Latitude and longitude

An address for anywhere on Earth

In Lesson 2 you described where things are relative to the boat: ahead, astern, on the port bow. That works on board, but it can’t tell anyone else where the boat actually is. The coastguard, a friend you are meeting, or you yourself plotting on a chart need a reference that everyone shares and that doesn’t move with the boat.

That reference is a grid of imaginary lines covering the whole Earth: latitude and longitude. Every place has one latitude, which says how far north or south it is, and one longitude, which says how far east or west it is. Put the two together and you have a position, a unique address for that spot — in the middle of an ocean just as much as in a harbour.

A position is two numbers: latitude (north or south) and longitude (east or west).

Latitude: north and south of the equator

The Earth spins around an axis through the North Pole and the South Pole. Halfway between them, around the Earth’s middle, runs the equator. Latitude measures how far north or south of the equator you are.

Latitude is an angle. Picture a line from the centre of the Earth to where you are: your latitude is the angle between that line and the plane of the equator. (This treats the Earth as a perfect sphere, which is close enough for chart work.) So the equator is 0°, the North Pole is 90°N and the South Pole is 90°S. A latitude can never be more than 90°.

All the places with the same latitude form a circle around the Earth, parallel to the equator. These circles are called parallels of latitude. The equator is the biggest; they get smaller towards the poles. On a chart, parallels run straight across from side to side.

Longitude: east and west of Greenwich

Lines running from pole to pole are called meridians. Each is a half-circle, and they all meet at the poles. The equator gives latitude a natural starting line, but meridians have no natural zero, so one had to be chosen. By international agreement in 1884, the meridian through Greenwich, in London, became the prime meridian: 0° longitude.

Longitude is the angle east or west of the prime meridian, measured around the Earth’s axis. It runs from 0° up to 180°E and 180°W. Those two are the same meridian, on the far side of the world from Greenwich. On a chart, meridians run straight up and down.

Degrees, minutes and decimal minutes

A whole degree is far too big a step to pinpoint a boat, so each degree is divided into 60 minutes, written with a single mark: 45' is forty-five minutes. Charts and GPS sets then show parts of a minute as a decimal; on charts it is usually tenths. So 45.2' means 45 minutes and 2 tenths of a minute. Degrees, minutes and decimal minutes is the standard way to give a position at sea.

Some land maps and websites divide each minute into 60 seconds (") instead. To turn seconds into decimal minutes, divide by 60: 30" is 0.5', 12" is 0.2' and 6" is 0.1'. Be careful: 45'30" is 45.5', not 45.3'. Phone apps sometimes show decimal degrees, such as 49.7533°. To convert, multiply the decimal part by 60: 0.7533 × 60 = 45.2', so 49.7533° is 49°45.2'.

Writing a position

Navigators write every position the same way, so that nobody can misread it:

1. Latitude first, then longitude.
2. Latitude degrees in two figures (07°, 49°); longitude degrees in three figures (004°, 012°, 145°). The leading zeros make it obvious which number is which, and stop a figure going missing.
3. Minutes with two figures and one decimal place: 04.6', 45.2'.
4. A hemisphere letter after each: N or S for latitude, E or W for longitude.

Put together, a position looks like this: 49°45.2'N 004°10.6'W — forty-nine degrees forty-five point two minutes north, four degrees ten point six minutes west.

The scales on the chart’s borders

Every chart is framed by scales. The latitude scale runs down the left and right borders; the longitude scale runs along the top and bottom. A handy way to remember: lines of latitude lie flat like the rungs of a ladder, and you read them on the ladder up the sides of the chart.

On the Isle of Kerran chart, latitude runs from 49°40'N at the bottom to 49°52'N at the top, and longitude from 004°00'W on the right to 004°24'W on the left. Each minute is shown as a black or white band, and small ticks divide it into tenths (the tick at the half-minute is a little longer). Every minute is labelled, with the full degrees and minutes printed every five minutes. Faint grid lines cross the chart every 2' of latitude and every 4' of longitude to help you line things up. The border prints the longitude as 4°10'W, but when you write a position you still use three figures: 004°10'W.

Reading a position off the chart

1. Latitude: from the object, go straight across (along a parallel) to the latitude scale at the nearer side. Count the whole minutes, then the tenths.
2. Longitude: go straight up or down (along a meridian) to the longitude scale. Count the minutes and tenths, in the direction the numbers increase.
3. Write it down in the standard form, latitude first.

On paper you don’t draw these lines. Instead, line your plotter’s edge up with a printed parallel and slide it to the object, or use dividers: put one point on the object and the other on the nearest printed parallel directly above or below it, then carry that span to the latitude scale, starting from the same parallel. Do the same for longitude with a printed meridian. A buoy’s charted position is the middle of the foot of its symbol, often marked with a small circle; a landmark such as a church or tower is charted at the small circle (or dot) at the foot of its symbol. Lesson 8 covers chart symbols in detail.

Plotting a position

Plotting is the same job in reverse. You are given a position — from the GPS, a pilot book or the coastguard — and you mark it on the chart.

Common mistakes, and a look ahead