I agree with Brian that not all locations along the US-Canada border have laser sensors. What I meant was that they are located all along the border in as many locations as possible, although this too needs to be clarified. Such video and laser systems are usually located in places where the border goes through urban settlements (i.e. east of the Great Lakes) and in potentially high-traffic (illegal) areas west of Lake of the Woods. Clearly it makes no sense for the Border Patrol to place sensors, trip wires, etc. at locations they cannot access within several minutes either by helicopter or ground vehicles. So, in many of the more remote locations, such sytems do not exist. Places like Lake of the Woods and Boundary Waters do not have laser systems, but they do have boat-based border patrols and air flyovers--presumably looking for something a little more suspicious than canoeing enthusiasts.
I also understand that these lasers and cameras are temporary in some places and can be moved from site to site, so that some parts of the border are not monitored that way for years at a time. As for the line segments Brian mentioned, most of the border is comprised of line segments, some short and some long. My understanding is that in places where lasers are in place, they are set up about every 1/2 mile or so to connect, but there are spaces between sections that do not have actual sensors--hence the dense shruberies, etc.
Incidentally, in Lake of the Woods, as in most similar situations I'm sure, fishers have to possess Ontario fishing licenses to be able to fish in Canadian waters. In my interviews with lodge owners in the Northwest Angle, I learned about many people who get in trouble because they don't know where the border lies in the lake, and are often fined for fishing without Minnesota or Ontario licenses respectively. To be safe, most fishers are encouraged to buy both licenses in case they unintentionally drift across the border.
As I have done a lot of research along the US-Canada border, I've tripped the lasers at several locations--east and west of the Great Lakes. It's usually sufficient to let the Border Patrol know that I'm on official government business and not trying to cross illegally or smuggle anything over.
Hope this clears up any misleading I might have done.
Cheers,
Dallen