5.1 Plotting Tools & Chart Setup

The Chart Plotting Module at a Glance

The Chart Plotting module (also called Navigation Problems or the Plotting exam) is short but unforgiving: 10 questions graded at 90%, so you may miss only one. You have 3½ hours, and every problem is worked with pencil, parallel rules, and dividers on a paper Mercator training chart — most commonly Chart 1210 TR. The TR stands for "training": the chart looks like a real NOAA chart of the waters between Martha's Vineyard and Block Island, but it is not maintained for live navigation. You will not memorize it; you will use it, extracting courses, distances, and positions exactly as you would at the helm.

The ten questions are drawn from a predictable set of skills: compass-correction (TVMDC) and compass-error problems, cross-bearing fixes, course and speed made good, set and drift, course to steer and leeway, distance off, and estimated time of arrival. This chapter walks each one with fully worked numbers.

Your Toolkit

A course plotter or a pair of drafting triangles is an acceptable substitute for parallel rules if you are faster with them, but the walking-parallel-rules technique below is what most schools teach.

Reading the Compass Rose

Each rose has two concentric scales. The outer ring is oriented to true north (marked by a star) and reads true directions. The inner ring is offset by the local variation and reads magnetic directions directly. At the center the rose prints the variation, the year it was measured, and an annual change, for example: "14°00′W (2016), annual increase 6′."

Worked example — updating variation. The rose reads 14°00′W for 2016 with an annual increase of 6′. For a 2026 problem that is 10 years × 6′ = 60′ = 1° of added westerly variation, giving 15°00′W. In practice most exam problems tell you to use the printed value from the nearest rose, and many editions ignore the annual change — but knowing how to update it protects you when a question demands it.

Walking the Parallel Rules

To read a course you have drawn: lay one edge of the rules exactly along your line, hold that half firm, and "walk" the other half toward the center of the nearest compass rose, alternately pinning each half until an edge passes through the rose's center. Read the degrees where the edge crosses the ring — the outer ring for true, inner ring for magnetic — and note the reciprocal at the opposite side. Always walk to the rose center; reading off-center introduces error.

Labeling Conventions (Memorize These)

Plotting is graded on neat, correct labels as much as on geometry:

• Course line: label the true course above the line as three digits — C 090°T — and the speed below — S 12.
• DR position: a semicircle with the four-digit time, e.g. a dot with a semicircle labeled 1130.
• Fix (from bearings, ranges, or GPS): a circle around the dot with the time.
• Estimated position (EP): a square with the time — a DR corrected for current.

Distances are always three digits of care: a sloppy 0.2-nm error compounds across a multi-leg problem.

Handling the Dividers

There are two ways to use dividers, and good candidates use both. To measure a leg, open them to span the two points, carry them to the latitude scale, and read the miles. To step off a known distance along a course — say, the 10 nm you will run in the next hour — open them to that distance against the latitude scale first, then "walk" them heel-over-heel along the course line. Keep the spread modest; a wide, loose divider drifts and reads long. Whenever a leg is longer than the widest comfortable span, set the dividers to a round figure such as 5 nm and step, counting the whole steps and measuring the leftover remainder separately.

Choosing Your Rose

A chart carries several compass roses. Always walk your rules to the nearest one to the line you are reading — the fewer inches you have to walk the rules, the less chance the angle creeps. When two roses are equally close, pick the one whose variation label matches the waters you are working, because variation changes slightly across a large chart.

Working a Problem — the Routine

With 3½ hours for 10 questions you have roughly 20 minutes each, so a disciplined routine beats speed:

1. Read the whole problem and mark the given position, times, courses, speeds, and any variation/deviation.
2. Plot the start point precisely from its latitude and longitude.
3. Lay off the course or bearing and read it against the nearest rose.
4. Measure distances on the latitude scale, and compute any speed–time–distance or TVMDC step on scratch paper.
5. Label everything and re-read the question to confirm which quantity it actually asks for.

Common Setup Traps

• Measuring distance on the wrong scale. Use the latitude scale on the side of the chart — never the longitude (top/bottom) scale, where a minute of arc is not a nautical mile on a Mercator projection.
• Reading the wrong ring of the rose (true vs magnetic) — the single most common careless error.
• Letting a divider point slip or the rules "creep" while walking, so the angle you read is not the angle you drew.
• Working in ink — you will re-plot, so keep it in pencil and keep the point sharp.