In this lesson you will learn
- What a nautical mile is, and why it is one minute of latitude (1,852 m)
- Cables and knots, and how to write distances (M) and speeds (kn) correctly
- Why distance is always measured on the latitude scale, never the longitude scale
- How to measure a distance with dividers, reading the scale level with the line
- How to walk the dividers along a long line, and total a route with several legs
The nautical mile
On land you measure distance in kilometres or miles. At sea, navigators use the nautical mile, and for a very good reason: it is built into the grid of latitude and longitude you met in Lesson 3.
A nautical mile is the length of one minute of latitude — the distance you cover going one minute (1′) north or south along a meridian. There are 60 minutes in a degree, so one degree of latitude is 60 nautical miles, and the 90° from the equator to either pole is 5,400 nautical miles.
The Earth is not a perfect sphere — it is slightly flattened at the poles — so a minute of latitude varies a little, from about 1,843 m at the equator to about 1,862 m near the poles. To avoid any argument, the nautical mile has been fixed internationally at exactly 1,852 metres. The difference is far too small to matter at the chart table: for every practical purpose, a minute of latitude is a nautical mile.
| Unit | Length | In nautical miles |
|---|---|---|
| Nautical mile (M) | 1,852 m | 1 |
| Cable | 185.2 m (about 185 m) | 0.1 |
| Statute (land) mile | 1,609 m | about 0.87 |
| Kilometre | 1,000 m | about 0.54 |
So a nautical mile is about 1.15 land miles, and not far short of two kilometres (1 M ≈ 1.85 km).
One minute of latitude = one nautical mile. That is why every chart carries its own scale of distance: the latitude scale down each side.
Cables, knots and how to write them
For short distances a mile is too coarse, so navigators also use the cable: one tenth of a nautical mile, about 185 m (roughly 200 yards). You will meet cables in pilot books and on the radio: “the entrance is two cables wide”, “anchor about three cables off the beach”. Ten cables make a mile, so 0.4 M is 4 cables and 7 cables is 0.7 M. On the chart’s latitude scale, each tenth of a minute is one cable.
Speed at sea is measured in knots. One knot is a speed of one nautical mile per hour: a yacht making 6 knots covers 6 miles in an hour, or 3 miles in half an hour. One knot is about 1.85 km/h, or 1.15 mph. Lesson 7 turns this into the navigator’s speed, time and distance formula.
The name comes from the days of sail, when speed was measured by streaming a small wooden “log” astern on a line with knots tied in it at regular intervals, and counting how many knots ran out while a small sand-glass emptied. That is also why the instrument that measures a boat’s speed is still called the log.
You can already find the M abbreviation on your practice chart. Gull Point lighthouse is labelled Fl(3)10s28m15M: the last part tells you the light has a range of 15 nautical miles (Module 3 decodes the rest).
Why we measure on the latitude scale
In Lesson 3 you saw that the chart has two scales in its border: the latitude scale down the left and right sides, and the longitude scale along the top and bottom. Both are marked in minutes and tenths of a minute, so it is tempting to think a minute is a minute. It isn’t — only one of them measures distance.
The meridians, the lines of longitude, are furthest apart at the equator and all meet at the poles. So the distance covered by one minute of longitude shrinks the further north or south you go. At the equator it is (almost exactly) a mile; at 60°N it is only half a mile; at the poles it is nothing at all. Minutes of latitude don’t shrink: the parallels stay the same distance apart all the way to the poles.
1′ of longitude ≈ cos(latitude) × 1 M
Roughly — you never need to work it out at the chart table, because you never measure distance on the longitude scale.
InteractiveA minute of longitude shrinks
Drag the slider, or pick a latitude, and watch the gap between the same two meridians shrink. The bars compare one minute of latitude with one minute of longitude at that latitude.
At 49°46′N the meridians are only 0.65 times as far apart as at the equator. A 3 M line running east–west here spans 4.6′ of the longitude scale — read that as miles and you would be 55% too far.
Worked example
How big is the practice chart?
- 1North–south, use the latitude scale: the chart covers 12′ of latitude (49°40′N to 49°52′N), so it is 12 M from bottom to top.
- 2East–west, the chart covers 24′ of longitude (004°00′W to 004°24′W) — but at 49°46′N each of those minutes is only about 0.65 M.
- 324 × 0.646 ≈ 15.5 M.
About 12 M from bottom to top and 15.5 M from side to side — not 24 M.
Measuring with dividers
Dividers are the navigator’s ruler. To measure the distance between two points:
- Open the dividers so that one point sits exactly on each end. Use each symbol’s precise position: the centre of a light’s star, the small circle at the foot of a buoy, the centre of a rock symbol.
- Carry them to the latitude scale without changing the spread, keeping them at about the same latitude as the line you are measuring. Use whichever side of the chart is nearer.
- Read the scale. Put one point on a whole minute and count the minutes and tenths to the other. Each minute is a mile; each tenth is a cable.
- Write it down, to one decimal place: for example, 2.9 M.
Worked example
The Kerran Approach buoy to Mew Rock
- 1Open the dividers from the small circle at the foot of the Kerran Approach buoy to the cross marking Mew Rock (just west of its beacon).
- 2Slide them, still open, straight across to the latitude scale on the left of the chart, level with the two marks.
- 3Put the lower point on 49°42′N and read where the upper point falls: 2.9′ further up the scale.
2.9′ of latitude = 2.9 M.
