Episode 3: when form takes precedence over area.

Distorting the world methodically.

From the family of projections, I'd like… conformal projections! That's the slightly more scientific name for our Angles team, which we discussed in this article.

The goal of this type of projection is to preserve the value of angles (i.e., the shape of countries) rather than the value of areas (the size of countries). The most famous projection in this group is Mercator's conformal cylindrical projection, named after its creator.

Mercator conformal projection, from the NASA Earth Observatory's "Blue Marble" series.

The challenge of right angles.

On the globe, meridians and parallels are drawn so that they always intersect at right angles. This is what this projection aims to preserve. One can well imagine the problems this can cause in terms of surface distortion. The Mercator projection is said to be cylindrical.

Mercator's conformal cylindrical projection.

As seen in the drawing above, the cylinder is in contact with the globe only at the equator. In this example, the equator is, therefore, the center of projection.

The representation of land is therefore accurate only at the equator. The further away one moves from it towards the poles, the more the areas will be distorted and amplified.

This is particularly evident in the depiction of the South Pole. The size of Greenland also poses a problem. On this projection, one might believe that its area is almost equivalent to that of the African continent, whereas in reality, the latter is 14 times larger... The larger the portion of land represented on the map, the more pronounced the distortions will be.

Lambert, the hikers' best friend.

For maps representing small portions of terrain, such as IGN's Top 25 maps for hiking, a conformal projection will also be used.

Thus, during your hike, when you look in the distance at the point where you want to go, it must be consistent with what you see on the map, that you indeed have in front of you what the map shows. Conformal projections are crucial for directional accuracy.

Excerpt from an IGN Top 25 collection map.

For this type of map, Lambert's conformal conic projection will be used (again, it's the cartographer's name. The egos of these guys, I'm telling you!). "Why conic?" you might ask. For this reason:

Lambert's conformal conic projection

In this example, there are two contact zones between the cone and the globe. This makes it a particularly well-suited projection for mid-latitude regions like ours.

But here too, the map will be accurate only at the projection centers. Even an IGN Top 25 map presents distortions due to its projection.

Although distortions are inevitable, Lambert's projection is designed to minimize distortions across the entire Hexagon. Surface and distance distortions are reduced, especially around the parallels in contact with the cone. The existing distortions on IGN Top 25 maps are therefore minimal and do not compromise the necessary accuracy for outdoor activities and navigation.

As this projection is particularly well-suited for mapping France, it is the one I use both for mapping French cities and the Brittany coast.

Did that whet your appetite? In this article, we cook up the surfaces team!