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# Trilinear interpolation in mathcad prime?

## Trilinear interpolation in mathcad prime?

(OP)
Hi all!

I'm attempting perform linear interpolation with 3 independent variables, like so:

The goal is to extract the rows corresponding to d0..d3 based on the inputs a, b and c.

Here is a simple test data set that I have put together:
Excel sheet here

I have done interpolation with 2 independent variables using the Bicubic2D function, but this get's me lost. My best attempt now is to do a simple linear interpolation ('a' to find 'b'), then use the Bicubic2D function to find the dependent variable.

Any tips or hints or resources to help me along the way?

Cheers

### RE: Trilinear interpolation in mathcad prime?

So d0 to d3 are the coefficients of the polynomial for each e. Can I guess that this is a factorial experiment with 2 factors at 2 levels and one at 5 levels.

I do not see how you can generate your values for each e without some underlying data or equation. That is, what are you fitting?

So I think you need to explain what you are trying to do. AT the moment all I've got is wall of numbers with no rationale. Using a cubic function when you say you want linear interpolation sounds a bit sus.

Cheers

Greg Locock

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### RE: Trilinear interpolation in mathcad prime?

That kind of interpolation of a tabled "function" in three independent variables was posted a few times in the native Mathcad forum at PTC https://community.ptc.com/t5/Mathcad/bd-p/PTCMathc...
It can be done using linear interpolation as well as cubic spline interpolation and the tabled data has to be provided in a special way.
You may search that forum but the search function often is of not much help and so you may consider posting your question there in a new thread. I am sure someone will be able to come up with the solution functions you need.

### RE: Trilinear interpolation in mathcad prime?

I might be wrong, but is this uniquely defined? Your second set of data and the seventh set have same a, b and c values, with different e values - is that correct?

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