Years ago somebody on VFTT posted a link to the documents that defined the Pemi wilderness (and possibly others, but that was the one I was interested in at the time). On a map, the boundaries follow the obvious ridgelines (e.g., Franconia ridge), but in fact they're defined as being X feet away from the ridge, where I forget what X was but it's just enough to be reasonably certain that the (preexisting) trail never crosses the boundary even though the trail may wriggle to either side of the ridgeline. There are also a couple of carve-outs around pre-existing structures, Guyot Shelter being one that shows plainly on the map.
A glance at the USFS maps shows that the same situation pertains to the Sandwich wilderness. Flat Pond Shelter is inside another carve-out. The US Forest Service maps show the trails, e.g. most of Flat Mountain Pond Trail, adjacent to the boundary, and draws them in their correct relation, i.e, the Flat Mountain Pond trail is outside the boundary until past the Flat Pond Shelter carve-out. (The boundary crossing happens when the trail has wandered west from the shelter and has turned north again to approach the west shore of the pond.)
