PHAST-Style Interpolation

PHAST has a built in interpolation method using the grid coordinate system that can be applied to some CHEMISTRY_IC, FLUX_BC, HEAD_IC, LEAKY_BC, MEDIA, and SPECIFIED_HEAD_BC properties (Parkhurst and others, 2004).  In PHAST-style interpolation, the user specifies a coordinate direction (X, Y, or Z), two distances, and two values.  (The coordinate direction is relative to grid so if the grid is at an angle these are actually the X', Y' and Z directions.) If the X', Y', or Z coordinate of the current node is less than or equal to the first distance, the first value is used. If the X', Y', or Z coordinate of the current node is greater than or equal to the second distance, the second value is used. For intermediate distances, linear interpolation between the two values is used.

A related capability in PHAST is the ability to specify mixtures between two chemical compositions.  In PHAST-style mixtures, the user specifies two values that represent chemical compositions and a series of proportions (between 0 and 1). The proportion represents the fraction of value 1 in the mixture.

The following controls are used to specify the values needed for PHAST-style interpolation.

PHAST-style interpolation in GoPhast can only be applied to data sets in PHAST that allow PHAST-style interpolation.  This includes all the data sets directly used in the PHAST transport data file except “Active”, “Print_Chemistry”, and “Print_XYZ_Chemistry”.  PHAST-style interpolation can not be used with data sets created by the user.  However, the user could specify a formula that would have the same effect as PHAST-style interpolation.  Either of the following will emulate PHAST-style interpolation when applied to a real-number data set.

In these formulas, the interpolation direction is X', the two distances are 1000 and 10,000, and the two values are 25 and 50.

Data sets that store integers and whose values are set by PHAST-style interpolation or mixtures should not be used in formulas for other data sets because the value used in the formula will be rounded to the nearest integer rather than representing a mixture between the two end member compositions.  The formulas used in GoPhast are strictly mathematical and are unable to interpret those numbers as the chemical compositions that they are.