Chart Plotting Exam Guide
TVMDC compass conversions, speed-time-distance, set and drift, dead reckoning, and required tools — everything for the 18-question chart plotting section, with worked problems.
Bring Your Own Tools — This Is Not Optional
The USCG testing center does not provide chart plotting tools. You must bring:
Practice with these exact tools on NOAA Training Chart 1210Tr before exam day — muscle memory matters.
TVMDC — Compass Conversions
TVMDC is the sequence for converting between True course (from the chart) and Compass course (what your steering compass reads). Every compass problem on the exam uses this chain.
The Direction Rule
Worked Example — True to Compass
True course: 090°. Variation: 15°W. Deviation: 3°E. Find Compass course.
Worked Example — Compass to True
Compass course: 245°. Deviation: 5°W. Variation: 12°E. Find True course.
Speed-Time-Distance (S-T-D)
Distance (NM) = Speed (knots) × Time (minutes) ÷ 60
Worked S-T-D Problem
You are traveling at 8 knots on a course of 035°T. After 1 hour 15 minutes, what is your DR position? Starting from 41°15.0'N, 71°30.0'W.
Step 1: Convert time to minutes: 1hr 15min = 75 minutes
Step 2: Calculate distance: D = 8 × 75/60 = 10 NM
Step 3: On the chart, from your starting position, draw a line in direction 035°T (use compass rose + parallel rulers)
Step 4: Mark off 10 NM along that line (use latitude scale with dividers — 1 minute of latitude = 1 NM)
Step 5: Label the DR position with a half-circle (⊙) and the time
Set and Drift
Set is the direction the current is flowing (where it's going, not where it's coming from). Drift is the current's speed in knots. Together, they push your vessel off the intended track.
Finding Set and Drift
When you know your DR position and your actual fix position:
- 1.Plot your DR position (where you should be)
- 2.Plot your actual fix position (where you are)
- 3.Draw a line from DR to actual fix
- 4.Measure the direction of that line = Set
- 5.Measure the distance = current displacement; divide by elapsed time = Drift
Correcting for Current (CMG)
When planning a course with known current:
- 1.From starting point, draw intended track to destination
- 2.Draw current vector (set/drift) from starting point for 1 hour
- 3.From current vector tip, draw arc of radius = vessel speed (1 hour)
- 4.Where arc intersects intended track = Course to Steer heading
- 5.Distance from start to intersection = Speed Made Good
Worked Set and Drift Problem
At 0800 your DR position is 41°12.0'N, 71°20.0'W. At 1000, your actual fix is 41°10.0'N, 71°16.0'W. Find set and drift.
Step 1: Plot both positions on the chart
Step 2: Measure the direction DR → actual fix using parallel rulers + compass rose → Set ≈ 135°T (southeast)
Step 3: Measure the distance using dividers on latitude scale → approximately 4 NM
Step 4: Elapsed time = 2 hours. Drift = 4 NM ÷ 2 hours = 2 knots
Current: Set 135°T, Drift 2 knots
Measuring Distance on a Chart
Critical Rule — Use the Latitude Scale
Always measure distances using the latitude scale (left or right sides of the chart). Never use the longitude scale (top or bottom). One minute of latitude = 1 nautical mile. The latitude scale is constant; the longitude scale varies with latitude and cannot be used for distance.
Using Dividers
- 1.Open dividers to span the distance you want to measure on the chart
- 2.Transfer dividers to the latitude scale at the same latitude as your plot
- 3.Count the minutes of latitude = nautical miles
- 4.For long distances, walk the dividers in equal steps
Common Mistakes
- ✗Using the longitude scale for distance
- ✗Reading latitude minutes at the wrong latitude band
- ✗Forgetting that 1 minute = 1 NM (not degrees)
- ✗Measuring a curved route as a straight line
Chart Plotting Best Practices
Every fix, DR, and estimated position should be labeled: time (4-digit, e.g., 0830), position type (⊙ for DR, △ for fix, ⊕ for estimated position).
Plot lightly so errors are erasable. Don't bear down — heavy lines are hard to erase and reduce accuracy.
When walking rulers across the chart, press firmly with the stationary ruler before moving the other. A slipped ruler costs you a question.
Transfer courses using the compass rose nearest your plot area. Long ruler walks increase error. The chart usually has multiple roses.
Draw every line, even if the answer seems obvious. The examiner may review your plot, and visible work demonstrates competence.
Speed-time-distance errors are the most common chart section mistakes. Re-work every calculation using a different method if time allows.
Frequently Asked Questions
What tools do I need for the OUPV chart plotting exam?
You must bring your own tools: parallel rulers (or a rolling plotter), dividers, pencils, and erasers. The testing center does not supply tools. Most candidates use Douglas-style parallel rulers or a triangular rolling plotter. Practice with the exact tools you'll use on exam day — muscle memory matters. The exam uses NOAA Training Chart 1210Tr; download and practice on it before the exam.
What is TVMDC in chart plotting?
TVMDC is the memory sequence for compass conversions: True → Variation → Magnetic → Deviation → Compass. To convert True to Compass, work left to right adding West values and subtracting East. To convert Compass to True, work right to left subtracting West and adding East. The mnemonic 'True Virgins Make Dull Companions' (or 'Can Dead Men Vote Twice') helps remember the order. Variation comes from the chart's compass rose; deviation from the vessel's deviation table.
What is the formula for speed-time-distance in nautical navigation?
Speed × Time = Distance, but in nautical navigation, time is expressed in minutes and converted using the factor 60. The three forms: Distance = Speed × Time/60, Speed = Distance × 60/Time, Time = Distance × 60/Speed. Where distance is in nautical miles, speed in knots, and time in minutes. Example: traveling 12 knots for 45 minutes covers 12 × 45/60 = 9 nautical miles.
Practice chart plotting problems
TVMDC conversions, S-T-D problems, set and drift — with detailed explanations. 1,628+ practice questions across all 4 OUPV sections.
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