Please create a function that can read dat files and export as a matrix. Then if you sort the dat file such that you have a matrix with 2 rows. One has the x coordinates going from 0 to 1 and the other has the y coordinates. Then what you might be able to do is loop through the elements of the matrix and basically check the x value at one entry, then we know the x value at the next entry will correspond to a bottom airfoil point (assuming the previous point corresponded to a top point) then you can just search for the x in the array that is closest to the x value in question. If the corresponding y value is larger than the previous y value, the en sort it in the bottom.