Read contour lines on IGN map

Read contour lines on IGN map

Here are many examples of IGN contour lines. Easy contour reading is ideal for determining elevations during a hike.

Definition of a contour

On a map, a contour is the path that should be followed to go up and down. A contour line joins all the points whose altitude is identical. A contour is thus “flanking” mountain or hill.

IGN map and contour lines

Contours are drawn in brown, gray or blue depending on the terrain:

countryside, forest, city: brunpierres: grisgaciers, eternal snow: blue

Below, different examples on the IGN map help to understand the contours.

Excerpt 1

A first example of an IGN map extract at 1: 25000 shows a terrain of Alsatian hills.

Level curves on IGN map (scale 1: 25000)

The easiest way is to find the altitude written on the contour (drawn in brown). Here we see written “200”, “225” and “250”. It is the altitude in meters. We already know that everywhere on these curves, we are at 200m, 225m or 250m altitude. It is deduced that the intermediate contour lines (finer brown) are spaced 5m apart. Between 225m and 250m, we have contour lines 230m, 235m, 240m and 245m.

We are helped by the altitude “224” which appears at the crossroads, towards the middle of the IGN map extract. This crossing is in fact very close to the 225m contour line.

If we move to the right (towards the east), we gain altitude: we go up.

Point A is placed on the contour line 230m. Its altitude is 230m.

Excerpt 2

This map extract IGN presents a gently sloping terrain in the Vosges, around 1200m above sea level.

IGN map extract 1: 25000

The elevations of the contours are not given explicitly, but we can read the altitudes 1200, 1226 and 1258. This makes it possible to determine the main contour lines 1200m and 1250m. The altitude 1258m corresponds to a summit which is quite flat. Intermediate contour lines are spaced 10m (on the previous extract, it was 5m!). The points of altitude 1226m are indeed between the two contour lines 1220m and 1230m.

This time, point A is not on a contour but between two. These are 1230m and 1240m curves. Its altitude is about 1235m. If it were closer to the 1240m curve, its altitude would be 1237 or 1238m, if it was closer to 1230m, its altitude would be 1232 or 1233m. In fact, it is assumed that the slope is regular between two contour lines.

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