with elevation if significant. | Moderate |
| Ed | Existence Doubtful | Shoal, rock, or obstruction reported but not confirmed. Treat as real hazard. | High |
Blue Shading on Charts
NOAA charts use blue shading to indicate shallow water — typically 0–18 feet (varies by chart scale). Deeper blue (darker) may indicate intermediate depths; white indicates deep water beyond a certain threshold. Always check the chart's depth legend. Never navigate in a blue-shaded area without checking individual soundings first.
Existence Doubtful (ED) Warnings
When a chart shows ED or PA (Position Approximate) next to a hazard, treat it as real and avoid it. Reported hazards not yet surveyed are often marked Rep (year) — reported in that year. These are unconfirmed but must be treated with caution.
Aids to Navigation — Ranges, Daymarks & Lighthouses
Beyond buoys, the U.S. aids to navigation system includes fixed structures that provide precise bearings, channel entry guidance, and visual landmarks.
Range Marks — Most Precise Visual Aid
A range consists of a front range mark (closer, shorter) and a rear range mark (farther, taller). When both appear vertically aligned, the vessel is exactly on the range bearing.
- ▸Front mark is always lower than the rear
- ▸Front mark to the left = you are to the right of the range — turn left
- ▸Front mark to the right = you are to the left — turn right
- ▸Depicted on chart as a dashed line with bearing
- ▸May be lighted (for night use) or unlighted (daytime only)
- ▸Bearing given is from the vessel toward the range (true bearing)
Lighthouses
Lighthouses are major fixed aids, identified on charts by a magenta star or flash symbol with the light characteristic. Key information printed on the chart includes:
- ▸Characteristic: e.g., Fl W 10s (flashing white, 10-second period)
- ▸Height: elevation of the light above MHW in feet
- ▸Range: nominal range in nautical miles (e.g., 16M)
- ▸Fog signal: horn, diaphone, or whistle noted if equipped
- ▸Racon: radar transponder beacon — shown in parentheses
Daymarks — Unlighted Daybeacons
Daymarks are fixed retroreflective signs on single pilings (daybeacons). They follow the same color and numbering system as buoys but are fixed structures on the bottom.
| Shape | Color | Meaning |
|---|---|---|
| Square | Green with reflective border | Port-hand mark (green/odd channel side) |
| Triangle | Red with reflective border | Starboard-hand mark (red/even channel side) |
| Diamond | Red & white or green & white | Preferred channel — diamond indicates junction |
| Rectangle (vertical) | White with orange border | Regulatory information mark |
| Circle (orange) | White with orange circle | Controlled area — speed limit or exclusion zone |
| Diamond (orange) | White with orange diamond | Danger area — hazard to navigation |
Shoreline & Land Features
Understanding how the coastline is depicted on NOAA charts helps you identify anchorages, hazards near shore, and landmarks for visual piloting.
Mean High Water (MHW) Line
The legal shoreline on NOAA charts — average of all high tides over 19-year epoch. Separates land from water on the chart.
Mean Lower Low Water (MLLW)
Chart datum for soundings. All depth figures are referenced to this level. Depths are slightly greater at most tidal stages.
Mud Flat / Tidal Flat
Area that dries at low tide. Shown in blue-green with dotted lines on NOAA charts. May be navigable at high tide only.
Mangroves / Marsh
Dense coastal vegetation. Shown with a hatching or stippled pattern at the shoreline edge. Generally impassable.
Sandy Beach
Shown as yellow shading along the coastline. Good for anchoring reference but offers no weather protection.
Cliffs / Bluffs
Steep terrain shown with hachure marks on the land side. Useful as visual landmarks for dead reckoning.
Cardinal Marks
Cardinal marks are used in offshore and international waters to indicate which side of a hazard safe water lies on — named for the four cardinal compass directions. While U.S. coastal waters primarily use the lateral system, cardinal marks appear in offshore waters and are tested on the USCG exam.
North Cardinal Mark
- Colors: Black over Yellow
- Cone orientation: Both cones point UP (↑↑)
- Safe water is to the NORTH of the mark
- Light: VQ or Q (uninterrupted quick flash)
South Cardinal Mark
- Colors: Yellow over Black
- Cone orientation: Both cones point DOWN (↓↓)
- Safe water is to the SOUTH of the mark
- Light: VQ(6)+LFl or Q(6)+LFl
East Cardinal Mark
- Colors: Black-Yellow-Black (stripes)
- Cone orientation: Cones BASE to BASE (◇)
- Safe water is to the EAST of the mark
- Light: VQ(3) or Q(3) — 3 quick flashes
West Cardinal Mark
- Colors: Yellow-Black-Yellow (stripes)
- Cone orientation: Cones POINT to POINT (⋄)
- Safe water is to the WEST of the mark
- Light: VQ(9) or Q(9) — 9 quick flashes
Cardinal Mark Memory Aid
North: cones up like a compass needle pointing north. South: cones down, opposite of north. East: cones base-to-base form a diamond shape (like an egg — E for East, Egg). West: cones point-to-point form an hourglass shape (W for West, Waist). The number of quick flashes corresponds to the clock position: East = 3 o'clock (3 flashes), South = 6 o'clock (6 flashes), West = 9 o'clock (9 flashes), North = continuous (no pause needed to count to 12).
