The map showing Mt. Cardigan State Forest in Alexandria, NH incorrectly labels the 0.6 mile trail segment between the Elwell Trail and Back 80 Trail junction to the Mowglis Trail junction as part of the Back 80.  It's actually part of the Elwell Trail.  The USGS Mt. Cardigan topo does not label the segment, but the AMC Southern NH Trails Guide map does.

This problem has also been reported to GAIA GPS.

FWIW, I'm the adopter of the Back 80 and am friends with the Elwell adopter.