It's a foolhardy skipper who leaves harbour with no knowledge of how to use his charts and nav instruments to keep track of where he is. Robert Avis introduces chartwork.

It’s a foolhardy skipper who leaves harbour with no knowledge of how to use his charts and nav instruments to keep track of where he is. Robert Avis introduces chartwork. (January 1998)

Lat and long

There are many ways of determining a position. Latitude and longitude are what mariners use to specify a point on the globe without reference to a well-known feature.

Latitude is the angle subtended between a point on the earth’s surface and the centre of a plane through the equator. It is expressed in degrees and minutes north or south of the equator in this form: 50°36’N.

Longitude is the angle subtended horizontally between any point on the earth’s surface east or west of the Greenwich meridian and is also expressed in degrees and minutes in this form: 002°46’E.

Note that as longitude can be expressed up to 180° east or west it should always be shown in three figures, even if the first two figures are zeros, to avoid confusion with latitude, which can only ever be up to 90°.

In giving a position using lat and long, it is customary to give lat first: i.e. “My position is 60°00’N 060°00’E”. This indicates only one place on the surface of the earth.

Helpful hints

Never measure distance off the top or bottom of a chart ­ always off the latitude scale on the sides.

Take great care when cruising along the South Coast not to forget that longitude changes from east to west or (vice versa) as you cross the Greenwich Meridian.

Always follow up a fix with a DR (dead reckoning) or EP (estimated position).

Bearings

If we established our position as that shown, it could be described in terms of latitude and longitude as follows: 51°30.0’N 004°20.0’E

We could also describe it as a distance and bearing from a well-known point ­ for example, four nautical miles south of Worms Head, or even 180° Worms Head 4M.

Note that positions should be given as bearings from well-known points and not to them.

However if, in a moment of panic, you need to give your position and you give a bearing from you, so long as you make it quite clear that: “Worms Head bears 000° from me four miles”, no one is going to criticise you for using this formula in the heat of the moment.

Laying off a course

Laying off a true course is very simple. A Breton or Portland plotter is probably the easiest, though not the only way, of drawing the course that you wish to follow onto the chart.

Place the plotter along the course you wish to follow. Check carefully that it does not go over any danger areas and that the arrow on the plotter is pointing in the direction you wish to go to.

Hold the plotter firmly with one hand and draw in the planned track.

Before removing the plotter, swivel the centre compass rose so that the north marker points up the chart towards north and finely adjust it, lining up the centre grid with any of the vertical or horizontal grid lines on the chart. Then lift the plotter off the chart and read off the course.

Fixes

It is important that we all use the same plotting symbols so that in the unlikely event of an accident, anyone can understand what is drawn on the chart.

A perfect fix is difficult to obtain. You are more likely to get a ‘cocked hat’ or area of uncertainty.

If you don’t have much success with a hand bearing compass but have a radar, try taking the ranges of three points from the radar screen. These can be plotted with a pair of compasses by drawing a position line with an arrow on each end of the line to indicate a range rather than a bearing.

It is easy to obtain a good fix with three or more radar ranges.

Having established your position you can then lay off your planned track: in this case, 110° true.

If you know your speed is ten knots you can predict where you will be by dead reckoning your position. At ten knots, you will travel one mile every six minutes. A dead reckoning or DR is shown by a small line at right angles to the track with the recorded time in minutes. At any time in the future, a quick glance at your watch will indicate your likely position from the DR.

Estimated position

Once you have mastered the art of dead reckoning, the next way of determining a future position is by calculating an estimated position or EP. This is more accurate than a DR because it accounts for tide and wind.

Providing you know the direction and speed of the tide ­ and, if appropriate, you can estimate the effect of the wind ­ it is not difficult to predict an EP, which is shown as an equilateral triangle ­ point up.

What a knot is

In the last example we showed our speed as “10 knots” but what does this mean?

The nautical measure of speed is a knot ­ a nautical mile per hour, not “a knot per hour”.

We measure a nautical mile as one minute of latitude at that latitude on the chart.

It is approximately 2,000yds and is divided into ten cables of around 200yds each.

Always measure distance off the Latitude scale on the side of the chart.

Never, never, never off the Longitude scale (top or bottom).

Transits

The ideal way to ensure that you are certain of your line of approach is to line up two charted marks. The line through two charted marks is known as a transit, and it can be used to check approach courses and compasses