Chart Correction — Notice to Mariners
Nautical charts must be kept corrected and up to date. The captain is legally responsible for navigating with the most current information available.
Notice to Mariners (NtM)
- ▸Published weekly by the National Geospatial-Intelligence Agency (NGA)
- ▸Covers all NOAA charts with corrections, new editions, and hazard updates
- ▸Available free at nauticalcharts.noaa.gov
- ▸Record corrections in chart margin in violet or magenta ink
- ▸Log the NtM number and year in the lower left margin of the chart
- ▸Never use black ink for corrections — it is indistinguishable from original printing
Local Notice to Mariners (LNtM)
- ▸Published by each USCG District
- ▸Covers local buoy moves, new hazards, temporary safety zones, and regatta closures
- ▸Published weekly or bi-weekly depending on district
- ▸Critical for local knowledge — inshore buoy positions change seasonally
- ▸Free download from your USCG district website
Chart Edition & Print Date
Every NOAA chart shows an edition number and print date in the lower margin. The edition date tells you the survey data cutoff; the print date tells you when the chart was printed. Corrections applied after the print date must be sourced from NtM and applied manually.
Edition 23, July 2023
Corrections through NtM 12/2024 applied
Electronic Chart Updates
- ▸RNC (Raster Navigational Chart) — digital image of paper chart, updated by download
- ▸ENC (Electronic Navigational Chart) — vector format, layered data, auto-updated by connected navigation systems
- ▸ENC is preferred for official navigation on commercial vessels
- ▸Always verify update date in your navigation software
- ▸Paper charts remain valid and required backup for many documented vessels
Captain's Responsibility
Under 33 CFR and navigation regulations, the master of the vessel is responsible for ensuring charts are current and corrected before departure. An uncorrected chart that contributes to a grounding or collision will be considered a contributing factor in any USCG casualty investigation.
Chart No. 1 — The Symbol Reference
NOAA Chart No. 1 is not a navigational chart — it is the definitive reference booklet listing every symbol, abbreviation, and term used on U.S. nautical charts. It is organized into sections: A (Chart number systems), B (Positions, distances), C (Natural features — coastlines), D (Cultural features), E (Landmarks), F (Ports and harbors), H (Tides and currents), K–L (Depths and bottom type), P (Lights), Q (Buoys and beacons), and more.
Section P
Lights — all light characteristics and structure symbols
Section Q
Buoys, beacons, and aids to navigation symbols
Section K
Depths, soundings, and chart datum definitions
Section L
Nature of the seabed (M = mud, S = sand, R = rock)
Section F
Harbors, anchorages, and port facilities
Section T
Services and facilities — marinas, fuel, repairs
Chart No. 1 is available free from NOAA at nauticalcharts.noaa.gov. Download and study it — the USCG exam pulls questions directly from its contents.
Exam Tips
Large scale = more detail
Questions often reverse the intuitive answer. A 1:10,000 chart is larger scale than a 1:500,000 chart — it covers less area but shows more detail. Harbor charts are large scale. Ocean charts are small scale.
Soundings are at MLLW
All charted depths are referenced to MLLW. Actual depth = charted depth + current tide height above MLLW. Bridge clearances are measured from MHW — a separate and higher datum.
Red Right Returning
Keep red aids on your starboard when heading toward port (returning from sea). Heading seaward: green on starboard, red on port. This is one of the most-tested chart/buoy concepts on the USCG exam.
Fl = light off longer than on
Flashing means the light is off longer than it is on. Occulting is the reverse — light on longer than off. Isophase is equal. The abbreviation Fl is the most common light characteristic on lighted buoys.
Magenta = lights and electronics
All light-related information on NOAA charts is printed in magenta — light circles, sector lines, radio beacon symbols, and fog signal notations. Black is for land features; blue for water.
Notice to Mariners = violet ink
Chart corrections from NtM must be applied in violet or magenta ink. Record the NtM number and year in the lower margin. The exam may ask who is responsible for corrections — the captain/master.
Frequently Asked Questions
What datum are depth soundings on NOAA charts referenced to?
Depth soundings on NOAA nautical charts are referenced to Mean Lower Low Water (MLLW) — the average of the lower of the two daily low tides over the National Tidal Datum Epoch. This represents approximately the lowest water level you can expect under normal conditions. Actual depths at any given time depend on the current tide height. At high tide, depths are greater than charted; at low tide, depths may be equal to or less than charted. Always add the current tidal height to the charted depth to get actual depth.
