Years in the making, Sovereign Limits is a database of international boundaries intended for use in the research, visualization, and mapping of the borders that divide up sovereignty on land and sea.

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The International Date Line is wrong

March 29, 2018
Tim Montenyohl

The other day Kevin was working on a project in the Pacific, and complained to me about those geometric, square lines that are commonly used to divide up the Pacific islands. “Where do those lines come from?” His question was more rhetorical than directed towards me, but it did get me thinking.

Suspecting the lines may be sourced from time zones, I peeked at a time zone map and realized: the International Date Line (IDL) is wrong. To be clear, the IDL is made up, with no established standard. It’s still however standardized enough that Natural Earth has a shapefile of it, which seems to match every other version one can find. This existing IDL is accurate in terms of dividing land up into proper time zones, but it completely ignores maritime sovereignty.

From wikipedia, so it must be true

Sovereignty is the driving force behind time zones, and therefore the IDL. If American Samoa decides it wants to switch from observing UTC–11, to observing UTC+13, that would redefine the IDL (it would also basically abolish a whole day for them). But American Samoa isn’t going to specify a different time zone for their maritime space. (I mean as a sovereign nation they could, but for what purpose?) When a nation adheres to a specific time zone, it’s pretty safe to assume that it also applies to its maritime space. So I came up with a thought experiment: What if we redefined the IDL using maritime sovereignty?

This wasn’t as clear-cut as I thought it would be. There’s a little problem with Kiribati. As a nation, it’s clumped into three groups; Gilbert Islands, Phoenix Islands, and Line Islands. The Line Islands are basically a time zone enclave, being well within the Western hemisphere, but observing a very Easternly UTC+14. (No other country observes that time zone.) It makes sense however, that the island nation as a whole would want to be on the same day.

The existing IDL has a somewhat awkward bridge between the Phoenix Islands and the Line Islands. I redrew the IDL using maritime boundaries and Exclusive Economic Zones (EEZs), and maintained that awkward bridge.

Awkward bridge, now with more detail!

I can’t call this experiment over after drawing this sovereignty-based IDL. See those gray hairlines in the above map? Those are Extended Continental Shelf (ECS) submissions. These are created unilaterally, so it’s questionable how much weight should be given to them. The problem with my new IDL is that the Awkward Bridge goes right across the ECS claim of Cook Islands, which is on the other side of the IDL as Kiribati. Alternatively, I can make an IDL that recognizes ECS submissions, but that means redirecting the bridge.

Including ECS submissions, Awkward Bridge is… still awkward

This map gives full weight to countries’ ECS submissions (remember, questionable). Regardless of validity, those submissions should probably be on the same side of the IDL as the submitting nation. So Awkward Bridge now reaches over Jarvis Island as opposed to running below it. But does Awkward Bridge even need to exist? If it didn’t, the Line Islands would truly be an IDL enclave, which I’d like to point out, is also an awkward situation.

“Existing” IDL vs. my 2 new versions

So the big question in redefining the IDL is how to incorporate ECS submissions, if at all. I’m fine with the all-or-nothing approaches presented above. The only artistic license I’ve taken is how I’ve connected Phoenix Islands & Line Islands. Hopefully a more sovereignty-based IDL, however drawn, will eventually replace the existing, rectilinear version.

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