1 /*! \file
2 A simple example of reading a netCDF file.
3
4 This is an example which reads some surface pressure and
5 temperatures. The data file read by this program is produced by the
6 companion program sfc_pres_temp_wr.c. It is intended to illustrate the
7 use of the netCDF C API.
8
9 Copyright 2006 University Corporation for Atmospheric
10 Research/Unidata. See COPYRIGHT file for conditions of use.
11
12 */
13
14 #include <stdio.h>
15 #include <string.h>
16 #include <netcdf.h>
17
18 /* This is the name of the data file we will read. */
19 #define FILE_NAME "sfc_pres_temp.nc"
20
21 /* We are reading 2D data, a 6 x 12 lat-lon grid. */
22 #define NDIMS 2
23 #define NLAT 6
24 #define NLON 12
25
26 #define LAT_NAME "latitude"
27 #define LON_NAME "longitude"
28 #define PRES_NAME "pressure"
29 #define TEMP_NAME "temperature"
30
31 /* These are used to calculate the values we expect to find. */
32 #define SAMPLE_PRESSURE 900
33 #define SAMPLE_TEMP 9.0
34 #define START_LAT 25.0
35 #define START_LON -125.0
36
37 /* For the units attributes. */
38 #define UNITS "units"
39 #define PRES_UNITS "hPa"
40 #define TEMP_UNITS "celsius"
41 #define LAT_UNITS "degrees_north"
42 #define LON_UNITS "degrees_east"
43 #define MAX_ATT_LEN 80
44
45 /* Handle errors by printing an error message and exiting with a
46 * non-zero status. */
47 #define ERR(e) {printf("Error: %s\n", nc_strerror(e)); return 2;}
48
49 int
main()50 main()
51 {
52 int ncid, pres_varid, temp_varid;
53 int lat_varid, lon_varid;
54
55 /* We will read surface temperature and pressure fields. */
56 float pres_in[NLAT][NLON];
57 float temp_in[NLAT][NLON];
58
59 /* For the lat lon coordinate variables. */
60 float lats_in[NLAT], lons_in[NLON];
61
62 /* To check the units attributes. */
63 char pres_units_in[MAX_ATT_LEN], temp_units_in[MAX_ATT_LEN];
64 char lat_units_in[MAX_ATT_LEN], lon_units_in[MAX_ATT_LEN];
65
66 /* We will learn about the data file and store results in these
67 program variables. */
68 int ndims_in, nvars_in, ngatts_in, unlimdimid_in;
69
70 /* Loop indexes. */
71 int lat, lon;
72
73 /* Error handling. */
74 int retval;
75
76 /* Open the file. */
77 if ((retval = nc_open(FILE_NAME, NC_NOWRITE, &ncid)))
78 ERR(retval);
79
80 /* There are a number of inquiry functions in netCDF which can be
81 used to learn about an unknown netCDF file. NC_INQ tells how
82 many netCDF variables, dimensions, and global attributes are in
83 the file; also the dimension id of the unlimited dimension, if
84 there is one. */
85 if ((retval = nc_inq(ncid, &ndims_in, &nvars_in, &ngatts_in,
86 &unlimdimid_in)))
87 ERR(retval);
88
89 /* In this case we know that there are 2 netCDF dimensions, 4
90 netCDF variables, no global attributes, and no unlimited
91 dimension. */
92 if (ndims_in != 2 || nvars_in != 4 || ngatts_in != 0 ||
93 unlimdimid_in != -1) return 2;
94
95 /* Get the varids of the latitude and longitude coordinate
96 * variables. */
97 if ((retval = nc_inq_varid(ncid, LAT_NAME, &lat_varid)))
98 ERR(retval);
99 if ((retval = nc_inq_varid(ncid, LON_NAME, &lon_varid)))
100 ERR(retval);
101
102 /* Read the coordinate variable data. */
103 if ((retval = nc_get_var_float(ncid, lat_varid, &lats_in[0])))
104 ERR(retval);
105 if ((retval = nc_get_var_float(ncid, lon_varid, &lons_in[0])))
106 ERR(retval);
107
108 /* Check the coordinate variable data. */
109 for (lat = 0; lat < NLAT; lat++)
110 if (lats_in[lat] != START_LAT + 5.*lat)
111 return 2;
112 for (lon = 0; lon < NLON; lon++)
113 if (lons_in[lon] != START_LON + 5.*lon)
114 return 2;
115
116 /* Get the varids of the pressure and temperature netCDF
117 * variables. */
118 if ((retval = nc_inq_varid(ncid, PRES_NAME, &pres_varid)))
119 ERR(retval);
120 if ((retval = nc_inq_varid(ncid, TEMP_NAME, &temp_varid)))
121 ERR(retval);
122
123 /* Read the data. Since we know the contents of the file we know
124 * that the data arrays in this program are the correct size to
125 * hold all the data. */
126 if ((retval = nc_get_var_float(ncid, pres_varid, &pres_in[0][0])))
127 ERR(retval);
128 if ((retval = nc_get_var_float(ncid, temp_varid, &temp_in[0][0])))
129 ERR(retval);
130
131 /* Check the data. */
132 for (lat = 0; lat < NLAT; lat++)
133 for (lon = 0; lon < NLON; lon++)
134 if (pres_in[lat][lon] != SAMPLE_PRESSURE + (lon * NLAT + lat) ||
135 temp_in[lat][lon] != SAMPLE_TEMP + .25 * (lon * NLAT + lat))
136 return 2;
137
138 /* Each of the netCDF variables has a "units" attribute. Let's read
139 them and check them. */
140 if ((retval = nc_get_att_text(ncid, lat_varid, UNITS, lat_units_in)))
141 ERR(retval);
142 if (strncmp(lat_units_in, LAT_UNITS, strlen(LAT_UNITS)))
143 return 2;
144
145 if ((retval = nc_get_att_text(ncid, lon_varid, UNITS, lon_units_in)))
146 ERR(retval);
147 if (strncmp(lon_units_in, LON_UNITS, strlen(LON_UNITS)))
148 return 2;
149
150 if ((retval = nc_get_att_text(ncid, pres_varid, UNITS, pres_units_in)))
151 ERR(retval);
152 if (strncmp(pres_units_in, PRES_UNITS, strlen(PRES_UNITS)))
153 return 2;
154
155 if ((retval = nc_get_att_text(ncid, temp_varid, UNITS, temp_units_in)))
156 ERR(retval);
157 if (strncmp(temp_units_in, TEMP_UNITS, strlen(TEMP_UNITS))) return 2;
158
159 /* Close the file. */
160 if ((retval = nc_close(ncid)))
161 ERR(retval);
162
163 printf("*** SUCCESS reading example file sfc_pres_temp.nc!\n");
164 return 0;
165 }
166