Reading an IGN map may seem complex at first, but with a little practice you will quickly feel at ease. Understanding landmarks and knowing how to use them to navigate is essential, especially when exploring unfamiliar areas. Here are some key tips for learning to read an IGN map, locating important information and navigating the terrain with confidence.
The IGN map scale
Each IGN map comes with a scale representing the ratio between distances on the map and those in the real world. For example, a scale of 1:25,000 means that one centimetre on the map corresponds to 250 metres in reality. IGN maps used for hiking, such as those in the Top 25 series, are at 1:25,000. This means that 1 cm on the map represents 250 metres on the ground, allowing you to measure distances precisely.
Mastering the scale is fundamental for accurately assessing distances on your hikes. It will help you plan your routes more effectively and estimate the time needed to cover a given area.
Orienting an IGN map
IGN maps are always oriented north. This means that geographic north will be at the top of the map when you hold it the right way up. However, it is important to note that the north shown on the map is geographic north, which may differ slightly from magnetic north (indicated by a compass). This difference, known as magnetic declination, is generally small, but it matters when travelling long distances or in areas near the poles. This information is shown on a diagram near the map legend.
Identifying roads, tracks, paths and marked routes
IGN maps are packed with useful information to help you navigate, including different types of roads, paths and other routes. Here is how to identify them:
- Main roads: These are shown by solid coloured lines. Their thickness varies according to the size of the road: a motorway will be the widest, while a small local road will be narrower.
- Drivable tracks: These tracks, accessible by vehicle, are shown by two dashed lines.
- Paths: Paths are generally shown as black dashed lines. You may also find marked routes such as long-distance hiking trails (GR) or local footpaths, marked by pink lines. The GR will be thicker than other marked trails.
These symbols help you navigate the map easily, identifying the paths best suited to your activity — whether hiking, trail running, or ski touring.
How to read contour lines on an IGN map
Contour lines indicate changes in altitude on the map. They are shown in light brown. Each line connects points at the same altitude. The interval between two contour lines is generally 10 metres in mountain terrain and 20 metres in flat terrain.
By observing the spacing of the contour lines, you can easily determine the gradient of the terrain. Widely spaced contour lines indicate relatively flat ground. Conversely, closely spaced contour lines indicate a steeper slope. This information is crucial for assessing the difficulty of the terrain and planning your ascents safely.
Other legend elements for reading an IGN map
The legend of an IGN map provides essential information to better understand the different terrain features:
- Water: Rivers and lakes are shown in blue. High-mountain glaciers are also represented in blue with contour lines.
- Forests: Forests are shown in green, with specific shades to distinguish vegetation types, such as conifers (represented by a triangle symbol).
- Shelters and mountain huts: Mountain huts are symbolised by a small pink house. They are also marked to indicate whether they are staffed or unstaffed, depending on whether a door is shown.
Using the GPS grid on an IGN map
Recent maps are often GPS-compatible. They include a grid, typically 1 km per side, with coordinates in the margin. This grid lets you pinpoint your location on the map precisely, even without a GPS device to hand.
To use the GPS grid, you first need to display your GPS coordinates. For example, a coordinate such as "30 T0695 420" tells you that you are 420 metres to the right of marker 695 on the map. You can then estimate your position by drawing a vertical and horizontal line on the map to find your exact location.
Using a compass to navigate with the map
A compass is a valuable tool for navigating with a map. To use it, simply orient the map to north, then sight a distinct landmark (such as a summit or antenna) using the compass. Once done, you can draw a line on the map to pinpoint your exact position.
For greater precision, you can use compass triangulation, which involves taking several visible landmarks around you to pinpoint your exact position on the map.
How to perform compass triangulation
First and foremost, take your time. Good compass triangulation requires a bit of attention and time. It is not recommended in emergency situations where time is short.
First, orient your map. Once oriented, do not move it.
Then take the time to study the landscape and choose clearly identifiable landmarks. You need at least two, but a third will give greater accuracy. Do not choose landmarks in the same direction, as this will make the reading imprecise and unreliable. Choose landmarks that are well spread out from one another.
You can choose a church steeple, a peak or summit, a relay antenna, or a water tower. You must also be able to identify these features on the map without making mistakes — for example, not confusing one peak with another.
Take a sighting towards the first landmark: without moving the map, orient the compass towards the point and use the straight edge of the compass as a guide to draw the line extending from that landmark. The map and the compass needle must of course remain oriented to north.
Then repeat the same process by sighting the second landmark, and a third if needed. Next, extend the lines sketched on your map. The point or area where these lines meet is where you are!
Don't hesitate to practise in a familiar area. Don't wait until you're in a critical situation to try this exercise. You can even do it from your flat or garden, or on a hiking route you know well.
