Chart Plotting Tips for the OUPV Exam
The Chart Plotting section is the #2 failure point on the OUPV exam. Only 10 questions, 90% to pass — you can miss just ONE. Here's how to nail it.
Why this section is dangerous
With only 10 questions and a 90% passing score, there's zero margin for error. One careless mistake on TVMDC, one misread compass rose, one wrong scale — and you fail. The good news: it's open book and the problems are predictable. Practice the techniques until they're automatic.
1. What to Bring
You work on an actual NOAA training chart (1210TR) with physical tools. Bring your own — don't rely on the testing center having extras.
NOAA Training Chart 1210TR
The exact chart used on the exam. Practice on this specific chart — learn its compass roses, landmarks, and depth contours. Available from Paradise Cay or Amazon (~$20).
Parallel rules or rolling plotter
For transferring bearings from the compass rose to your course line. Weems & Plath is the standard. Practice until 'walking' the parallel rules is second nature.
Dividers
For measuring distances on the latitude scale. Get quality brass dividers that hold their setting — cheap ones slip.
Sharp pencils (#2)
Mechanical pencils (0.5mm) work well for precise lines. Bring several.
Eraser
A good eraser is essential. Consider FriXion erasable pens for clean corrections.
Simple calculator
For 60 D ST calculations, tide heights, and ETA computations. Nothing fancy needed.
2. TVMDC Conversions
TVMDC (True, Variation, Magnetic, Deviation, Compass) is the foundation of chart plotting. Get this wrong and every subsequent answer is wrong too.
The Formula
CAE = Correcting Add East. When converting from Compass to True (correcting), add easterly errors and subtract westerly errors. When going from True to Compass (uncorrecting), do the opposite.
Correcting (C → T)
Add East, Subtract West
Uncorrecting (T → C)
Add West, Subtract East
Exam Tip
Write TVMDC as a column on scratch paper. Fill in what the problem gives you. Variation comes from the chart's compass rose. Deviation comes from the deviation table (based on the ship's heading). Solve for the unknown.
3. Dead Reckoning (DR)
Dead reckoning is plotting your estimated position based on course and speed from a known position, without accounting for current. It's the baseline for all navigation.
Steps
- Plot the starting position (fix) and mark it with a circle and time
- Draw the course line from the fix in the direction of the true course
- Calculate distance: D = S x T / 60 (speed in knots, time in minutes)
- Measure the distance along the course line using dividers on the latitude scale
- Mark the DR position with a half-circle and the time
Critical Rule
Always use the latitude scale (sides of the chart) for distance — NEVER the longitude scale (top/bottom). One minute of latitude = one nautical mile. This is the most common beginner mistake.
4. Fixes & Running Fixes
Visual Fix (Two Bearings)
Take bearings on two or more identifiable landmarks. Plot each Line of Position (LOP) on the chart. Your fix is where they intersect. Mark it with a circle and the time.
Running Fix
When you can only see one landmark, take a bearing at Time 1, continue on course, then take another bearing at Time 2. Advance the first LOP along the course line by the distance traveled. Where the advanced LOP crosses the second LOP is your running fix.
How to advance an LOP:
- Calculate distance traveled between the two bearings (D = S x T / 60)
- From any point on the first LOP, measure that distance along the course line
- Draw a new line through that point, parallel to the original LOP
- Where this advanced LOP crosses the second bearing is the running fix
5. Set & Drift
Set is the direction the current pushes you. Drift is the speed. You find these by comparing your DR position (where you should be) to your actual fix (where you are).
Finding Set & Drift
- Plot your DR position for the time of the fix
- Plot your actual fix at the same time
- Draw a line from DR position to fix — the direction of this line is the set
- Measure the length of this line — this distance divided by the time interval gives the drift (in knots)
Course to Steer (Current Sailing)
To compensate for known current, use the vector triangle: plot the current vector from your starting position, then swing an arc of your speed from the end of the current vector to your destination course line. Where the arc intersects is the course to steer.
6. Speed, Time & Distance (60 D ST)
One formula handles all speed-time-distance problems on the exam:
60 x D = S x T
D = distance (nautical miles), S = speed (knots), T = time (minutes)
Find Distance
D = S x T / 60
Find Speed
S = 60 x D / T
Find Time
T = 60 x D / S
7. Tide Calculations (Rule of Twelfths)
The Rule of Twelfths estimates the tide height at any time between high and low water by dividing the tidal range into 12 equal parts:
1/12
1st hr
2/12
2nd hr
3/12
3rd hr
3/12
4th hr
2/12
5th hr
1/12
6th hr
Remember: 1-2-3-3-2-1. The tide changes fastest in hours 3 and 4 (3/12 each = half the total range).
Exam Tip
First find the tidal range (high water minus low water). Then figure out how many hours have elapsed since the last tide change. Add up the twelfths for each elapsed hour and multiply by the range. Add to low water (or subtract from high water) to get the height at your time.
8. Labeling Standards
Proper labeling is part of the answer. Incorrect labels can cost you points even if the plot is right.
Position Symbols
Fix
Circle
DR Position
Half-circle
EP
Square
Course Line Labels
- • Above the line: 3-digit course with T or M (e.g., "C 045 T")
- • Below the line: Speed in knots (e.g., "S 8.5")
- • At positions: 4-digit time in 24-hour format (e.g., "1430")
9. Common Mistakes That Cause Failures
Using the longitude scale for distance
Always use the latitude scale (sides of the chart). 1 minute of latitude = 1 nautical mile. The longitude scale changes with latitude and is NOT accurate for distance.
Reading the reciprocal bearing (180 degrees off)
The compass rose has numbers on both the inner and outer edges. Make sure you're reading the direction FROM your vessel TO the landmark, not the reverse.
Wrong compass rose ring (True vs Magnetic)
Outer ring = True. Inner ring = Magnetic. If the problem asks for True course, use the outer ring. If it asks for Magnetic, use the inner ring.
TVMDC direction error
CAE: Correcting (Compass to True) Add East. If you're going the other direction (True to Compass), reverse the signs. Write it out as a column every time — don't do it in your head.
Using the wrong deviation from the table
Deviation depends on the ship's compass heading, not the true course. Look up deviation for the compass heading you're converting FROM.
Sloppy parallel rule walking
If your parallel rules slip even slightly, your course is wrong. Press firmly, walk in small steps, and verify by reading the bearing at the compass rose. Consider a rolling plotter for more precision.
Not labeling positions correctly
Circle = fix, half-circle = DR, square = EP. Time labels in 4-digit 24-hour format. Course above the line with T/M, speed below. Missing labels = wrong answer.
Final Exam Day Tips
- ✓ Practice on chart 1210TR — it's the exact chart used on the exam. Familiarity saves time.
- ✓ Read each question twice — note whether it asks for True or Compass, and the direction of conversion.
- ✓ Write TVMDC every time — even simple conversions. It takes 10 seconds and prevents sign errors.
- ✓ Check units — speed in knots, time in minutes for 60 D ST. Mixing units is an instant wrong answer.
- ✓ Use scratch paper — work calculations outside the chart to keep your plot clean and legible.
- ✓ Verify answers — if you have time, re-measure one course and one distance. A small verification can catch a costly error.
Practice Chart Plotting Questions
NailTheTest has 150+ chart plotting practice questions covering every problem type on the exam, plus structured worksheets and an AI chart work checker.