1. NOAA Nautical Chart Types and Scales
Know which chart to use — and why scale matters
NOAA (National Oceanic and Atmospheric Administration) publishes nautical charts for all U.S. coastal waters, the Great Lakes, and major inland waterways. Charts are classified primarily by their scale — the ratio between a distance on the chart and the actual distance on the water.
Understanding Scale Ratios
A scale of 1:80,000 means one unit on the chart equals 80,000 of the same units on the water. At 1:80,000, one inch on the chart equals 80,000 inches — about 1.1 nautical miles — on the water.
Memory tip: "Large scale = large detail" — a 1:10,000 chart covers a small area in great detail. "Small scale = small detail" — a 1:1,200,000 chart covers a huge area with minimal detail. This is counterintuitive; the larger the denominator, the smaller the scale.
| Chart Type | Scale Range | Primary Use | Detail Level |
|---|---|---|---|
| Sailing Chart | 1:600,000 – 1:1,200,000 | Ocean passages, coast-to-coast planning | Major ports, offshore dangers, major light characteristics |
| General Chart | 1:150,000 – 1:600,000 | Coastal navigation, offshore approaches | Coastal features, offshore soundings, major buoys |
| Coastal Chart | 1:50,000 – 1:150,000 | Inshore navigation, entering bays | Channels, shoals, inshore aids to navigation |
| Harbor Chart | 1:10,000 – 1:50,000 | Port navigation, docking, tight passages | Full buoyage, dock layouts, precise depths, obstructions |
| Small-Craft Chart | Various | Recreational boating in specific waterways | Simplified format, bridge clearances, marinas, anchorages |
Chart 1210Tr — The USCG Training Chart
The USCG uses NOAA Training Chart 1210Tr (formerly Chart No. 1210) for all chart-plotting exam questions. This fictional chart depicts a coastal area at approximately 1:80,000 scale. It includes all chart elements tested on the exam: compass rose, depth soundings, buoys, lights, anchorages, and hazards.
Exam Tip
Download Chart 1210Tr from the NOAA website or the NailTheTest resource library and practice every type of problem on the actual chart. Familiarize yourself with its specific compass rose values, buoy positions, and depth contours before exam day. You cannot bring an actual NOAA chart to the exam — the testing center provides Chart 1210Tr — but practicing on it builds the spatial familiarity that saves time under pressure.
2. Chart Datum — MLLW and MHW
The reference planes that make depth and clearance numbers meaningful
Every number on a nautical chart is relative to a reference plane called the chart datum. NOAA uses two different datums on U.S. charts depending on whether the measurement is below or above the water.
Used for soundings (water depths). MLLW is the average of the lower low-water height over a 19-year National Tidal Datum Epoch. Depths printed on the chart are measured from this plane downward.
- • Charted depth of 6 ft at MLLW
- • If tide is +2 ft above MLLW, actual depth = 8 ft
- • If tide is -0.5 ft below MLLW, actual depth = 5.5 ft
Water CAN be shallower than charted depth during spring low tides when water falls below MLLW.
Used for overhead clearances — bridge heights, cable heights, and the elevation of features above water. MHW is the average of all high water heights over the same 19-year epoch.
- • Bridge clearance of 65 ft at MHW
- • Mast height of your vessel: 62 ft
- • At high water you have 3 ft to spare
- • At low water you have MORE clearance
Overhead clearances are at their minimum at high water — the most dangerous time to transit under a bridge.
Calculating Actual Depth
Actual depth at any moment = Charted Depth (MLLW) + Tide Height (from tide tables)
Vessel draft (hull depth below waterline) must be less than actual depth to avoid grounding. Always add a safety margin — aim for 2× your draft minimum.
Tidal Datum Hierarchy
3. Compass Types
Magnetic, gyro, and fluxgate — what each reads and why it matters
The USCG exam tests not just how to use a compass, but which type you are dealing with — because the corrections required differ fundamentally. A magnetic compass needs both variation and deviation corrections; a gyrocompass needs neither. Understanding why each type works differently is the foundation for all compass math questions.
