Coordinates and map grids: stating a position precisely

Geographic coordinates – latitude and longitude – measure angles. Latitude says how far north or south of the equator a point lies, longitude how far east or west of an agreed prime meridian. This is the same worldwide, but impractical on a paper map, because degrees are awkward for measuring distances.

That is why many topographic maps carry a grid of squares measured in metres. A widespread system is UTM: the Earth is divided into 60 zones, each 6 degrees of longitude wide. Within a zone you state how many metres east a point lies (easting) and how many north (northing). On hiking maps the grid squares are often one kilometre across. Some countries also have their own national grids.

A grid reference is read from coarse to fine. On a one-kilometre grid, two plus two digits identify a whole square. If you also estimate how far into the square the point lies to the right and up – in tenths – you add one digit each and are accurate to about 100 metres. A romer, printed on many baseplate compasses, helps you measure precisely.

The most common mistake is the wrong order. Always the easting first (to the right), then the northing (up). And always take the line to the left of the point and the line below it as your starting point – not the ones to the right or above.

The second big mistake is mixed-up formats. A phone can show the same position, for example, as decimal degrees, as degrees and minutes, or as degrees, minutes and seconds. The numbers look alike but mean different things: 47° 30′ is 47.5 degrees, not 47.30 degrees. This mix-up alone produces an error of more than 20 kilometres north–south, because one degree of latitude is roughly 111 kilometres.

The third point is the reference system, also called the map datum. It describes which mathematical model of the Earth the coordinates refer to. Satellite navigation mostly uses WGS84; older or national maps sometimes use another. If the map states a different datum from the one set in the device, change the device – otherwise positions can be noticeably offset.

In an emergency call, coordinates are the quickest way to describe a place that has no name – such as a spot in the middle of a forest. First name the format ('UTM, zone …' or 'latitude and longitude in decimal degrees'), then the numbers in groups, slowly and clearly. Have them read back. If you are unsure, also describe what you see: path, stream, hut, summit.

In a group, coordinates let you agree meeting points that everyone can find on their own map. And when switching between paper map and GPS, waypoints can be transferred – provided format and datum match.

Coordinates are only useful if you know where you are. Without a known position you cannot read them off the map. Your phone often knows your position anyway – which is why it is worth practising at home where the app shows the coordinates.

Start by reading the map margin: many maps explain there with an example how to build a reference and which zone and datum apply. Then find a known point on the map, such as your house or a hut, and work out its reference.

Next, set your phone or GPS to the same format and datum, go to that point and compare. If the values roughly match, you did it right. If not, check the order, the format and the datum.

A good game for two: one person names a coordinate, the other finds the point on the map and describes what is there. It trains exactly what matters in an emergency.