Multiple Column Curves by Modifying Factors

There are two principal disadvantages in the use of multiple column curves as they are currently formulated. First, they are represented either by equations which are cumbersome to use or by tables with a new table for each column curve. Second, the change from one column curve to another is a large step change so that it may be difficult to decide which curve should represent some sections and it is possible for anomalies to arise. These two disadvantages are removed in the proposed method, in which the three SSRC curves are represented by a single curve and a single linear modifying function. The modifying function has been chosen to allow smooth transitions between the existing SSRC curves and to make step changes between the curves unnecessary.