Latitude, longitude and coordinates: how to read and share a location

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Coordinates let you describe any point on Earth with two numbers, with no need for a street address. They are invaluable for meeting friends at a trailhead, reporting a fault, finding a parked car or giving emergency services your position. Here is how they work and how to share them without mistakes.

Latitude and longitude

Latitude measures how far north or south of the equator you are, from 0 degrees at the equator to 90 degrees at each pole. Lines of latitude run east to west and are called parallels.
Longitude measures how far east or west you are of the prime meridian, which passes through Greenwich in London. It runs from 0 to 180 degrees east or west. Lines of longitude run from pole to pole and are called meridians.

By convention, latitude is always given first, then longitude. Mixing up the order is the most common mistake people make, and it can put a point on the other side of the world.

The three common formats

The same location can be written in several ways:

Degrees, minutes and seconds (DMS): for example 51 degrees 30 minutes 26 seconds N, 0 degrees 7 minutes 39 seconds W. There are 60 minutes in a degree and 60 seconds in a minute.
Degrees and decimal minutes (DDM): for example 51 degrees 30.433 minutes N. Common on marine charts and many GPS units.
Decimal degrees (DD): for example 51.5072, -0.1276. Used by most phone apps and websites. In this format, south and west are written as negative numbers rather than with letters.

To convert DMS to decimal degrees, divide the minutes by 60 and the seconds by 3,600, then add them to the degrees. Most map apps will accept any of the three formats if you type them carefully.

How precise do you need to be?

A degree of latitude is roughly 111 kilometres everywhere. A degree of longitude is about the same at the equator but shrinks towards the poles. As a rough guide in decimal degrees:

Two decimal places narrow a location to around a kilometre, enough for a town.
Four decimal places get you to roughly ten metres, enough for a building.
Five decimal places reach about a metre, which is already finer than a typical phone GPS reading.

Quoting eight or ten decimal places does not make your position more accurate. It only repeats noise.

Datums: the hidden detail

Coordinates are measured against a model of the Earth's shape called a datum. Phones and GPS receivers use WGS 84, and almost every web map does too. Some older paper maps and national grids use a different datum, and the same numbers can then point to a spot hundreds of metres away. If you copy coordinates from an old map, check which datum it uses. National grid references, such as those on British Ordnance Survey maps, are a separate system of eastings and northings rather than latitude and longitude. Do not mix the two.

How to find and share your coordinates

On a phone map app: press and hold on a spot, or tap the blue dot showing your position, and the coordinates usually appear and can be copied.
In a compass or GPS app: many show your current latitude and longitude on screen.
When sharing: send decimal degrees, latitude first, separated by a comma, and check the minus signs. Pasting them back into a map before you send them is a good habit.
In an emergency: read the numbers out slowly, say north or south and east or west, and repeat them. Many countries' emergency services can also receive your location automatically from a mobile phone call.

Alternatives to raw coordinates

Short location codes, such as the open Plus Codes system, turn coordinates into a compact string that works where there are no street addresses. They are handy for sharing, but raw latitude and longitude remain the most universal format, understood by every map, GPS unit and rescue service. How do you usually share a meeting point or a remote spot? Share your method in the comments.