Reading an IGN map may seem complex at first, but with a little training and practice, you'll quickly become confident. Understanding landmarks and knowing how to use them to navigate is essential, especially when exploring lesser-known areas. Here are some key tips for learning to read an IGN map, locating important information and finding your way on the ground with ease.
The IGN map scale
Each IGN map comes with a scale that shows the ratio between distances on the map and those in reality. 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 to accurately assessing distances on your hikes. It will help you better plan your routes and estimate the time needed to cover a given area.
Orienting an IGN map
IGN maps are always oriented to the north. This means that geographic north will be at the top of the map when you hold it the right way up. However, it's important to note that the north shown on the map is geographic north, which may differ slightly from magnetic north (as indicated by a compass). This difference, known as magnetic declination, is generally small, but it matters when travelling over long distances or in areas close to the poles. This information is shown on a diagram near the map legend.
Identifying roads, tracks, trails and marked routes
IGN maps are packed with useful information to help you find your way, including the different types of roads, trails and other routes. Here's how to recognise them:
- Main roads: These are shown as 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 thinner.
- Tracks: These tracks, accessible by vehicle, are shown by two dashed lines.
- Trails: Trails are generally shown as dashed black lines. You may also find marked routes such as long-distance hiking paths (GR) or local footpaths, indicated by pink lines. GR routes 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 or even ski touring.
How to read contour lines on an IGN map
Contour lines show altitude variations on the map. They are represented in light brown. Each line connects points at the same altitude. The interval between two contour lines is generally 10 metres in the mountains and 20 metres on flat ground.
By observing the spacing of the contour lines, you can easily determine the gradient of the terrain. If the lines are widely spaced, the terrain is relatively flat. Conversely, closely spaced lines indicate a steeper slope. This information is crucial for assessing the difficulty of the terrain and for planning your ascents safely.
Other legend elements for reading an IGN map
The legend of an IGN map provides essential information to help you better understand the different features of the terrain:
- Water: Rivers and lakes are shown in blue. Glaciers at high altitude are also represented in blue with contour lines.
- Forests: Forests are shown in green, with specific shades to distinguish vegetation types, such as conifers (triangle symbol).
- Shelters and mountain huts: Mountain huts are shown by a small pink house symbol. 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 compatible with GPS devices. They include a grid, typically 1 km across, with coordinates in the margin. This grid allows you to locate yourself precisely on the map, 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 located 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 on a map
A compass is a valuable tool for navigating with a map. To use it, simply orient the map to the 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.
If you want greater precision, you can use compass triangulation, which involves taking multiple visible landmarks around you to pinpoint your exact position on the map.
How to carry out a compass triangulation?
First and foremost, take your time. A good compass triangulation requires a little attention and time. It is not suitable for emergency situations where time is of the essence.
First, take your time. A good compass triangulation requires a little attention and time. It is not recommended in emergency situations where time is short.
Next, take time to observe the landscape and choose clearly identifiable landmarks. You need at least two, but a third will give greater accuracy. Don't choose landmarks in the same direction, as the measurement will be imprecise and highly uncertain. Choose landmarks that are well spread apart.
You can choose a church steeple, a peak or summit, a relay antenna, or a water tower. You also need to be able to identify these points on the map and not confuse them. Don't mistake one peak for another, for example.
Then take a bearing towards the first landmark: that is, without moving the map but orienting the compass towards the point, use the straight edge of the compass as a guide and draw the line extending from the landmark. The map and the magnetic needle of the compass both remain oriented to North, of course.
Then repeat the same process for the second landmark and, if necessary, the third. 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 out. You can even do it from your flat or your garden at home, or on a hiking route you know well.