Magnetic Compass
Magnetized needle aligns with Earth's magnetic field
No power required, always functional, simple
Subject to deviation from vessel magnetism, slow settling, swinging in rough seas
Primary required compass; must be corrected for variation and deviation
Gyrocompass
Fast-spinning gyroscope aligns with Earth's rotational axis (True North)
Points to True North — no variation or deviation correction needed
Requires continuous power, 15–30 min warm-up, error near poles, expensive
Reads True North; no compass corrections needed but power dependent
Fluxgate Compass
Solid-state sensor detects magnetic field electronically
No moving parts, feeds autopilot and chartplotter digitally
Still subject to deviation; requires compensation/calibration
Electronic magnetic compass; same corrections as traditional magnetic compass
Key Exam Point: Gyrocompass vs. Magnetic
If an exam question states you are using a gyrocompass, no TVMDC conversion is needed — the gyrocompass reads True North directly. If the question involves a magnetic compass or a fluxgate compass, you must apply deviation (from the deviation card) and variation (from the chart compass rose) to convert to or from True.
4. Variation vs. Deviation
Two different errors — two different sources — one combined correction
Variation
Variation (also called magnetic declination) is the angle between True North and Magnetic North at a specific geographic location. It is caused by the fact that Earth's magnetic poles do not coincide with its geographic poles.
- Source: Earth's magnetic field
- Location: Chart compass rose (inner circle)
- Changes: With geographic position AND slowly over years
- Same for: All vessels at that location
- Range: 0° to 20°+ depending on location
Deviation
Deviation is the error in a magnetic compass caused by the vessel's own magnetic field — from engines, alternators, steel fittings, electronics, and cargo. Unlike variation, deviation changes as the vessel's heading changes.
- Source: Vessel's own magnetic field
- Location: Deviation card aboard the vessel
- Changes: With vessel's heading
- Different for: Every vessel (and every compass position)
- Range: Usually 0° to 10° on a well-swung compass
Reading the Compass Rose
Every NOAA chart has at least one compass rose — a printed circle divided into 360 degrees. Most roses have two concentric rings:
Referenced to geographic True North. The 0°/360° point aligns with the geographic meridian. Use the outer ring when plotting true courses and bearings from the chart.
Offset from the outer ring by the local variation. The annual change rate is printed inside the rose (e.g., "Var 15°00' W (2020) Annual Increase 7'"). Use the inner ring to transfer magnetic courses directly.
5. TVMDC — The Compass Correction Sequence
The backbone of all compass math on the captain exam
TVMDC is the ordered sequence of compass corrections every navigator must know. It is not just a mnemonic — it is the logical chain from the chart (True) to the compass (Compass) and back. All USCG compass conversion questions follow this exact chain.
| Letter | Name | Definition | Where to Find It |
|---|---|---|---|
| T | True | Direction referenced to geographic True North — what you plot on the chart | Calculated / plotted |
| V | Variation | Angle between True North and Magnetic North at your location | Chart compass rose |
| M | Magnetic | Direction after correcting True for Variation | Calculated |
| D | Deviation | Error caused by the vessel own magnetic field on a given heading | Vessel deviation card |
| C | Compass | What the steering compass actually reads aboard your vessel | Compass instrument |
Mnemonic: True to Compass
Work LEFT to RIGHT when converting True to Compass. At each step: Add West, Subtract East.
Mnemonic: Compass to True
Work RIGHT to LEFT (Compass to True) — or equivalently, at each step: Add East, Subtract West.
The Four Rules — Memorize These
6. Worked Conversion Examples
Step-by-step solutions for every question type
Example 1: True to Compass
Given: True course 270, Variation 8W, Deviation 3E
Example 2: Compass to True
Given: Compass reading 045, Variation 12E, Deviation 4W
Example 3: Finding Deviation from Known True and Compass
Given: True bearing to landmark: 180. Compass bearing to same landmark: 176. Variation: 5W.
Example 4: Full TVMDC Chain
Given: True course 315, Variation 10W, Deviation 5W. Find Compass course.
| Step | Value | Operation | Correction | Result |
|---|---|---|---|---|
| T | True | Starting point | — | 315° |
| V | Variation | West = ADD (T to C) | +10 | 325° Magnetic |
| D | Deviation | West = ADD (T to C) | +5 | 330° Compass |
| C | Compass | Final answer | — | 330° |
7. Compass Deviation Card
How to read and use the vessel deviation table
The deviation card is a table (or card) posted near the compass showing how much the compass deviates from magnetic north for each heading the vessel might steer. It is created by a compass adjuster who "swings the ship" — taking compass bearings to a known object from multiple headings and computing the difference from the true bearing corrected for variation.
