Originally Posted by Sil_liS
Where do they start counting the 100 miles from, a line in the sand?
Where land and sea (or Great Lake) meet, or from the land border. They take all the points along that line and draw arcs across US territory, and everything that falls inside the joined lines of the arcs is "within 100 miles". That's why there's a bump above Mobile, AL, for example, since they draw the arc up 100 miles from the northern end of Mobile Bay.
Basically, if you start from anywhere on US soil, and can get to any point on a land border or to the edge of a large body of water (Atlantic, Pacific, Gulf of Mexico, Great Lakes) by moving 100 miles in any direction, then you are within the area of operation.
They use the land border instead of the actual international border with the Great Lakes because of the operational difficulties of covering the open water of the lakes. They want 100 miles they can reach by vehicle, not boat. That's not to say they do not operate on those waters, they do - especially, for example, along the St Lawrence Seaway / River where the border runs along it, since there is a lot of illegal activity they are concerned about there, and in that terrain there is a very short time to find someone before they disappear.
They used to use drones on the Mexican border that were borrowed from the military, but DHS kept crashing them, so the military was becoming reluctant to keep letting them use them. I don't know how many are still in use today.
The justification for the whole thing is that they can be more effective with an "in-depth" defense using the resources available rather than lining everyone up right on the line. And there are secondary issues like small aircraft that land at airfields not far inland, but they would get complaints if they just shot them down right at the beach. And that there are about 20 million people who are already here illegally, and they might catch a few that they wouldn't otherwise. The last, though, is becoming a big no-no, for political reasons neither party can ignore.