midline of a surface

Hello,

I have a 3D surface with (x,y,z) coordinates on a number of nodes over the surface and a value (A) assigned to each node. I want to find the closest midline-node coordinates along the surface length and A at those nodes. The attached file contains all nodes. Any suggestion is appreciated.

x y z A

-5.878 2.593 -3.271 1.09E-13

-5.877 2.594 -3.273 1.51E-13

-5.880 2.593 -3.271 6.56E-14

-5.882 2.592 -3.269 0.00E+00