[allemagne]

>The Mercator projection was indeed originally designed for compass navigation, but the reason it's still used for web mapping is slightly different. The Mercator projection is conformal.

Just speculating here, but it's also a big rectangle that is easy on the eyes. Non-rectangular online maps would fit awkwardly in websites. As for Gall-Peters, that map is just irredeemably ugly to me, and I'm ignorant of other rectangular projections that could compete with Mercator.

[mewo2]

Yes, absolutely! The Mercator projection isn't the only conformal projection, but it is the only one for which north-south maps to the vertical axis, and east-west to the horizontal axis. That means that it maps the whole world onto a rectangle, which works well both for computers and interior decorating.

As for other rectangular projections, the most common is the equirectangular (or plate carrée), which simply maps (longitude, latitude) to (x, y). This is a pretty common representation for gridded global data.

[Retric]

Gall-Peters is also a conformation projection that maps north-south maps to the vertical axis, and east-west to the horizontal axis. As to online maps one major advantage is the X-Y scale does not get nearly as messed up at multiple zoom levels.

However, online maps alter the projection as you zoom so this is not much of an issue.

[mewo2]

Gall-Peters is definitely not conformal. It preserves area, but greatly distorts angles, apart from at the two standard parallels of 45 degrees north and south. This results in the shapes of land masses getting very squished towards the poles (and to a lesser extent, the equator). For an example, look at the shape of Greenland, which appears twice as wide (east-west) as it is tall (north-south), when in reality it's the other way around.

I'm also not aware of any of the main online maps changing projection as you zoom. It seems like that would be more pain than it's worth.

[Retric]

A Mercator projection (ed: is a Conformal map, but) it also greatly distorts both angles and shapes as you approach the poles. The trivial example is to walk a square mile (N,W,S,E) you would expect the opposite sides to be at 45deg angle, but on Mercator the sides get stretched so a square in the real world can have a 10+:1 or even 100:1 side lengths on a Mercator map.

The property they both preserve is points north, south, east, or west of them on a globe are also north, south, east, or west of them in the projection. (Plenty of rectangular projections don't have this property ex: http://geographer-at-large.blogspot.com/2011/08/fun-with-map...)

The issue is interior angels of a triangle on a sphere don't add up to 180 degrees so you literally can't preserve all angles on a projection.

PS: If you actually walk exactly 1 mile north, 1 mile west, 1 mile south, then 1 mile east you don't necessarily end up on exactly the same place on a globe. (It does work if you start half a mile below the equator.)

Edit: Not that the earth is even a sphere...

[vilhelm_s]

It preserves angles locally. https://en.wikipedia.org/wiki/Conformal_map

[Retric]

Yea, nm I was wrong. Shapes do get distorted near the poles, but at fixed latitude the distortion decreases as their size decreases.