FOLD AMPLITUDE

Amplitude of a fold is half of the least distance between adjacent hinge
points or half of the least distance between the top and bottom enveloping
surfaces. In other words, it is the least distance between a median
plane\surface and one of the two enveloping surfaces of a fold train. The
enveloping surface can be treated as a form surface since it gives approximate
orientation of the layers before folding. *see
enveloping surface and median surface and geometry of folded surfaces*