I am trying to find an average plane of a surface.

my data include X, Y, Z.

I run analyze->fit model->Y variable=Z, Model effects=X,Y.

then i choose factor profiling-> surface profiler to see the plot+ average plane.

this is my plot when i run scatterplot 3D:

and this is my plot when i run fit model:

there is some transformation that change my data from dome to cylinder.

any idea what to do?