Chart plotting simulator. Real training charts, real instruments.

They are the same sheets. 1210 Tr (Martha's Vineyard to Block Island) is the one most US navigation schools issue. 12221 Tr (Chesapeake Bay Entrance) and 13205 Tr (Block Island Sound) are the two the Coast Guard publishes its own worked plotting problem sets against, so you can take an official question set and work it here. All three come from NOAA's historical chart collection, so they are US government works in the public domain. Like the paper originals they are printed for instructional purposes only and must never be used for navigation: each is frozen at its edition year, so aids to navigation, depths and magnetic variation have all moved on.

Because the real ones cannot. Parallel rules are two arms held by pivoting links, and an arm can only travel perpendicular to itself — that is the whole mechanism. To get a course from the compass rose across to your position you open one arm, hold it, bring the other up to it, and repeat: you walk them. The simulator enforces the same constraint, because the transfer is the skill the exam is testing. Dragging the rule freely across the sheet would be quicker and would teach you nothing.

That is the single most common lost mark in the plotting module, and it is a property of the Mercator projection rather than a mistake in the chart. Mercator stretches east–west more and more as you go north, so the longitude scale along the top and bottom borders is not a distance scale at all. Distance is only ever measured on the latitude border at the sides, where one minute of latitude is one nautical mile. The dividers readout in this tool always computes distance the correct way, so if your hand measurement disagrees with it, check which border you stepped it off on.

Off the compass roses printed on the chart, the same as on paper — there are three on this sheet. Each rose gives the variation, the year it was correct for, and an annual change, and the exam expects you to bring it up to date. The tool deliberately does not do that for you: it reports true courses only, and leaves the true-to-magnetic step where it belongs. If you want the arithmetic checked afterwards, the magnetic variation calculator works the T-V-M-D-C ladder step by step.

Yes, which is why it is switched off by default and labelled as not being on the real instrument. With it off you read the course off the compass rose exactly as you would on paper. Turn it on when you want to check an answer or when you are learning the motion of the rules and do not yet want to fight the rose at the same time.

Yes. Load any image of a chart and calibrate it by clicking two points whose position you can read and typing those positions in — a graticule crossing at opposite corners works best. The image never leaves your browser: it is stored locally and nothing is uploaded, which is also why we can let you use a chart we would have no right to host ourselves. The calibration assumes a north-up Mercator sheet, and it cross-checks itself: on a true Mercator the horizontal and vertical scales must come out the same, so if they disagree the tool tells you by how much rather than quietly plotting wrong courses.

Yes. The exam is worked by hand on a paper chart, and there is a feel to walking rules and stepping dividers that a mouse does not reproduce. A combined kit with parallel rules, dividers and a training chart runs about $30–50. Use this simulator to learn the method, to practise anywhere, and to check your work — not as a replacement for the paper.