The chart table: the right chart, a position, a course and a DR

What you do on the bridge

1. The right chart
2. Plot the position
3. Lay off the course
4. A DR ahead
5. Check the DR

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. Which chart should you be using, and how do you know it is fit to use?Often asked

   A good answer covers:

   • The largest-scale chart that covers where the ship is, as the passage plan lists them
   • Corrected up to date from the latest notices to mariners, with the corrections recorded on it and in the chart correction log
   • The latest edition, checked against the catalogue or the cumulative list
   • Check the datum and units, and any notes, cautions and the source data diagram before relying on it

   Show me on the bridgeThe chart table and the folio. Which chart covers where we are at the largest scale?

2. How are depths, heights and dangers shown on a chart, and where do you look a symbol up?Often asked

   A good answer covers:

   • Depths are soundings below chart datum, usually about the lowest astronomical tide; drying heights are underlined
   • Heights of lights and land are above a high water level, so clearance under a bridge or a light's range must allow for the tide
   • Wrecks, rocks and obstructions have their own symbols, with a dotted danger line round those that matter
   • Every symbol and abbreviation is explained in Chart 5011, the INT 1 symbols booklet
3. What is the difference between a fix, a DR and an EP, and how do you mark each?Often asked

   A good answer covers:

   • A fix is an observed position, from position lines or a positioning system: a dot in a circle, with the time
   • A DR is worked from a known position using the course steered and the speed through the water: a cross on the course line
   • An EP also allows for the tidal stream and leeway: a dot in a triangle
   • Each is labelled with the time, and the method for a fix
4. How do you measure a distance and lay off a course on a Mercator chart?

   A good answer covers:

   • Courses and bearings with a parallel ruler or plotter, from the compass rose or a meridian
   • Distance with dividers against the latitude scale at the side, level with where you are measuring
   • Never use the longitude scale for distance: a minute of latitude is a nautical mile, a minute of longitude is not
   • Use a soft pencil and a soft rubber, so the chart survives the corrections and erasures

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   We're heading south at eight knots, about three miles off the New Jersey shore, with the ECDIS working. The paper chart is the back-up, and you need to be quick with it before you ever need it.

   It's slack water, so for now the ship goes where she is pointed. We'll take out the right chart, plot our position, lay off the course and run a DR ahead.

2. The right chart

   Go to the chart table, key four, and open the folio. Take out the largest-scale chart that covers where we are. A small-scale chart leaves out the detail that matters close to land.

3. Plot the position

   Read our position from the ECDIS Sensors tab and plot it on the chart as a GNSS fix, with the time. Latitude off the side scale, longitude off the top and bottom.

4. Lay off the course

   From the position, lay off our course, one nine zero, for a few miles. On paper you would use the parallel ruler from the compass rose; here, give the course and a length.

5. A DR ahead

   Now a dead reckoning position fifteen minutes ahead: from the fix, at one nine zero and eight knots. That's two miles down the course line. Measure distance on the latitude scale, never the longitude scale.

6. Check the DR

   Let the time run on to the DR time. Time compression is fine here: open water, nothing close. Then plot the GNSS position again.

   Compare the fix with the DR. With no stream they sit almost together. When they do not, the difference over the time run is the set and drift, and that comes in the coastal lessons.