Chart Types by Scale
Scale = ratio of chart distance to actual distance. Larger ratio number = smaller scale = less detail = bigger area covered.
| Chart Type | Scale Ratio | Best Used For | Detail Level |
|---|---|---|---|
| Harbor charts | 1:20,000 or larger | Entering ports, anchorages, marinas | Maximum |
| Approach charts | 1:20,000–1:80,000 | Approaching coastlines, entering bays | High |
| Coastal charts | 1:80,000–1:150,000 | Offshore passage within sight of land | Moderate |
| General charts | 1:150,000–1:600,000 | Offshore passage planning | Low |
| Sailing charts | 1:600,000+ | Ocean passage planning | Minimal |
Latitude, Longitude & Position
Latitude
- • Horizontal lines (parallels)
- • Measured N/S from equator (0°) to poles (90°)
- • Read from the side margins of the chart
- • 1 minute of latitude = 1 nautical mile (always)
- • Use the latitude scale to measure distance
Longitude
- • Vertical lines (meridians)
- • Measured E/W from Prime Meridian (0°) to 180°
- • Read from the top/bottom margins of the chart
- • 1 minute of longitude ≠ 1 NM (varies by latitude)
- • Do NOT use longitude scale to measure distance
Compass Rose & TVMDC
Compass Rose Structure
Outer ring: True bearings (referenced to True North)
Inner ring: Magnetic bearings (offset by variation)
Center label: Variation value and annual change rate
Example: "VAR 15°30′W (2020) — Annual Decrease 4′"
TVMDC Conversion
True
± Variation (W = add, E = subtract)
Magnetic
± Deviation (W = add, E = subtract)
Compass
Memory: "True Virgins Make Dull Companions"
Worked Example
True course 285°T, Variation 12°W, Deviation 3°E → Compass course?
True: 285°T
+ Variation 12°W (west = add going T→C): 285 + 12 = 297°M
− Deviation 3°E (east = subtract going T→C): 297 − 3 = 294°C
Depth Soundings & Datums
MLLW
Chart Datum
Mean Lower Low Water — the average of the lowest daily tides. All chart soundings reference this. Actual depth = charted depth + tidal height above MLLW.
MHW
Overhead Clearance
Mean High Water — used as the datum for bridge clearances and overhead obstacles shown on charts. At mean high tide, a bridge with 30′ clearance has exactly 30 feet.
Units
Feet or Fathoms
NOAA charts are in feet or fathoms (1 fathom = 6 feet). Always check the chart title/note for the unit used. Some older charts use fathoms with feet as subscripts.
Contour Lines (Depth Curves)
Depth contour lines connect points of equal depth, similar to topographic contour lines on a land map. The 6-foot and 12-foot curves are critical for shoal-draft navigation. Blue shading typically indicates shallow water (0–18 feet on NOAA charts).
IALA Buoy System (U.S. — Region B)
The U.S. follows IALA Region B: red right returning. Green buoys mark the port (left) side of the channel when returning from sea.
| Buoy / Mark | Color | Shape | Light (if lit) | Pass On |
|---|---|---|---|---|
| Port (left) side | Green | Can (cylindrical) | Green — any rhythm | Starboard (right) outbound |
| Starboard (right) side | Red | Nun (conical) | Red — any rhythm | Port (left) outbound |
| Safe water (mid-channel) | Red & White vertical | Ball / Pillar / Spar | White — Mo(A) Morse | Pass on either side |
| Danger (isolated) | Black & Red bands | Pillar / Spar | White — Fl(2) | Pass around (do not approach) |
| Special (anchorage, etc.) | Yellow | Any | Yellow — any rhythm | Follow local rules |
| Preferred channel (junction) | Red & Green | Can or Nun | Red or Green — Fl(2+1) | Top color = preferred channel |
Common Chart Symbols & Abbreviations
| Symbol | Meaning | Notes |
|---|---|---|
| Wk | Wreck (exact position) | Dashed if position uncertain |
| Rk | Rock (submerged or awash) | Danger to navigation |
| Obstn | Obstruction | Depth unknown |
| PA | Position Approximate | Not exactly charted |
| PD | Position Doubtful | Existence uncertain |
| Rep | Reported (depth/hazard) | Not yet verified |
| ( ) | Depth in parentheses | Less water than surrounding area |
| † | Drying height | Height above MLLW at low water |
| Fl | Flashing light | On longer than off |
| Occ | Occulting light | Off longer than on |
| Iso | Isophase light | Equal on/off |
| Q | Quick flash | 50–80 flashes/minute |
Full symbol reference: NOAA Chart No. 1 (free download at nauticalcharts.noaa.gov)
Exam Strategy
Always identify the chart datum first
Check the chart title block for units (feet or fathoms) and the tide table to find how much to add to charted depths for your transit time.
Use dividers + latitude scale for distance
The OUPV chart plotting exam gives you parallel rulers and dividers. Set dividers on your course line, then walk to the latitude (side) scale at the same latitude to read nautical miles.
Read buoy numbers for channel side
Even-numbered buoys are red (right returning). Odd-numbered are green. Buoy numbers increase as you go upstream or toward port.
Variation is west in eastern U.S. waters
In East Coast and Gulf of Mexico waters, variation is westerly — meaning compass heading will be greater than true heading. Know your local variation value.
Frequently Asked Questions
What do depth soundings on a chart represent?
Depth soundings are the numbers printed on a nautical chart showing water depth at that location. On NOAA charts, soundings are given in feet or fathoms (1 fathom = 6 feet). They are referenced to Mean Lower Low Water (MLLW) — the average of the lower of the two daily low tides — so actual depth may be greater at higher tidal stages.
What is the difference between a large-scale and small-scale chart?
A large-scale chart shows a small geographic area in great detail (harbor chart, scale 1:20,000 or larger). A small-scale chart shows a large area with less detail (coastal chart 1:80,000; offshore chart 1:150,000+). Use large-scale charts for close-in navigation and small-scale charts for passage planning.
What is variation on a nautical chart?
Variation (also called magnetic declination) is the angular difference between True North (geographic north pole) and Magnetic North (where a compass points). It is shown in the compass rose on every nautical chart, along with the annual rate of change. Apply variation using the TVMDC rule: True ± Variation = Magnetic ± Deviation = Compass.
Related Study Guides
Chart Plotting Exam Guide
TVMDC, dead reckoning, set & drift, and speed/time/distance for the chart plotting portion.
Navigation Study Guide
TVMDC, tides, anchoring, weather, fog signals, and celestial navigation overview.
ColRegs Lights & Shapes
Navigation light rules, arcs, power and sailing vessel lights, shapes and daymarks.
Practice Chart Reading Questions
Work through real OUPV-style chart reading and navigation questions with instant feedback and AI explanations — free on NailTheTest.
Start Practicing Free