Module 02 · The chart table
Reading notes · Module 02 · Direction, distance and time
The tools on the table before any sum: the dividers on the latitude scale, the plotter, the pencil and the rubber.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.
The chart table
What is on the table
In the exam, and on any boat worth sailing, the chart is paper, and the navigator works it with a few things: a pair of dividers to measure distance, a plotter to measure and lay off directions, a pencil and a rubber. Some navigators keep a parallel rule too.
The dividers come first, because you will use them on every leg and every estimate.
The tools on the screen
Under this text are the tools, in the same place on every chart of the course. A screen switches on the ones it needs; the others stay greyed. Try them: nothing here is marked.
The dividers
Two points and a hinge
Dividers are two steel points on two legs, joined by a hinge. They do one thing: they hold a distance. You open them between two places on the chart, and the opening stays the same wherever you carry them.
Hold them by the hinge, with one hand. The points are sharp on purpose: they go exactly on the spot, not near it.
Read them on the latitude scale
A chart has two scales round its edge. Distance is read on the latitude scale, up the side: one minute of latitude is one nautical mile.
Latitude, never longitude
The scale along the top and bottom is longitude, and its minutes are not miles. A minute of longitude is a mile only at the equator; it shrinks as you go north. On this chart one minute of longitude is 0.69 of a mile: measure on it and every distance comes out 45 per cent too long.
A minute of latitude is a mile anywhere on earth. Distance comes off the side.
Swing them round a point
Off the scale the dividers come upright, and the track hardly ever is. You turn them onto it without opening them: one point stays down, the other swings round it.
The dividers are open to 2 miles on the latitude scale, as they come off it. Carry them by the hinge and put one point on S.
Press Lock the opening, in the tools above. Then a point cannot come closer or go further: it can only go round the other one.
Drag the free point round until it sits on the track. The opening stays at 2 miles: that is step 1 of the walk.
Walk them along a track
A long leg does not fit in one opening. Set the dividers to a round figure on the latitude scale — here 5 miles — and walk them along the track: pivot on the front point, swing the other one over, again and again, counting.
The quick way, without the dividers
On the screen there is a short cut: touch the leg, and its length is laid beside the latitude scale, starting on a whole minute. You still read it yourself: count the minutes it covers.
On paper there is no short cut. Use this to check yourself, not instead of the dividers.
The plotter
A rule with a rose on it
The plotter is a long clear rule with a small rose that turns in its middle. This is a Breton; the Portland does the same job. The exam lets you use either.
Three things matter: the dotted centre line with its arrow, the rose with its grid, and the index, where the rose meets the rule, where you read the answer.
Measure a direction, step by step
Watch. The edge of the rule lies along the line from A to B — the dotted centre line on it would do the same.
The arrow on the rule points the way you are going: from A towards B. The other way round, it would give the reciprocal.
Now the rose is turned, not the rule, until its grid lies along a meridian — the dashed lines of longitude on the chart — with its red arrows to the north.
The figure at the index, where the rose meets the rule on the arrow's side, is the direction: 115°.
Pencil and rubber
Lines you can take back
A soft pencil and a soft rubber. Lines go on lightly and every one gets its label. A line you no longer believe comes off.
Laying off a course
From a point, on a course, for a distance
Measuring a direction has a twin, and it matters as much: laying one off. From where you are, on the course you will steer, for the distance you will run — and the end of it is where you will be.
Watch. The course, 143°, is set on the rose at the index; the rule can lie anyhow for now.
The edge goes through P, and the whole plotter — not the rose — is turned until the rose's grid lies along a meridian, the arrow the way you are going.
The pencil runs along the edge, from P, the way the arrow points.
The dividers are opened to 2.5 miles on the latitude scale at the side, and the opening is locked.
Carried by the hinge, one point goes on P and the other swings onto the line. Where it lands is where you will be after 2.5 miles on 143°.
The parallel rule
If there is one on the table
Some navigators carry directions across the chart with a parallel rule: two rules joined by arms that stay parallel. Lay one edge across the compass rose on the figure you want, then walk it — hold one rule down, move the other, never both — until an edge touches your point, and draw.
It does the same job as the plotter, and on this course you use the plotter. It is here so you know it when you see one.
What you can do now
The table, ready
You can open the dividers on the chart, read them on the latitude scale, swing them and walk them along a leg, measure a direction with the plotter and lay one off, and keep the chart clean with a pencil and a rubber.
The tools are the same on every chart of the course.
Open the interactive chapter →The same material with the chart, the drawings and the exercises.