If the points that are generated by the horizontal variables tend to lie along a curve, a poor interpolation or spline can result. In such cases, the vertical variable(s), and one of the horizontal variables should be modeled as a function of the remaining horizontal variable. A scatter plot of the two horizontal variables enable you to determine the appropriate function.

If the horizontal variable points are collinear, the procedure interpolates the function as constant, along lines perpendicular to the line in the plane that is generated by the input data points.