2D interpolation with GSL in C

2dinterp_gsl.c

#include <stdio.h>
#include <stdlib.h>

#include <gsl/gsl_math.h>
#include <gsl/gsl_interp2d.h>
#include <gsl/gsl_spline2d.h>

int main(int argc, char const *argv[])
{
  const gsl_interp2d_type *T = gsl_interp2d_bicubic;
  double xa[] = {-2.0, -1.0, 0.0, 1.0, 2.0};
  double ya[] = {-2.0, -1.0, 0.0, 1.0, 2.0};
  double za[] = {0.0, 3.0, 4.0, 3.0, 0.0, -3.0, 0.0, 1.0, 0.0, -3.0, -4.0, -1.0, 0.0, -1.0, -4.0, -3.0, 0.0, 1.0, 0.0, -3.0, 0.0, 3.0, 4.0, 3.0, 0.0};
  // double xval[] = {1.0, 1.5, 2.0};
  // double yval[] = {1.0, 1.5, 2.0};
  // double zval[] = {1.2, 1.3, 1.4};
  size_t nx = sizeof(xa) / sizeof(xa[0]);
  size_t ny = sizeof(ya) / sizeof(ya[0]);
  // size_t test_size = sizeof(xval) / sizeof(xval[0]);

  gsl_spline2d *spline = gsl_spline2d_alloc(T, nx, ny);
  gsl_interp_accel *xacc = gsl_interp_accel_alloc();
  gsl_interp_accel *yacc = gsl_interp_accel_alloc();

  gsl_spline2d_init(spline, xa, ya, za, nx, ny);

  printf("-1.5 and 1.0 : %f\n", gsl_spline2d_eval(spline,-1.5,1.0,xacc, yacc));
  printf("1.0 and -1.5 : %f\n", gsl_spline2d_eval(spline,1.0,-1.5,xacc, yacc));
  
  return 0;
}

// OUTPUT
// -1.5 and 1.0 : -1.339286
// 1.0 and -1.5 : 1.339286

Dear Sir/Madam!
The example is cool. I am dealing with 2D Interpolation in GSL. Please, do you have another examples like above? I really need to learn them for my research. Now I am frozen at the gsl_spline2d_set function. otabek.umarov@science.unideb.hu. This is my email. (I am a PhD student at DU.) I will be very very happy if you can help or contact with me.