Sample Deviation Card
| Compass Heading | Deviation | Compass Heading | Deviation |
|---|---|---|---|
| 000 (N) | 2° E | 180 (S) | 3° W |
| 015 | 3° E | 195 | 2° W |
| 030 | 5° E | 210 | 1° W |
| 045 (NE) | 6° E | 225 (SW) | 0° |
| 060 | 5° E | 240 | 1° E |
| 075 | 3° E | 255 | 3° E |
| 090 (E) | 1° E | 270 (W) | 5° E |
| 105 | 1° W | 285 | 4° E |
| 120 | 3° W | 300 | 2° E |
| 135 (SE) | 4° W | 315 (NW) | 1° E |
| 150 | 4° W | 330 | 0° |
| 165 | 4° W | 345 | 1° E |
How to Use the Deviation Card
- 1.Determine your compass heading (the direction you are steering)
- 2.Find the nearest compass heading listed in the deviation card
- 3.Note the deviation for that heading (E or W)
- 4.Apply the correction per TVMDC rules
Interpolating Between Entries
If your heading falls between two listed values, interpolate proportionally. Heading 052 is halfway between 045 (dev 6E) and 060 (dev 5E): interpolated deviation is approximately 5.5E, rounded to 6E. On most exam problems, the heading will match a listed value exactly — interpolation is rarely tested but good seamanship to understand.
Critical Point: Heading-Dependent Deviation
Deviation is always looked up using the compass heading— the direction you are actually steering by the compass. If you are converting from True to Compass, you do not yet know the compass heading, so in strict practice you may need to iterate. On the exam, this complication is usually avoided by providing the deviation directly or stating the compass heading.
8. Taking Bearings and Determining Position
Lines of position, fixes, running fixes, and dead reckoning
Knowing where you are is the fundamental task of navigation. A bearing to a charted object creates a Line of Position (LOP) — your vessel is somewhere on that line. One LOP tells you a line; two intersecting LOPs give you a fix; three give you a high-confidence fix.
Three-Bearing Fix
ExcellentTake simultaneous or rapid-succession bearings to three charted objects. Plot each bearing as a line of position (LOP) on the chart. The intersection is your fix. Three LOPs rarely intersect perfectly — a small triangle (cocked hat) is normal. Your position is within or near the triangle; assume the worst-case corner nearest danger.
Two-Bearing Fix (Cross Bearing)
Good when bearings are 60–120 degrees apartTwo LOPs from two landmarks. The intersection is the fix. Accuracy is best when bearings are approximately 90 degrees apart; degrades when bearings are nearly parallel.
Running Fix
Degrades with errors in speed or course during the runTake bearing to one object, record time and log. Run a known course for a measured time/distance. Advance the first LOP by the distance run, parallel to the course steered. Take a new bearing; intersection with the advanced LOP is the running fix.
DR Position (Dead Reckoning)
Degrades with time; errors accumulateProject position forward from a known fix using course steered and speed through water for elapsed time. No external bearings required. Does not account for current, leeway, or compass error.
Estimated Position (EP)
Better than DR but worse than a fixA DR position that has been corrected for estimated set and drift (current). Better than a pure DR but still not a fix. Marked with a square on the chart; a fix gets a circle.
Chart Symbols for Position Types
9. COG vs. Heading — CMG vs. SMG
Understanding the difference between where you point and where you go
One of the most frequently misunderstood concepts on the exam is the distinction between what a vessel is headed (heading) and where it is actually going (Course Over Ground). Current, wind, and leeway all push a vessel off its heading. Knowing these differences is critical for set-and-drift calculations and for understanding what GPS reports.
Heading vs. COG
The direction the bow points. Measured from North by the vessel compass. Does not account for external forces.
The actual direction of travel relative to the seabed. Reported by GPS. Includes the effect of all current, wind, and leeway.
CMG vs. SMG
The actual direction from the departure point to the arrival point over the entire voyage. Combines all legs, course changes, and drift into one resultant direction.
The effective speed of advance from departure to destination, considering all delays, diversions, and current effects. Always referenced to the ground (earth).
Set and Drift — Measuring Current Effect
The direction the current is flowing — expressed as a true bearing toward which the water is moving. A set of 090 means the current is pushing you eastward.
