##csa (Cubic Spline Approximation)
csa is a C code for cubic spline approximation of 2D scattered data. It provides a C library and a command line utility csabathy.
csa uses a fast O(N) and robust algorithm that works nicely for uniformly distributed data. It is described in this paper. For clustered data you may prefer Natural Neighbours interpolation.
Checkout csa by running git clone https://github.com/sakov/csa-c
or svn checkout https://github.com/sakov/csa-c
.