It's an excellent question. The Wikipedia citations don't actually lead to much, and there's no indication they use the same methodology.

Best I can find is the CIA World Factbook [1] which lists France's border with Spain at 646 km (under "France" and "Spain", same value), and Brazil's border with French Guiana at 649 km (under "Brazil").

So, already a radical difference -- from a 45km difference to a 3km difference (just 0.5%). But there's more:

> When available, official lengths published by national statistical agencies are used. Because surveying methods may differ, country border lengths reported by contiguous countries may differ.

But there's no indication whether these particular measurements are made by the CIA using the same technique with maps of the same resolution... or, being so close to begin with, whether different resolutions would change the asnwer... or if these are official lengths derived using totally different and ultimately incomparable procedures.

So maybe it's not so cut-and-dried that France's longest border is with Brazil...?

[1] https://www.cia.gov/the-world-factbook/about/archives/2022/f...

When the measurement was taken is important too because any border based on natural features is in constant flux. A big storm could cause the Oyapock River to straighten or create new bends or both.

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