The speed of the current in knots. A drift of 1.5 knots means the water is moving at 1.5 knots in the direction of set.
10. GPS and GNSS
Accuracy, HDOP, datum, and the limitations every mariner must know
GPS (Global Positioning System) is a U.S. government satellite navigation system. GNSS (Global Navigation Satellite System) is the umbrella term covering GPS, Russia's GLONASS, Europe's Galileo, and others. Modern chartplotters often use multiple GNSS constellations simultaneously for better accuracy and reliability. The exam primarily tests GPS knowledge.
Consumer GPS uses WGS-84 datum. NOAA charts are referenced to WGS-84. If your GPS is set to a different datum (NAD-27, for example), displayed positions will be offset from chart positions — sometimes by hundreds of feet.
Horizontal Dilution of Precision — a dimensionless number indicating satellite geometry quality. HDOP below 1 is excellent; below 2 is good; above 5 is poor. High HDOP (few satellites or bad geometry) means larger position error. The exam expects you to know that low HDOP = better accuracy.
Civilian GPS without WAAS: approximately 15 meters (50 ft) CEP. With WAAS (Wide Area Augmentation System): sub-3-meter accuracy. WAAS corrects for atmospheric delays using ground stations — enabled automatically on most recreational chartplotters.
GPS fails under: signal blockage (canyons, bridges, tall structures), atmospheric interference, intentional jamming or spoofing, receiver failure, low battery. GPS is also only as accurate as the chart it is plotted on — chart errors translate directly to navigation errors.
GPS reports Course Over Ground (COG) and Speed Over Ground (SOG), not compass heading. COG is the actual track relative to the earth. Your compass heading may differ from COG due to current, leeway, and wind. Never use GPS COG as your compass course without correcting for these forces.
USCG regulations and good seamanship require independent verification of GPS position. Use visual bearings, radar ranges, or depth soundings to confirm GPS-derived positions, especially near hazards.
HDOP Reference Table
| HDOP Value | Rating | Approximate Accuracy | Practical Implication |
|---|---|---|---|
| 1 | Ideal | ~5 m (16 ft) | Excellent for coastal navigation |
| 1–2 | Excellent | ~10 m (33 ft) | Fully adequate for all navigation |
| 2–5 | Good | ~15–25 m (50–80 ft) | Acceptable for most purposes |
| 5–10 | Moderate | ~50 m (165 ft) | Use caution near hazards |
| >10 | Poor | >100 m (330 ft) | Cross-check with visual means |
GPS Limitations — Exam Critical Points
- ◉GPS shows where you ARE, not what is around you — a GPS position is useless without an accurate chart
- ◉Chart errors are invisible to GPS — a wreck or shoal in the wrong place on the chart is wrong on your chartplotter too
- ◉GPS does not measure depth — it cannot detect a shoal directly below you
- ◉GPS can be jammed, spoofed, or lost at the worst moment — always maintain situational awareness by traditional means
- ◉WGS-84 datum mismatch can place you hundreds of feet from your actual position
11. NOAA Chart Symbols
Buoys, lights, depths, hazards, and obstructions
NOAA Chart No. 1 ("Nautical Chart Symbols, Abbreviations and Terms") is the reference for all chart symbols. It is available free from NOAA and is an allowed reference on the USCG exam. The exam tests the most common symbols — particularly light characteristics, hazard markers, and position quality qualifiers.
Light Characteristics
Lights are described by their characteristic (pattern), color, period (seconds for one cycle), and range (nautical miles visibility). Example: Fl G 4s 15ft 4Mmeans Flashing Green every 4 seconds, 15 feet above MHW, visible 4 nautical miles.
