NAVIGATION · Lesson 3 of 6
Latitude, Longitude and Position
25 min · Beginner friendly
You’ll learn toExpress a position in degrees, minutes and decimal minutes · Plot and read positions using dividers and the latitude scale · Explain why one minute of latitude equals one nautical mile
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Every position at sea is two numbers: how far north or south you are, and how far east or west. Your speaks this grid, the almanac's tide tables use it, and a Mayday call is built around it — so a navigator must be able to read it, write it and plot it without hesitation.
Take a guess
Decide before you read: you have spanned 3 miles of coast with your dividers and need to read the distance off. Which chart edge do you hold the span against?
Commit to an answer before reading on — a wrong guess here helps the right idea stick.
The Grid Around the Globe
- Latitude
- Angular distance north or south of the equator, 0° to 90°, written with N or S.
- Longitude
- Angular distance east or west of the Greenwich , 0° to 180°, written with E or W.
- Parallel
- A line of constant latitude, running east–west.
- Meridian
- A line of constant longitude, running from pole to pole.
- Minute of arc
- One sixtieth of a degree. Written with a prime mark, and subdivided into decimals in modern practice.
The grid is angular: degrees, divided into 60 minutes of arc, divided in modern practice into decimals rather than seconds. Angles, not distances — with one glorious exception coming below.
Knowledge check
From the globe above: which set of lines squeezes closer together as you travel towards the poles?
Writing a Position
Marine practice writes positions in degrees, minutes and decimal minutes — not seconds. A position off the fictional of Westhaven might be written 50° 42.50' N, 001° 15.30' W, read aloud as fifty degrees, forty-two point five zero minutes north; one degree, fifteen point three zero minutes west. One decimal place of a minute is a tenth of a — 185 metres — and two decimal places is about 18 metres, more precision than most chartwork needs.
- Latitude always comes first, then longitude.
- Always attach the hemisphere letter: N or S for latitude, E or W for longitude — 50° N is a world away from 50° S.
- Longitude degrees are conventionally written with three figures (001°, 010°, 120°) to avoid ambiguity.
- Minutes never reach 60 — sixty minutes rolls over into the next degree.
Knowledge check
Which of these is a correctly written marine position?
One Minute of Latitude = One Nautical Mile
The most elegant fact in navigation: the nautical mile was defined as one minute of latitude, standardised at 1,852 metres. The consequence is that the latitude scale running up each side of your chart doubles as a distance scale — span two points with , hold the span against the latitude scale, and read minutes as miles. A tenth of a minute is a , about 185 metres. Because Mercator stretches latitude up the sheet, always use the part of the scale level with the area you are measuring.
Knowledge check
Why must distance always be measured on the latitude scale rather than the longitude scale?
Plotting a Position
Plotting turns a written position into a pencil mark on the chart. You need dividers, a soft pencil, and a plotter or to carry straight lines. The process is mechanical — which is exactly what you want when you are tired, cold, and the boat is bouncing.
- On the latitude scale at the side of the chart, find the latitude — degrees, then minutes, then decimals — and set one divider point on it.
- Transfer that latitude into the chart area by laying a straight edge horizontally through it, or by measuring with dividers from the nearest printed .
- On the longitude scale at the top or bottom, find the longitude the same way.
- Carry the longitude vertically to meet the latitude line; the intersection is your position.
- Mark it neatly with a small dot and label it — you will learn the standard notation in Lesson 6.
Worked example
Plotting 50° 44.30' N, 001° 12.60' W
You want to plot this position on the Westhaven approach chart, where the latitude scale is subdivided into whole minutes and tenths.
- On the side (latitude) scale, find 50° 44'. Each small subdivision is 0.1', so count three subdivisions upwards for 44.30'. Mark it with a divider point.
- Lay the plotter edge horizontally through that point — horizontal meaning parallel to the printed parallels of latitude.
- On the bottom (longitude) scale, find 001° 12', then count six tenths towards the west for 12.60'.
- Bring that longitude up vertically (parallel to the meridians) until it crosses the pencil line from step 2.
- The intersection is the position. Mark it with a small dot and write the time beside it.
The position lies at the intersection — about 1.5 nm north-east of the Westhaven fairway buoy on this chart.
Your turn
Reverse the process: a charted rock lies at the intersection of the 50° 41.20' N parallel and the 001° 18.75' W meridian. Describe, step by step, how you would read that position off the chart using dividers, starting from the pencil dot on the rock.
Work it through on paper first — then check yourself against the solution. Reading a worked answer feels like learning; producing one is learning.
Key points
- Latitude measures north–south from the equator (0–90°); longitude measures east–west from Greenwich (0–180°).
- Positions are written latitude first, in degrees and decimal minutes, always with hemisphere letters.
- One minute of latitude = one nautical mile = 1,852 m; the latitude scale is your distance scale.
- Never measure distance on the longitude scale, and always use the latitude scale level with your working area.
- Plotting and reading positions are the same divider technique run in opposite directions.
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