This function scales both x and y, as well as the surface value (z),
to between 0 and 1 to best match their units. This is done because
most satellite data have units where the x, y units do not equal the
z units. The surface area represents the surface area of the sample
area (N-1, M-1).
Note that the surface object may have NA values around the edges,
but should not have any missing values within the main area.