I am using Thin plate Spline to interpolation rainfall data. The problem is that I get negative values for rainfall which is impossible!! Is that normal?
What parameters are you using?
Controlling the passage of the interp through the data points, does not guarantee values won't fall outside the range of input data in the intervening space.
In light of your parameters, can you consult How Spline works—Help | ArcGIS for Desktop and report back as to whether the data are suitable for this type of interpolator and what the pattern of the data are and where the -ve values fall with respect to it
There are other methods that can give you what you want, for example by using a LogEmpirical transformation in the EBK tool.
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