USCG Chart Plotting: How to Read and Plot on a Nautical Chart for the Exam
Nautical chart plotting questions appear on every USCG captain's license exam. A licensed captain explains how to read charts, use parallel rulers, and solve the problems the exam actually tests.
The Chartroom Captain
Licensed USCG 100-Ton Near Coastal Master
What the USCG Exam Actually Tests on Charts
When students hear "nautical chart plotting," they picture complex offshore navigation with sextants and parallel rulers. The USCG exam chart work is more targeted than that. For the OUPV, you need to solve specific problem types — speed/time/distance calculations, current corrections, finding positions using bearings, and reading chart symbols. For the 100-Ton, add tide calculations and more advanced current work.
None of it is beyond anyone willing to learn the framework. Here's what you need to know.
Anatomy of a Nautical Chart
Before you can plot on a chart, you need to read it. The key elements:
Latitude and longitude lines. Latitude lines run east-west (horizontal); longitude lines run north-south (vertical). On a standard Mercator chart, one minute of latitude equals one nautical mile — this is how you measure distances on a chart using the latitude scale on the sides, never the longitude scale at top and bottom.
Depth soundings. Numbers printed on the water areas showing depth at mean lower low water (MLLW). The chart's title block tells you the units (feet, fathoms, or meters). Know which unit your chart uses before reading any depth.
Chart symbols. The full reference is Chart No. 1, published by NOAA — it's a free download and the authoritative source. For the exam, know the most common: anchorage areas, rocks (above and below water), wrecks, restricted areas, buoy symbols, and lighthouse symbols.
Variation. The compass rose printed on the chart shows both true north (outer ring) and magnetic north (inner ring). The difference between them is the variation for that area, along with the annual rate of change. This is the variation value you use in compass math problems.
Scale. The chart's scale determines how much detail it shows. Large-scale charts (like 1:10,000) cover a small area in high detail — good for harbors. Small-scale charts (like 1:1,200,000) cover large areas with less detail — good for offshore passage planning. The exam will specify which chart to use.
The Tools You Need to Know
Parallel rulers. Used to transfer a course line to the compass rose to read the bearing or course in degrees. You "walk" the rulers across the chart from your plotted line to the center of the nearest compass rose. Practice this — it's awkward at first and requires a feel for keeping the rulers parallel as you step them across.
Course plotter (Portland plotter or similar). An alternative to parallel rulers that many students find easier. You align the plotter's grid with the chart's latitude/longitude lines and read the course directly. Ask your REC whether they provide plotting tools or whether you should bring your own.
Dividers. Used to measure distances on the chart against the latitude scale. Set the dividers to the distance between two points on the chart, then transfer to the latitude scale on the side of the chart to read the nautical miles.
Calculator. For speed/time/distance math and current calculations. The REC allows non-programmable calculators — bring one.
Speed, Time, Distance: The Foundation
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The most fundamental navigation calculation on the exam is the speed/time/distance relationship:
Distance = Speed × Time Speed = Distance ÷ Time Time = Distance ÷ Speed
Time must be in hours (or decimal hours) for these formulas to work. 30 minutes = 0.5 hours. 45 minutes = 0.75 hours.
Example: A vessel travels at 8 knots for 2 hours 30 minutes. How far does it travel?
Time = 2.5 hours
Distance = 8 × 2.5 = 20 nautical miles
These questions appear in the Navigation Problems section and are some of the most straightforward points available if you have the formula memorized and use your calculator correctly.
Current Corrections
Current problems add a layer of complexity to basic course and speed calculations. When a current is pushing your vessel, your actual track over the ground differs from the course you're steering through the water.
Key terms:
Course steered (heading): The direction the bow is pointed
Speed through water: Your vessel's speed log reading
Current set: The direction the current flows toward
Current drift: The speed of the current in knots
Course made good (CMG): Your actual track over the ground
Speed made good (SMG): Your actual speed over the ground
The exam tests whether you understand how current affects your actual track versus your intended course. Vector diagrams are the traditional method — drawing the current vector and vessel vector to find the resultant track. Some students find the graphical method easier; others prefer the mathematical approach. Either works on the exam.
Finding Position by Bearings
A common chart problem: you take compass bearings to two charted objects (a lighthouse, a buoy, a radio tower) and plot those lines of position on the chart to find your location. Where the lines intersect is your fix.
Steps:
Convert compass bearings to true bearings using deviation and variation
At each charted object, draw a line in the direction of the bearing (the reciprocal — add or subtract 180°)
The intersection of the two lines is your position
The exam will give you the bearings and ask you to identify your position from multiple-choice answers — you don't need to be perfectly precise, just close enough to identify the correct answer from the options given.
Chart Symbols Worth Memorizing
The exam tests recognition of common chart symbols. The ones that appear most frequently:
Rocks awash at mean water level: asterisk symbol
Submerged rocks (dangerous): + symbol with dot
Wrecks (dangerous): specific wreck symbol — know the difference between a wreck that covers and one that doesn't
Anchorage area: anchor symbol within a circle
Restricted area: magenta (pink) hatching
Shoal water (shallow): blue shading on the chart
Lights: magenta circle with light characteristics described in abbreviated format (e.g., "Fl R 4s 15ft 4M" = flashing red, 4-second period, 15 feet above water, 4 nautical mile visibility)
Chart No. 1 is the complete reference — download it free from NOAA and scan the symbols section. Spend an hour with it before your exam and most symbol questions become easy.
Practicing Chart Work Effectively
Chart plotting is one area where physical practice matters. Reading about parallel rulers is not the same as using them. Get a practice chart — the NMC’s current exam room materials list names 12221TR, 12354TR and 13205TR, and your REC can tell you which one your exam uses, a set of parallel rulers or plotter, and dividers, and work through 10–15 plotting problems before exam day.
The navigation math — speed/time/distance, current corrections, compass conversion — can be drilled in practice question format. The Navigation General practice module includes these problem types with step-by-step explanations. Start with 50 free questions to see which problem types you're already comfortable with and which need more work before your REC appointment.
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