According to the VNC, the MEF represents the highest feature in each quadrangle. It refers to
- AIM RAC 5.4 and CAR 602.15(2b) which basically says "lower than 500 AGL (CAR 602.14(2b)) ok, when for aerial application or aerial inspection, photography, ..."
- AIM AIR 1.5 which is about "Pressure Altitude" which I guess is because the heights are ASL
Now, the sample quadrangle here shows a MEF of 2700'. Looking at that quadrangle, the highest obstacle I would say is the mountain in the very east, north of 50°35' which is 2290' high. I wonder where that difference comes from.
I think that both figures, 2700' MEF and 2290' are ASL. Therefore it can't be the difference between ASL and AGL. It always seems to be ~400' more than the highest feature.