How to get maximum value of a gridfunction on a clipping plane?

Hi there,

Is there an efficient way to get the maximum value of a gridfunction on a clipping plane. For example, my gridfunction is defined on a cub mesh of 1x1x1. I want to know the maximum value of the gridfunction lying on the cross section of the mesh and clipping plane of z=0.5.

Thanks and regards,
H.A