I believe contiguous and continental can be interchanged. Continental should include Ak, but I think it's one of those language things where over time something that's wrong is accepted as correct. In 1977 I visited the marker near Lebanon, KS that marked the center of the Continental US. That didn't include AK. -- you can tell, as it would be considerably more NE (the center of the US including AK & HI is about 10 miles E. of Belle Fourche, SD and unaccessable by road). Center of N.A. (U.S. & Canada) is near Center, ND (Used to be at Rugby, ND and the statue still stands, but modern technology nixed that).
Geographical center of Canada is somewhat S. of Baker Lake, Northwest Territories.
East-West middle near the border is East of Winnipeg on highway TCH1 (there is a sign).
I'll take N.A. Geography for 2000 Alex.