8 CFR 287.1 - Definitions:
(1) External boundary. The term external boundary, as used in section 287(a)(3) of the Act, means the land boundaries and the territorial sea of the United States extending 12 nautical miles from the baselines of the United States determined in accordance with international law.
(2) Reasonable distance. The term reasonable distance, as used in section 287(a) (3) of the Act, means within 100 air miles from any external boundary of the United States or any shorter distance which may be fixed by the chief patrol agent for CBP, or the special agent in charge for ICE, or, so far as the power to board and search aircraft is concerned any distance fixed pursuant to paragraph (b) of this section.
8 USC § 1357 - Powers of immigration officers and employees:
(a) Powers without warrant
Any officer or employee of the Service authorized under regulations prescribed by the Attorney General shall have power without warrant—
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(3) within a reasonable distance from any external boundary of the United States, to board and search for aliens any vessel within the territorial waters of the United States and any railway car, aircraft, conveyance, or vehicle, and within a distance of twenty-five miles from any such external boundary to have access to private lands, but not dwellings, for the purpose of patrolling the border to prevent the illegal entry of aliens into the United States;
Would the definition of "external boundary" be read as not applying to airports? I googled a lot, but mostly came up with people asking about airports and no one saying that they were 100% either way.
One criticism of the map that I saw was that it should not be following the land lines in the Michigan area. I found some other "100 mile boundary" maps that where created when Congress tried to pass some laws that would allow DHS to ignore environmental laws within 100 miles of the border (HR1505 and HR2578). This map goes across Lake Michigan, as do this map and this map (although the last two do not include the east/west coasts for some reason).
So the short answer is that I have lots of maps, but no definite answer :/