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167 lines
5.1 KiB
C
167 lines
5.1 KiB
C
/* Copyright 2018 University Corporation for Atmospheric
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Research/Unidata. See COPYRIGHT file for conditions of use. */
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/**
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* @file
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* A simple example of reading a netCDF file.
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*
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* This is an example which reads some surface pressure and
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* temperatures. The data file read by this program is produced by the
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* companion program sfc_pres_temp_wr.c. It is intended to illustrate
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* the use of the netCDF C API.
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*
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* @author Ed Hartnett
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*/
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#include <stdio.h>
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#include <string.h>
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#include <netcdf.h>
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/* This is the name of the data file we will read. */
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#define FILE_NAME "sfc_pres_temp.nc"
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/* We are reading 2D data, a 6 x 12 lat-lon grid. */
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#define NDIMS 2
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#define NLAT 6
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#define NLON 12
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#define LAT_NAME "latitude"
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#define LON_NAME "longitude"
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#define PRES_NAME "pressure"
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#define TEMP_NAME "temperature"
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/* These are used to calculate the values we expect to find. */
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#define SAMPLE_PRESSURE 900
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#define SAMPLE_TEMP 9.0
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#define START_LAT 25.0
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#define START_LON -125.0
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/* For the units attributes. */
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#define UNITS "units"
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#define PRES_UNITS "hPa"
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#define TEMP_UNITS "celsius"
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#define LAT_UNITS "degrees_north"
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#define LON_UNITS "degrees_east"
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#define MAX_ATT_LEN 80
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/* Handle errors by printing an error message and exiting with a
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* non-zero status. */
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#define ERR(e) {printf("Error: %s\n", nc_strerror(e)); return 2;}
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int
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main()
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{
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int ncid, pres_varid, temp_varid;
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int lat_varid, lon_varid;
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/* We will read surface temperature and pressure fields. */
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float pres_in[NLAT][NLON];
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float temp_in[NLAT][NLON];
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/* For the lat lon coordinate variables. */
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float lats_in[NLAT], lons_in[NLON];
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/* To check the units attributes. */
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char pres_units_in[MAX_ATT_LEN], temp_units_in[MAX_ATT_LEN];
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char lat_units_in[MAX_ATT_LEN], lon_units_in[MAX_ATT_LEN];
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/* We will learn about the data file and store results in these
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program variables. */
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int ndims_in, nvars_in, ngatts_in, unlimdimid_in;
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/* Loop indexes. */
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int lat, lon;
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/* Error handling. */
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int retval;
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/* Open the file. */
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if ((retval = nc_open(FILE_NAME, NC_NOWRITE, &ncid)))
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ERR(retval);
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/* There are a number of inquiry functions in netCDF which can be
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used to learn about an unknown netCDF file. NC_INQ tells how
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many netCDF variables, dimensions, and global attributes are in
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the file; also the dimension id of the unlimited dimension, if
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there is one. */
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if ((retval = nc_inq(ncid, &ndims_in, &nvars_in, &ngatts_in,
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&unlimdimid_in)))
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ERR(retval);
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/* In this case we know that there are 2 netCDF dimensions, 4
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netCDF variables, no global attributes, and no unlimited
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dimension. */
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if (ndims_in != 2 || nvars_in != 4 || ngatts_in != 0 ||
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unlimdimid_in != -1) return 2;
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/* Get the varids of the latitude and longitude coordinate
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* variables. */
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if ((retval = nc_inq_varid(ncid, LAT_NAME, &lat_varid)))
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ERR(retval);
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if ((retval = nc_inq_varid(ncid, LON_NAME, &lon_varid)))
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ERR(retval);
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/* Read the coordinate variable data. */
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if ((retval = nc_get_var_float(ncid, lat_varid, &lats_in[0])))
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ERR(retval);
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if ((retval = nc_get_var_float(ncid, lon_varid, &lons_in[0])))
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ERR(retval);
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/* Check the coordinate variable data. */
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for (lat = 0; lat < NLAT; lat++)
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if (lats_in[lat] != START_LAT + 5.*lat)
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return 2;
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for (lon = 0; lon < NLON; lon++)
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if (lons_in[lon] != START_LON + 5.*lon)
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return 2;
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/* Get the varids of the pressure and temperature netCDF
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* variables. */
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if ((retval = nc_inq_varid(ncid, PRES_NAME, &pres_varid)))
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ERR(retval);
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if ((retval = nc_inq_varid(ncid, TEMP_NAME, &temp_varid)))
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ERR(retval);
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/* Read the data. Since we know the contents of the file we know
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* that the data arrays in this program are the correct size to
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* hold all the data. */
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if ((retval = nc_get_var_float(ncid, pres_varid, &pres_in[0][0])))
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ERR(retval);
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if ((retval = nc_get_var_float(ncid, temp_varid, &temp_in[0][0])))
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ERR(retval);
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/* Check the data. */
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for (lat = 0; lat < NLAT; lat++)
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for (lon = 0; lon < NLON; lon++)
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if (pres_in[lat][lon] != SAMPLE_PRESSURE + (lon * NLAT + lat) ||
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temp_in[lat][lon] != SAMPLE_TEMP + .25 * (lon * NLAT + lat))
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return 2;
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/* Each of the netCDF variables has a "units" attribute. Let's read
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them and check them. */
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if ((retval = nc_get_att_text(ncid, lat_varid, UNITS, lat_units_in)))
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ERR(retval);
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if (strncmp(lat_units_in, LAT_UNITS, strlen(LAT_UNITS)))
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return 2;
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if ((retval = nc_get_att_text(ncid, lon_varid, UNITS, lon_units_in)))
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ERR(retval);
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if (strncmp(lon_units_in, LON_UNITS, strlen(LON_UNITS)))
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return 2;
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if ((retval = nc_get_att_text(ncid, pres_varid, UNITS, pres_units_in)))
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ERR(retval);
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if (strncmp(pres_units_in, PRES_UNITS, strlen(PRES_UNITS)))
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return 2;
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if ((retval = nc_get_att_text(ncid, temp_varid, UNITS, temp_units_in)))
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ERR(retval);
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if (strncmp(temp_units_in, TEMP_UNITS, strlen(TEMP_UNITS))) return 2;
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/* Close the file. */
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if ((retval = nc_close(ncid)))
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ERR(retval);
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printf("*** SUCCESS reading example file sfc_pres_temp.nc!\n");
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return 0;
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}
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