| Symbol | Light Type | Description |
|---|---|---|
| Fl | Flashing | Single flash at regular intervals; off period longer than on |
| Fl(2) | Group Flashing | Two flashes in a group, then darkness |
| Oc | Occulting | Light more on than off; brief eclipse |
| Iso | Isophase | Equal periods of light and darkness |
| Q | Quick | Rapid flashing, approximately 60 per minute |
| VQ | Very Quick | Very rapid flashing, approximately 120 per minute |
| Mo(A) | Morse Code A | Dot-dash pattern (di-dah) |
| F | Fixed | Continuous uninterrupted light |
| F Fl | Fixed and Flashing | Fixed light with brighter flash superimposed |
| Abbreviation | Meaning | Chart Color |
|---|---|---|
| Rk | Rock (above MHW) | Red |
| Wk | Wreck (dangerous) | Magenta |
| Obstn | Obstruction | Magenta |
| PA | Position Approximate | Black |
| PD | Position Doubtful | Black |
| ED | Existence Doubtful | Black |
| Rep | Reported (unverified feature) | Black |
| Fl | Flashing light (on less than off) | Magenta/Green/Red |
| Oc | Occulting light (on more than off) | Magenta/Green/Red |
| Iso | Isophase (equal on and off) | Magenta/Green/Red |
| Mo(A) | Morse Code light (letter A: dot-dash) | Magenta |
| Q | Quick flashing (about 60 per minute) | Magenta/Green/Red |
IALA Buoyage System — Region B (U.S.)
The United States uses IALA Region B buoyage. The key rule: "Red Right Returning" — red buoys (even numbers) are kept to starboard when entering port from seaward.
Depth Soundings and Contours
Individual numbers printed on the chart showing water depth at MLLW in feet or fathoms (1 fathom = 6 feet). The unit is stated in the chart title. A depth printed in italic indicates the sounding is less reliable.
Lines connecting points of equal depth, similar to topographic contour lines. The 6-foot (1-fathom) and 12-foot (2-fathom) contours are particularly important for shoal-draft vessels. Blue tinting typically indicates shoal areas.
12. Chart Plotting Tools
Parallel rulers, dividers, and plotters — what they do and how to use them
The USCG chart plotting exam section requires physical tools. You must supply your own — the testing center does not provide them. Understanding each tool's function is tested in exam questions as well.
Parallel Rulers
Two rulers connected by pivoting arms that maintain parallelism as one ruler is walked across the chart. Used to transfer a bearing line from the compass rose to any position on the chart (or vice versa). Walk the parallel rulers from the course line to the center of the compass rose, read the bearing on the rose. Traditional Douglas-style parallel rulers or rolling plotters serve the same purpose.
- ‣Walk rulers carefully — the hinge can slip on a wavy chart or ship motion
- ‣Always confirm which side of the rose you read (0-180 or 180-360)
- ‣Practice transferring lines at angles from all four quadrants before exam day
Dividers
A compass-like instrument with two pointed legs used to measure distances on the chart. Open the dividers to span a distance, then compare to the latitude scale (one degree of latitude = 60 nautical miles; one minute of latitude = 1 nautical mile). Always measure distance on the latitude scale at the same latitude as the feature — on Mercator charts, the scale expands toward the poles.
- ‣Always use the latitude scale (vertical), never the longitude scale, for distance
- ‣For large distances, step the dividers in known increments (e.g., 5 NM per step)
- ‣Set dividers before exam; transport carefully to avoid bending the points
Protractor Plotter (Course Plotter)
A transparent plastic plotter combining a straight edge and a printed compass rose or degree scale. Align the center hole over a meridian (vertical line on the chart) and read the bearing directly from the degree scale. Douglas, Weems and Plath, and Spratt plotters are common types. Unlike parallel rulers, they do not need to be walked across the chart — they are used in place.
- ‣Align the vertical line on the plotter with a true meridian (not magnetic)
- ‣Read from the correct side — 0-360 clockwise from North
- ‣Some plotters have both True and Magnetic scales; use True for chart work then convert
Pencils and Erasers
Sharp HB pencils for plotting. Soft erasers that do not damage the chart surface. Plot all lines lightly — exam scorers deduct for multiple conflicting lines left on the chart. Use a 0.5mm mechanical pencil for precision. Never use ink on a chart.
- ‣Bring multiple sharp pencils — points break under pressure
- ‣Erase all construction lines that are not part of your final answer
- ‣A kneaded eraser works well on vellum chart surfaces
Exam Day: Tools Checklist
13. Frequently Asked Questions
Common exam questions answered precisely
What is the difference between variation and deviation on a compass?+
What does MLLW mean and why does it matter for navigation?+
What is the TVMDC mnemonic and how do I apply it?+
What is the GPS datum WGS-84 and why does it matter?+
What is a three-bearing fix and why is it preferred?+
What is the difference between a DR position and an estimated position?+
How do I calculate actual depth under the keel at a given tide?+
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