What is the difference between a small-scale and large-scale nautical chart?
A small-scale chart covers a large geographic area with less detail — used for ocean passage planning. A large-scale chart covers a smaller geographic area with much more detail — used for harbor entry, anchorage, and coastal piloting. The scale number itself is the key: a 1:80,000 chart is larger scale (more detail) than a 1:1,200,000 chart. NOAA chart types: Sailing Charts (1:600,000+) for offshore; General Charts (1:150,000–1:600,000) for coastal passage; Coastal Charts (1:50,000–1:150,000) for inshore navigation; Harbor Charts (1:50,000 or larger) for ports and anchorages.
What do the colors green and red mean on U.S. lateral buoys?
In the U.S. lateral mark system (IALA-B), red marks the right side of a channel returning from sea (heading toward port), and green marks the left side. The memory aid is 'Red Right Returning.' Red buoys are even-numbered; green buoys are odd-numbered. Red buoys are can-shaped at the top (nun buoys) and green buoys are cylindrical (can buoys) when unlighted. Lighted buoys use red or green lights matching their color. A red-and-green horizontally banded buoy marks a junction — the top band color indicates the preferred channel.
What does 'Fl 4s' mean on a nautical chart?
Fl 4s means the light has a single flash characteristic with a period of 4 seconds — the light flashes once and is obscured for the remainder of the 4-second period. Common light characteristic abbreviations: Fl = flashing (light is off longer than on); Oc = occulting (light is on longer than off); Iso = isophase (equal light and dark periods); Q = quick (about 60 flashes per minute); VQ = very quick (about 120 flashes per minute); F = fixed (continuous light); Fl(2) = group flashing with 2 flashes per period. The number of seconds after the abbreviation always represents the total period from the start of one cycle to the start of the next.
What do the symbols for rocks and wrecks look like on a nautical chart?
Rocks on NOAA charts use a plus sign (+) for rocks that are always submerged (with depth value), an asterisk (*) for rocks awash at the chart datum level, and a plus sign in a circle for rocks that cover and uncover. Wrecks are shown with different symbols based on navigational danger: a stylized wreck symbol with masts suggests a dangerous wreck with minimal clearance, while a wreck symbol with depth sounding is considered non-dangerous. Dangerous wrecks in navigable waters are often marked with a buoy and carry the notation 'Wk' or 'Wkd' on the chart. Shoal areas are indicated by blue shading or dotted lines.
What is a range on a nautical chart and how is it used?
A range consists of two fixed structures (daymarks, lights, or beacons) aligned so that when a vessel sees both marks in line — one behind the other — the vessel is on a specific charted bearing. Ranges are among the most accurate visual navigation aids. When the front mark is directly in front of and below the rear mark (they appear vertically aligned), you are on the range line. When the front mark appears to the left of the rear mark, you are to the right of the range; move left. Ranges are depicted on charts as a dashed line with a bearing notation. The rear mark is always taller than the front mark.
What do magenta circles on a nautical chart indicate?
Magenta is the standard color for all items related to lights and marine electronics on NOAA nautical charts. A magenta circle or dot at a buoy or beacon position indicates the aid is lighted. Magenta flashes or sectors around a lighthouse indicate the area illuminated by the light and any sector colors (red, green, white). Magenta-colored symbols are also used for radio beacons, fog signal stations, and radar beacons. The color magenta was chosen because it is visible on both the white water areas and the blue-shaded shallow areas of the chart.
How do you update a nautical chart using Notice to Mariners?
Notice to Mariners (NtM) is published weekly by the National Geospatial-Intelligence Agency (NGA) and locally by USCG districts. To update a paper chart: read the NtM, locate the relevant chart number in the corrections section, apply the indicated changes to the chart in magenta or violet ink (never black), and record the NtM number and edition date in the chart's correction log in the lower margin. Local Notice to Mariners (LNtM) — issued by USCG districts — covers local aids to navigation changes, new hazards, and temporary safety zones. Electronic charts (ENCs and RNCs) can be updated automatically through navigation software. Skippers are responsible for ensuring their charts are corrected and up to date before every voyage.
What are cardinal marks and when are they used?
Cardinal marks are aids to navigation used in some international waters and offshore locations to indicate the safest water relative to a hazard — named for the four cardinal directions (North, South, East, West). A North cardinal mark (two cones pointing up, black-over-yellow) means safe water is to the north of the mark. A South cardinal mark (two cones pointing down, yellow-over-black) means safe water is to the south. East and West cardinal marks indicate safe passage accordingly. While IALA-B (the U.S. system) primarily uses lateral marks in inland and coastal waters, cardinal marks appear in COLREGS waters and are tested on the USCG exam. Cardinal marks use specific quick-flash and very-quick-flash light patterns to identify direction at night.
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