The type of function to be used is set using the parameter TRCFIT. (Default value is POLY for polynomial.)
When TRCFIT=POLY then the number of coefficients should be set to degree of polynomial + 1.
When TRCFIT=SPLINE the number of coefficients should be set to 2x(number of knots + 7).
For example, setting TRC_NPOLY to 4 would permit the fitting of polynomials of the form:
constant + ax + bx squared + cx cubed Initial suggested value: 4.