Accepted answer

Most of countries have their own geographical reference system, often several. In France, for instance, we use or used 3 of them: NTF, ED50 and WGS84.

However, there is no official projection and every cartographer produces its own map with its favourite projection. Of course, some projecions may more suitabe for some countries. For instance, you won't use Mercator's projection to map the south pole... but otherwise you're rather free to use whatever please you or what conveys your message.

Anyway, at small scale (i.e. country-wise), most of the projections will agree.

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