precision.c

precision.c How to control precision when coding a grib field.

/*
 * C Implementation: precision
 *
 * Description: how to control decimal precision when packing fields.
 *
 *
 * Author: Enrico Fucile <enrico.fucile@ecmwf.int>
 *
 *
 */
#include <stdio.h>
#include <stdlib.h>
#include <math.h>

#include "grib_api.h"

int main(int argc, char** argv) {
  int err = 0;
  size_t size=0;

  FILE* in = NULL;
  char* infile = "../../data/regular_latlon_surface.grib1";
  FILE* out = NULL;
  char* outfile = "out.grib1";
  grib_handle *h = NULL;
  const void* buffer = NULL;
  double* values1=NULL;
  double* values2=NULL;
  double maxa=0,a=0;
  double maxv=0,minv=0;
  double maxr=0,r=0;
  long decimalPrecision;
  long bitsPerValue1=0, bitsPerValue2=0;
  int i=0;

  in = fopen(infile,"r");
  if(!in) {
    printf("ERROR: unable to open file %s\n",infile);
    return 1;
  }

  out = fopen(outfile,"w");
  if(!in) {
    printf("ERROR: unable to open file %s\n",outfile);
    return 1;
  }

  /* create a new handle from a message in a file */
  h = grib_handle_new_from_file(0,in,&err);
  if (h == NULL) {
    printf("Error: unable to create handle from file %s\n",infile);
  }

  /* bitsPerValue before changing the packing parameters */
  GRIB_CHECK(grib_get_long(h,"bitsPerValue",&bitsPerValue1),0);

  /* get the size of the values array*/
  GRIB_CHECK(grib_get_size(h,"values",&size),0);

  values1 = malloc(size*sizeof(double));
  /* get data values before changing the packing parameters*/
  GRIB_CHECK(grib_get_double_array(h,"values",values1,&size),0);

  /* changing decimal precition to 2 means that 2 decimal digits
     are preserved when packing.  */
  decimalPrecision=2;
  GRIB_CHECK(grib_set_long(h,"changeDecimalPrecision",decimalPrecision),0);
   
  /* bitsPerValue after changing the packing parameters */
  GRIB_CHECK(grib_get_long(h,"bitsPerValue",&bitsPerValue2),0);

  values2 = malloc(size*sizeof(double));
  /* get data values after changing the packing parameters*/
  GRIB_CHECK(grib_get_double_array(h,"values",values2,&size),0);

  /* computing error */
  maxa=0;
  maxr=0;
  maxv=values2[0];
  minv=maxv;
  for (i=0;i<size;i++) {
     a=fabs(values2[i]-values1[i]);
     if ( values2[i] > maxv ) maxv=values2[i];
     if ( values2[i] < maxv ) minv=values2[i];
     if ( values2[i] !=0 ) r=fabs((values2[i]-values1[i])/values2[i]);
     if ( a > maxa ) maxa=a;
     if ( r > maxr ) maxr=r;
  }
  printf("max absolute error = %g\n",maxa);
  printf("max relative error = %g\n",maxr);
  printf("min value = %g\n",minv);
  printf("max value = %g\n",maxv);

  printf("old number of bits per value=%ld\n",(long)bitsPerValue1);
  printf("new number of bits per value=%ld\n",(long)bitsPerValue2);

  /* get the coded message in a buffer */
  GRIB_CHECK(grib_get_message(h,&buffer,&size),0);

  /* write the buffer in a file*/
  if(fwrite(buffer,1,size,out) != size) 
  {
     perror(argv[1]);
     exit(1);
  }

  /* delete handle */
  grib_handle_delete(h);

  fclose(in);
  fclose(out);

  return 0;
}

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