Data Fairing with a Smoothing Spline

Compute the coefficients of a cubic spline that approximates a set of data.

When wind tunnel data is presented in a report, it is usually shown plotted on Cartesian axes with the data points shown as symbols. A curve is usually presented along with these points; this curve does not go precisely through each point, but it is offered as a smooth curve that represents the data. This curve is called the fairing of the data or sometimes the fair curve through the data. The old way to create these curves required the test engineer or data technician to plot the data points, usually on a large piece of paper, and to use pencil, eraser, plastic curves and judgment to draw the fairing of the data. In the modern digital world, we seem to be agreed that we are not going to do it the old way, but it is not entirely obvious what new technique is going to replace the hours and hours of hand labor required for the traditional plots.

One approach is that of a smoothing spline as described in NASA TN D-7397 by Robert Smith, Joseph Price and Lona Howser of NASA Langley. This program was not released by NASA through COSMIC. The source code was obtained by a scan of TN D-7397.