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Hi Dennis, the autoconversion from short-_FillValue to float-0.0 is done in the netcdf-c library when reading with shorts with nc_get_vara_float. It does not happen with ncdump, since ncdump doesn't use type-conversion https://www.unidata.ucar.edu/software/netcdf/netcdf-4/newdocs/netcdf/Type-Conversion.html#Type-Conversion I attach an example file and C-program: gcc simple_xy_rd.c -lnetcdf -o simple_xy_rd ./simple_xy_rd out.nc | less Output will be many -0.0 where I would expect -32767., which I get when I change float to short in the defines at the beginning of the file: #define TYPE float #define NC_GETVARA nc_get_vara_float Heiko On 2017-01-16 20:11, dmh@xxxxxxxx wrote: > Ok, the first thing to check is if the problem is in the netcdf-c > library or in ncview. Try dumping the variable in question using > ncdump and the -v option. > =Dennis Heimbigner > Unidata > > p.s. If the netcdf file is not too large, then you can send it to me > at dmh at ucar dot edu. > > On 1/16/2017 7:12 AM, Heiko Klein wrote: >> Hi, >> >> I just was surprised by strange results from ncview when reading a >> variable with 'short' values, where all data is undefined. ncview reads >> all data with nc_geta_vara_float. >> >> >> _FillValues are defined as -32767s. Reading undefined data with >> nc_get_vara_float results in values = -0 or 0 for all _FillValues. I had >> expected a value like -32767.f (or more generally, the _FillValue >> translated into float) >> >> Is the 0-result really the expected behaviour of nc_get_vara_float? I >> have then no idea how to handling _FillValues correctly. >> >> Best regards, >> >> Heiko >> >> >> >> >> >> _______________________________________________ >> NOTE: All exchanges posted to Unidata maintained email lists are >> recorded in the Unidata inquiry tracking system and made publicly >> available through the web. Users who post to any of the lists we >> maintain are reminded to remove any personal information that they >> do not want to be made public. >> >> >> netcdfgroup mailing list >> netcdfgroup@xxxxxxxxxxxxxxxx >> For list information or to unsubscribe, visit: >> http://www.unidata.ucar.edu/mailing_lists/ >> > > _______________________________________________ > NOTE: All exchanges posted to Unidata maintained email lists are > recorded in the Unidata inquiry tracking system and made publicly > available through the web. Users who post to any of the lists we > maintain are reminded to remove any personal information that they > do not want to be made public. > > > netcdfgroup mailing list > netcdfgroup@xxxxxxxxxxxxxxxx > For list information or to unsubscribe, visit: > http://www.unidata.ucar.edu/mailing_lists/ -- Dr. Heiko Klein Norwegian Meteorological Institute Tel. + 47 22 96 32 58 P.O. Box 43 Blindern http://www.met.no 0313 Oslo NORWAY
/* This is part of the netCDF package. Copyright 2006 University Corporation for Atmospheric Research/Unidata. See COPYRIGHT file for conditions of use. This is a simple example which reads a small dummy array, which was written by simple_xy_wr.c. This is intended to illustrate the use of the netCDF C API. This program is part of the netCDF tutorial: http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-tutorial Full documentation of the netCDF C API can be found at: http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-c $Id: simple_xy_rd.c,v 1.9 2006/08/17 23:00:55 russ Exp $ */ #include <stdlib.h> #include <stdio.h> #include <netcdf.h> #include <math.h> /* This is the name of the data file we will read. */ /* We are reading 2D data, a 6 x 12 grid. */ #define NX 3600 #define NY 1801 #define NZ 49 #define NT 10 /* Handle errors by printing an error message and exiting with a * non-zero status. */ #define ERRCODE 2 #define ERR(e) {printf("Error: %s\n", nc_strerror(e)); exit(ERRCODE);} #define TYPE float #define NC_GETVARA nc_get_vara_float int main(int argc, char* argv[]) { if (argc < 2) { printf("usage: %s filename\n", argv[0]); exit(1); } /* This will be the netCDF ID for the file and data variable. */ int ncid, varid; //short data_in[NX][NY][NZ]; TYPE *data_in; data_in = (TYPE*) malloc(NX*NY*NZ*sizeof(TYPE)); for (int i = 0; i < NX*NY*NZ; i++) { data_in[i] = -111; } /* Loop indexes, and error handling. */ int x, y, z, t, retval; /* Open the file. NC_NOWRITE tells netCDF we want read-only access * to the file.*/ if ((retval = nc_open(argv[1], NC_NOWRITE, &ncid))) ERR(retval); /* Get the varid of the data variable, based on its name. */ if ((retval = nc_inq_varid(ncid, "air_temperature_ml", &varid))) ERR(retval); size_t start[4], count[4]; start[0]=0; start[1]=0; start[2]=0; start[3]=0; count[0]=1; count[1]=NZ; count[2]=NY; count[3]=NX; for (t = 0; t < NT; t++) { start[0] = t; /* Read the data. */ if ((retval = NC_GETVARA(ncid, varid, start, count, data_in))) ERR(retval); /* Check the data. */ for (z = 0; z < NZ; z++) for (y = 0; y < NY; y++) for (x = 0; x < NX; x++) if (data_in[x+NX*y+NX*NY*z] != -32767) { printf("%d %d %d %d: %f\n", x, y ,z, t, data_in[x+NX*y+NX*NY*z]); // return ERRCODE; } } /* Close the file, freeing all resources. */ if ((retval = nc_close(ncid))) ERR(retval); printf("*** SUCCESS reading example file %s!\n", argv[1]); return 0; }
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