Coordinates: latitude, longitude and UTM
Coordinates are the postal address of any point on Earth. This guide explains the two systems you will see on maps, how to read them and how to use the grid on a topographic map.
Latitude and longitude
The best-known way to give a position is with two angles. Latitude says how far north or south of the equator a point is: 0° at the equator, 90° at the poles. Longitude says how far east or west of the Greenwich meridian: from 0° to 180° on each side.
Lines of equal latitude are called parallels and run around the Earth from east to west; lines of equal longitude, meridians, run from pole to pole. Madrid's Puerta del Sol, for example, is at about 40.417° north and 3.703° west.
A degree of latitude is always about 111 km. A degree of longitude, on the other hand, is 111 km at the equator and narrows towards the poles: about 85 km in Madrid, about 55 in Oslo. That is why a one-degree by one-degree “square” is not square on the ground.
How they are written
There are two notations. The classic one uses degrees, minutes and seconds: 40° 25' 1" N, 3° 42' 13" W. A degree has 60 minutes and a minute 60 seconds, like hours.
The decimal one, used by phones and online maps, uses only degrees with decimals and a sign: positive to the north and east, negative to the south and west. The same point is 40.4170, -3.7036. Watch the order: latitude almost always comes first and then longitude, though some programs do it the other way round.
How many decimals do you need? With two, precision is about a kilometre; with three, about a hundred metres; with four, about ten metres, enough for a house; with five, a metre. More decimals add nothing outside professional surveying.
In PlotMyMap you can paste decimal coordinates into the search box to go straight to that point.
Why UTM exists
Degrees are perfect for a globe but awkward on a flat map: they do not measure the same in every direction, and working out a distance with them takes trigonometry. To work on paper you want a grid in metres.
UTM (Universal Transverse Mercator) divides the Earth into 60 zones 6° of longitude wide, numbered 1 to 60 eastwards from the antimeridian. Each zone is projected separately onto a plane, with very little distortion, and a kilometre grid is drawn on that plane.
Mainland Spain falls in zones 29 (Galicia and the west), 30 (most of the centre) and 31 (Catalonia, Valencia and the Balearics); the Canary Islands, in 28. The official maps of Spain's Instituto Geográfico Nacional use UTM, and so do PlotMyMap's topographic maps.
Reading a UTM coordinate
A UTM coordinate has three parts: the zone (and hemisphere), the X or “easting” and the Y or “northing”, both in metres. For example, the Puerta del Sol is 30N 440,310 E, 4,474,270 N.
X is counted from a line 500 km west of the zone's centre, so it is never negative: values below 500,000 lie west of the centre and above it, east. Y, in the northern hemisphere, is the distance in metres from the equator: 4,474 km in Madrid.
The great advantage is that distances are worked out by subtraction. If a hut is at X = 352,400 and a summit at X = 354,900, the summit is 2.5 km to the east. With Y, the same northwards. Nothing needs converting.
The drawback is that each zone has its own coordinates. On the border between two zones, a point has different coordinates depending on which zone you use, and maps always say which.
Using the grid on a topographic map
On a topographic map the UTM grid appears as fine lines labelled in the margin with their kilometres (often abbreviated: “352” instead of 352,000). The spacing depends on the scale: every kilometre on a 1:25,000, every 500 m or 100 m on a more detailed one.
To locate a point, find the square it falls in and read the lines at its bottom-left corner: that is the coordinate rounded to the kilometre. Then measure with a ruler how far the point is from those lines and convert to metres with the scale. At 1:25,000, each millimetre is 25 metres.
An example: the point is 18 mm to the right of line 352 and 31 mm above line 4,785. That is 450 m and 775 m: the coordinate is X = 352,450, Y = 4,785,775. You can pass that to a GPS or to a companion.
The reverse works for finding on the map a point you have been given: locate the square and measure from its corner.
Grid north and true north
The vertical lines of the UTM grid do not point exactly to geographic north, except at the centre of each zone. Towards the edges they tilt slightly, up to about 3° at latitudes like Spain's. That difference is called grid convergence.
On top of that, a compass does not point to geographic north either, but to magnetic north, which moves over the years; the difference is the magnetic declination. In Spain it is now under one degree and can be ignored on a hike, but in other parts of the world it exceeds 10°.
PlotMyMap draws each map on the UTM projection of the zone its centre falls in, so the grid lines up with the edges of the paper. On a map a few kilometres across, the difference from geographic north is small and, for finding your way on the ground, it does not matter.
Coordinates and PlotMyMap
When you choose a place in the tool, the centre of the map is stored as decimal latitude and longitude: if you share the link, whoever opens it sees exactly the same framing.
On topographic maps you can switch the grid on or off, choose its spacing and its style: full lines or just crosses at the intersections, which get in the way of the drawing less. On aesthetic maps, made for hanging, the grid rarely adds anything and is better left off.
If you want the map to carry written coordinates as a decorative detail, put them in the subtitle: they are the classic finishing touch for a map given as a gift.
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