[dane-pgp]

Which doesn't work well for land-locked countries.

I'm sure that some sensible (albeit still arbitrary) geometric rule could be invented, such as requiring that the line bisects the country into two regions which have areas that differ by at most 50%, for example.

Alternatively, or additionally, the length of the line could be required to be greater than some specific fraction of the country's total border/coastline (assuming that has a rigorous definition).

For countries composed of multiple islands (or a mainland with some islands), the rules should probably require that the line pass through the biggest island / mainland, but perhaps multiple islands would need to be crossed (on the same straight line?) in order to meet the "bisecting area" rule above.