capitainenemo
4 days ago
Hm. I feel like the french republic can still make the claim though, at least according to this wikipedia map. https://upload.wikimedia.org/wikipedia/commons/6/67/Mapadefr...
seszett
4 days ago
I measured it a while ago (well, 11 years now) and both France and the UK made the cut[0].
France has much more margin though, IIRC no single territory becoming independent would make the sun set on France.
arethuza
3 days ago
France also has the longest internal scheduled flight - 9,349km
https://en.wikipedia.org/wiki/World%27s_longest_domestic_fli...
diggan
3 days ago
Had*, seems it was added to work around American Covid restrictions, and it was only active for ~1 month.
seszett
3 days ago
You're thinking of the (temporary) Tahiti flight, but that was much longer than 9349 km.
The person you're responding to was talking about the Paris-Réunion flight that has been active for a long time and still active today. I took it 15 years ago when I had to go to Réunion.
diggan
3 days ago
Oh, seems indeed I confused the two, thanks for the correction!
madcaptenor
4 days ago
I'm glad someone did this, because for a moment I was thinking that I was going to have to do the math on France.
wolvesechoes
4 days ago
It is always funny to ask people what country has the longest land border with France.
lugao
4 days ago
It's funny because you can't measure borders due to the coastline paradox.
falcor84
3 days ago
I'm not aware of any 2 countries having their border be a natural coastline
retsibsi
3 days ago
Same principle applies to rivers, though, and there are lots of river borders.
falcor84
3 days ago
It isn't the same principle. Firstly, coastlines are more jagged because they are hit by waves perpendicularly, while rivers are shaped by water flowing along the banks, smoothing them. Secondly, the borders are typically defined by either the thalweg (greatest depth) or median line, either being smoother lines than the banks. Thirdly, river borders are then in practice defined by measuring the coordinates of particular sample points along the idealized line and then using straight lines or simple mathematical curves to connect these, forming a simple non-fractal boundary.
ndileas
3 days ago
You could say the exact same set of objections to shoreline paradox.
Jensson
3 days ago
But most borders are not defined as "on the shoreline", they are defined using something reliable.
ndileas
3 days ago
Exactly. The coastline paradox is a mathematical curiousity, not a practical objection to measuring things. Coastlines are not infinite length in practice. You define a system of measurement then a length in that system
falcor84
3 days ago
What? Neither of those three applies to a shoreline.
ndileas
3 days ago
Physical shorelines instantiations of a true fractal are always limited. I'd go so far as to say that there is no such real object in the world.
falcor84
2 days ago
I think I'm in agreement with you, but not sure if I'm agreeing that the are no fractals in the world, or that there are no shorelines.
Anyway, true fractal shorelines definitely never put sugar on their porridge.
wolvesechoes
3 days ago
Doesn't matter in this context.
jcattle
3 days ago
Eh. If you care about relative measurements it doesn't matter much. Pick a sensible resolution, stick with it, you got yourself a ranking.
heavenlyblue
3 days ago
It also makes sense to pick a resolution because coastline changes on an hourly basis (and minute-basis, for rivers during rainfall), so that these differences would not massively affect the measurement every single second
fakedang
4 days ago
For those wondering, it's Brazil.
crazygringo
4 days ago
> The Franco-Spanish border runs for 685.42 kilometres (425.90 mi) between southwestern France and northeastern Spain. [1]
> The Brazil–France border is the line, located in the Amazon Rainforest, that limits the territories of Brazil and France. The border is located between the Brazilian state of Amapá and French Guiana. It is 730 kilometres (450 mi) in length. [2]
I'll be damned!
schoen
4 days ago
Is there a standard for measuring borders for these purposes, in light of the coastline paradox?
I don't mean to suggest that there's no sensible way to do it; I just wonder if people might be using inconsistent methods sometimes, leading to not-very-comparable estimates.
crazygringo
4 days ago
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...
bragr
4 days ago
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.
jojobas
4 days ago
[flagged]
tomhow
4 days ago
Please don't do this here. If a comment seems unfit for HN, please flag it and email us at hn@ycombinator.com so we can have a look.
throwup238
4 days ago
Borders are not like coastlines because they’re abstract delineations, not physical things, even though they’re frequently defined using geographic features.
In this case, the length of the border is dominated by the length of the thalweg of the Oyapock river. Using thalwegs is SOP in international law when using rivers as the natural border and the choice of river is due to treaties that are hundreds of years old.
crazygringo
4 days ago
That works for smooth vector lines, like the border of Colorado, but not for rivers. The thalweg of a river is the same as a coastline -- it has the same fractal nature to it. The more you zoom in, the more it wiggles back and forth.
So yes, the length of the border is dominated by the length of the river, but that's just repeating the question, precisely because the thalweg is a physical thing, not a geometric delineation.
throwup238
4 days ago
You’re thinking of a hydrological thalweg.
In international law (w.r.t. borders) thalwegs are not dependent on coastlines but on navigable channels with a finite precision. The boundary monuments are often kilometers apart which creates a straight line regardless of the shifting coastline (which is a much bigger problem than the coastline paradox, since rivers can change on a dime).
crazygringo
4 days ago
Fascinating, I wasn't aware, thank you.
But when I look at Google Maps, the Oyapock river is extremely meandering. Major 180° bends within just 500 ft, e.g.:
https://www.google.com/maps/@2.3210582,-52.7667375,16z/data=...
Are you sure there's an official survey of every twist and turn, composed of "boundary monuments"? Is there a link to these things or something? It's not really clear to me there's any official "navigable channel" at all.
Is there anything you can link to that shows the actual legal boundary if it's made of vector segments? Or do we know if that's what Google Maps uses directly, or if that's what's being used for the length calculation?
heavenlyblue
3 days ago
However, it's quite improbable that bends like that would repeat at a 1-metre scale, at least because the river itself has a minimum width (or maximum depth it can go, if we assume some incredibly narrow gorge)
bragr
4 days ago
Maybe some borders are that way but not all. The thalweg is the US-Mexico border along the Rio Grande and the International Boundary and Water Commission semi-regularly swaps territory to deal with the changing border.
imtringued
3 days ago
No, there isn't. You can be sure that the methods are inconsistent unless they come from the same author.
Rexxar
4 days ago
The other funny thing is the land border with Netherlands ... in the Caribbean.
mihaic
3 days ago
It's pretty nice that EU agreements of free travel apply to it as well.