Unlike other spline interpolation functions, the “tpaps” function appears to be the command that can interpolate to values at ungridded data sites in the plane, as mentioned in the following documentation:
A possible workaround to use scattered data with “csaps” function is to first convert the non-gridded data to gridded data using the “griddatan” function. You can refer to the following documentation to know more about this function:
In addition, please refer to the “Tip” tab in the above documentation that states scattered data interpolation with “griddatan” function can be sensitive to scaling issues which can be resolved using normalization.
Once the gridded data is obtained, the “csaps” function can be used to return the cubic smoothing spline interpolation in ppform. Please refer to the following example to know more about how to use the multivariate (2 in this example) gridded data as a cell array:
I hope this helps resolve the issue.
