5.1 Plotting Tools & Chart Setup

Why Chart Plot Is a Career Filter

On the ONC06 Master Less Than 100 GRT path, Q163 Chart Plot is the shortest module and often the most failed: 10 questions, 90% required. That means 9 of 10 must be correct—one wrong answer is the maximum miss. There is no averaging with a strong Rules or Deck General score. You either plot and compute accurately under time pressure, or you retest the module.

The module is open-book with charts and tools, not a pure memory test. Success is procedure + neatness + arithmetic discipline. A Master carrying passengers under a COI lives this skill every inlet crossing; the exam simply compresses it into ten graded problems.

Time envelope: chart-plot sessions are often allotted up to about 3.5 hours for the 10 items (confirm your REC schedule). That is roughly 20 minutes per question if you use the full window—plenty if you are methodical, deadly if you re-plot the same leg four times because labels are missing.

Exam-Room Materials Awareness (Deck)

The National Maritime Center publishes an Examination Room Materials list for deck applicants. In general for navigation/plot modules (not Rules of the Road):

TR means training: the chart looks and works like a NOAA coastal sheet for plotting practice, but it is not maintained for live navigation. You will not memorize the chart; you will use it—lat/long scales, roses, and charted objects as given.

Before test day: download the current NMC exam-room materials PDF, practice on the same TR charts the exam uses, and bring tools you have already drilled with. Do not invent “allowed” electronics; chart plot is classical paper technique.

Core Toolkit

Parallel-ruler technique

1. Align one edge exactly with the line you drew (course or bearing).
2. Hold that half firm; walk the free half toward the nearest compass rose.
3. Continue until an edge passes through the center of the rose.
4. Read the degree mark where the edge crosses the correct ring.

Always through the center. Reading off-center is a silent degree error that fails a 90% module.

Divider technique

• Measure a leg: open points on both ends of the track, carry to the latitude scale (side of chart), read NM.
• Step a known distance: set dividers to (for example) 5.0 NM on the latitude scale, walk heel-to-heel along the course, count steps, measure remainder.
• Keep the spread firm and modest; a wide, loose pair sags and reads long.

Pencil habits that save points

• Sharp point for thin lines and small position dots.
• Light pressure so erase does not scar the chart.
• No ink.
• After each construction, label before you leave the problem—memory fades across ten items.

Compass Rose: True vs Magnetic

Each rose has (typically):

• Outer ring oriented to true north (often a star at true north) → true courses and bearings.
• Inner ring offset by variation → magnetic directions.
• Center text: variation, year, annual change (e.g., 14°00′W (2016), annual increase 6′).

Worked variation update. Rose: 14°00′W (2016), annual increase 6′. For a 2026 problem: 10 years × 6′ = 60′ = 1° more west → 15°00′W. Many exam items simply say “use nearest rose” with a stated variation; still know how annual change works when asked.

Default exam habit: plot and label in true (C 090°T) unless the question explicitly demands magnetic or compass. Convert with TVMDC when you must steer by compass.

Variation and Deviation — Application Order

Recall the ladder from Navigation General:

T → V → M → D → C (TVMDC)

• Variation (chart/rose, location) sits between true and magnetic.
• Deviation (vessel table, heading) sits between magnetic and compass.

Uncorrecting (true → compass): “East is least, West is best” — subtract easterly errors, add westerly errors.
Correcting (compass → true): reverse — add easterly, subtract westerly.

Mini drill. True course 100°T, Var 12°W, Dev 0° → uncorrecting: 100 + 12 = 112°C to steer.
Mini drill. True course 220°T, Var 14°W, Dev 3°E → T→M: 220 + 14 = 234°M; M→C: 234 − 3 = 231°C.

Never apply deviation before variation on the ladder, and never take deviation from the rose.

Chart Orientation and Scale Selection

• Align the chart so north is “up” relative to your work surface unless a problem forces a different layout; keep roses readable.
• Prefer the largest-scale training chart that covers the working area when more than one sheet is relevant—detail reduces measurement error.
• Measure distance on the latitude scale at about the latitude of the work, not across a distant part of the sheet where Mercator scale stretch differs.
• Never use the longitude (top/bottom) scale for distance on a Mercator chart.

Labeling Conventions (Graded Neatness)

Label every course, speed, LOP time, and position type. Unlabeled geometry is how candidates “know” the answer and still pick the wrong multiple-choice option because they misread their own plot.

Accuracy Routine for 90%

With only one miss allowed, treat each question as a mini checklist:

1. Read the entire stem—what quantity is asked (true course? compass CTS? set? ETA? SMG?).
2. Plot start position carefully from lat/long or charted object.
3. Draw, walk to nearest rose center, read correct ring.
4. Measure on latitude scale; write arithmetic on scratch paper.
5. Label; re-read the question; match the option to your labeled number, not a guess.

Setup traps that fail Q163

• Longitude scale used for distance.
• True/magnetic ring swap on the rose.
• Parallel rulers not through rose center.
• Course labeled without T/M/C qualifier.
• Deviation applied as if it were variation (or vice versa).
• Sloppy 0.2–0.3 NM divider error that compounds across multi-leg problems.

Master the setup until tools feel automatic. The remaining sections turn this setup into courses, current triangles, and timed fixes—the skills that convert a passenger-vessel Master’s daily craft into a 9/10 exam performance.