InteractiveDividers on the practice chart
Pick two features, drag the points A and B onto them (the bar under the chart shows the spread), then carry the dividers to the latitude scale. Try East Teeth → Gull Point: both are on the same latitude, so it shows the longitude-scale trap clearly.
- 1. Drag A onto the Kerran Approach buoy and B onto Mew Rock (both are pulsing). Zoom in with + to place the points exactly on the symbols.
- 2. Carry the dividers, without changing their spread, to the latitude scale at the same latitude, and read the minutes and tenths.
- 3. For comparison, see what the longitude scale would (wrongly) tell you.
Move the dividers to start. The bar under the chart shows their spread, like reading it off the scale.
Laying off a distance is the same job in reverse. To mark a point 1.5 M from a buoy, open the dividers to 1.5′ on the latitude scale (level with the buoy), put one point on the buoy and make a small pencil mark where the other falls along your line. You will do this all the time from Module 2 onwards, when you plot where the boat has got to.
Measure level with the line
As you saw in Lesson 4, your practice chart, like most charts used for coastal navigation, is drawn on the Mercator projection — it says so in the title block. Mercator draws the meridians as parallel straight lines, which spreads them further apart than they really are, and to keep coastlines the right shape it stretches the chart north–south by the same amount. As a result the latitude scale itself gets longer the further you are from the equator: on a chart of northern waters, a minute near the top is printed slightly longer than a minute near the bottom.
On a chart like the practice chart, which covers only 12′ of latitude, the difference between the top and the bottom of the scale is less than half of one per cent — far too small to see. But on a small-scale chart covering many degrees, such as an ocean passage chart, it really matters: on a chart running from 40°N to 60°N, a minute near 60°N is printed about half as long again as a minute near 40°N.
Longer distances: walking the dividers
Dividers only open so far, and the wider they are the harder they are to hold steady. For a longer distance, walk them:
- Set the dividers to a round distance on the latitude scale, level with the line: 1 M, 2 M or 5 M, to suit the chart.
- Put one point on the start and step the dividers along the line, swinging them over the leading point each time and counting the steps.
- When less than a full step is left, close the dividers onto the gap and measure it on the latitude scale.
- Add up: the number of steps × the setting, plus the gap.
Worked example
The Kerran Approach buoy to the East Teeth buoy
- 1Set the dividers to 1 M (1′ on the latitude scale, level with the line).
- 2From the Kerran Approach buoy, step them towards the East Teeth buoy: 6 full steps fit before the next would overshoot. That is 6 M.
- 3Close the dividers onto the gap that is left and measure it: about 0.8′, or 0.8 M.
6 + 0.8 = 6.8 M.
Most passages have several legs: straight lines between turning points, called waypoints, that you choose. Measure each leg separately, write its distance beside it on the chart, and add them up for the total. The widget below walks a three-leg passage from the Kerran Approach buoy, east-about past The Teeth, to the anchorage at Kerran Roads, turning at a waypoint in open water north-east of the island (WP1, 49°50.3'N 004°06.0'W).
InteractiveWalk the dividers
Choose 1 M or 2 M, then step the dividers along each leg. When less than a full step is left, measure the gap. Each leg’s distance is written on the chart as you finish it.
Leg 1 of 3: Kerran Approach buoy → East Teeth buoy
Put one point on Kerran Approach buoy and the other along the line, then step the dividers along it, counting as you go.
| Leg | From → to | Distance |
|---|---|---|
| 1 | Kerran Approach buoy → East Teeth buoy | 6.8 M |
| 2 | East Teeth buoy → WP1 | 3.5 M |
| 3 | WP1 → Kerran Roads anchorage | 4.1 M |
| Total | 14.4 M |
Common mistakes
| Mistake | What goes wrong | Instead |
|---|---|---|
| Measuring on the longitude scale | Distances come out half as long again as they really are, or more | Always use the latitude scale at the side |
| Reading the scale far from the line | On a small-scale chart the scale there is stretched differently | Read it level with the line |
| Points not on the exact positions | Errors of a cable or more creep in | Use each symbol’s precise position |
| Forgetting the gap when walking | The distance comes out too short | Count the steps, then always measure what is left |
| Saying “knots per hour” | It doesn’t mean anything | Speeds in kn, distances in M |
Next, in Lesson 6, you will add the other half of chart work: measuring the direction of a line with a plotter. Distance and direction together let you describe any passage on the chart.
Key terms
- Nautical mile (M)
- The length of one minute of latitude, fixed at exactly 1,852 m. Also written nm.
- Cable
- One tenth of a nautical mile — about 185 m.
- Knot (kn)
- A speed of one nautical mile per hour. Never “knots per hour”.
- Statute mile
- The land mile, 1,609 m — about 0.87 of a nautical mile.
- Latitude scale
- The scale down each side of the chart. One minute on it is one nautical mile: use it for every distance.
- Longitude scale
- The scale along the top and bottom. Its minutes shrink with latitude, so never use it for distance.
- Dividers
- A hinged, two-pointed instrument for measuring and laying off distances on a chart.
- Walking the dividers
- Stepping dividers set to a round distance along a line, then measuring the gap that is left.
- Mercator projection
- The projection most charts use. Its latitude scale stretches away from the equator, so read it level with the line.
- Leg
- One straight line of a route, between two turning points (waypoints).
Exercises
Check your understanding. You get a second try at each question, then the worked answer. Score 70% or more to complete the lesson — your progress is saved in this browser.
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Distance practice
Five fresh questions each round: two measurements on the chart, a conversion, a technique question and a quick check of the theory.