1 | |
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2 | NAME: |
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3 | |
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4 | ncdf_quickwrite |
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5 | |
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6 | |
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7 | SYNOPSIS: |
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8 | |
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9 | This is a routine for writing variables from an IDL session to a |
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10 | NetCDF file, in just a very few commands. |
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11 | |
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12 | It is designed to be quick to *use*, once you are familiar with the syntax. |
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13 | |
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14 | It may possibly also be slightly quicker to *learn* than writing NetCDF |
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15 | files the standard way, but this is not particularly the objective. |
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16 | |
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17 | |
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18 | DESCRIPTION: |
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19 | |
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20 | 1) Ensure that the supplied routines are installed in your IDL_PATH. |
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21 | |
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22 | ncdf_quickwrite.pro |
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23 | ncdf_quickwrite_helper1.pro |
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24 | ncdf_quickwrite_helper2.pro |
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25 | ncdf_quickwrite_helper3.pro |
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26 | |
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27 | 2) Set the string variable NCFILE to the NetCDF filename you wish to write, |
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28 | For example: |
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29 | |
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30 | ncfile='myfile.nc' |
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31 | |
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32 | The file should not already exist. If you wish to permit overwriting, put |
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33 | "!" before the filename. For example: |
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34 | |
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35 | ncfile='!myfile.nc' |
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36 | |
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37 | 3) Set the string variable NCFIELDS as follows. |
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38 | NB there are a number of options but you do not need to use them all! |
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39 | Skip down to "USAGE EXAMPLE" below for a simple example. |
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40 | |
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41 | a) For each variable, construct the VARIABLE SPECIFICATION as follows: |
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42 | |
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43 | => For an array, use the name of array, with the names of the |
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44 | dimensions in square brackets ([]) and separated by commas. |
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45 | For example: |
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46 | |
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47 | pressure[longitude,latitude] |
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48 | |
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49 | => For a 1-dimensional array, whose dimension name is the same |
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50 | as the array name, you can omit the dimension name if you want. |
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51 | For example: |
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52 | |
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53 | longitude[] |
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54 | |
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55 | => For each scalar, do not use square brackets, e.g.: |
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56 | |
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57 | g |
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58 | |
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59 | => If any of variables you want to use in the NetCDF file is not held |
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60 | in a variable of the same name in your IDL session, then put |
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61 | follow the variable specification with "=expression". |
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62 | |
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63 | Examples: |
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64 | |
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65 | 1) if pressure is in IDL variable "p", you have: |
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66 | |
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67 | pressure[longitude,latitude]=p |
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68 | |
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69 | 2) if g is not stored in a variable but should |
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70 | have value 9.8, you have: |
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71 | |
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72 | g=9.8 |
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73 | |
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74 | => If any of the variables have attributes to be stored, follow |
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75 | the specification with a colon (:) followed by the name of |
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76 | a variable containing a structure of attributes. For example: |
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77 | |
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78 | pressure[longitude,latitude]=p:patts |
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79 | |
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80 | where "patts" is a structure containing: |
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81 | {units:'pascal',missing_value:1e10} |
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82 | |
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83 | b) Join up all the variables with semicolons (;) to make the NCFIELDS |
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84 | string. For example: |
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85 | |
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86 | NCFIELDS = $ |
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87 | 'pressure[longitude,latitude]=p:patts;longitude[];latitude[];g=9.8' |
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88 | |
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89 | c) If there is a dimension, which you want to be of UNLIMITED size, then |
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90 | precede its name with "*" somewhere where it is mentioned (or where |
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91 | you omit the dimension name because it matches the variable name, |
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92 | nonetheless still include the "*" in the square brackets). For example |
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93 | the "time" dimension in any of the following: |
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94 | |
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95 | NCFIELDS = $ |
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96 | 'pressure[longitude,latitude,*time];longitude[];latitude[];time[]' |
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97 | |
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98 | NCFIELDS = $ |
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99 | 'pressure[longitude,latitude,time];longitude[];latitude[];time[*time]' |
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100 | |
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101 | NCFIELDS = $ |
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102 | 'pressure[longitude,latitude,time];longitude[];latitude[];time[*]' |
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103 | |
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104 | (Note that if you put "*" before the same dimension name more than once, |
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105 | that is accepted. But if you put "*" before more than one dimension |
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106 | name, that is an error, because only one dimension can be UNLIMITED.) |
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107 | |
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108 | d) If there are global attributes, follow the specification with an |
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109 | at sign (@) followed by the name of a variable containing a structure |
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110 | of attributes. For example: |
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111 | |
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112 | NCFIELDS = $ |
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113 | 'pressure[longitude,latitude];longitude[];latitude[]@globatts' |
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114 | |
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115 | where "globatts" is a structure containing: |
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116 | {source:'my program',version:2} |
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117 | |
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118 | e) You may insert whitespace in NCFIELDS for readability. |
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119 | |
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120 | 4) Type: @ncdf_quickwrite |
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121 | |
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122 | |
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123 | USAGE EXAMPLES: |
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124 | |
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125 | 1) SIMPLE EXAMPLE. |
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126 | |
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127 | => You have the following variables: |
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128 | |
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129 | LATITUDE FLOAT = Array[73] |
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130 | LONGITUDE FLOAT = Array[96] |
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131 | PRESSURE DOUBLE = Array[96, 73] |
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132 | |
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133 | => You issue the following commands: |
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134 | |
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135 | ncfile='my.nc' |
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136 | ncfields='latitude[];longitude[];pressure[longitude,latitude]' |
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137 | @ncdf_quickwrite |
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138 | |
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139 | => This makes "my.nc". "ncdump -h my.nc" reports the following: |
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140 | |
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141 | netcdf my { |
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142 | dimensions: |
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143 | latitude = 73 ; |
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144 | longitude = 96 ; |
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145 | variables: |
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146 | float latitude(latitude) ; |
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147 | float longitude(longitude) ; |
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148 | double pressure(latitude, longitude) ; |
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149 | } |
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150 | |
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151 | 2) MORE COMPLEX EXAMPLE. |
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152 | |
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153 | => You have the following variables: |
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154 | |
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155 | LATS FLOAT = Array[73] |
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156 | LONS FLOAT = Array[96] |
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157 | P DOUBLE = Array[96, 73, 100] |
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158 | UBAR DOUBLE = Array[73, 100] |
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159 | YR LONG = Array[100] |
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160 | |
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161 | => You issue the following commands. |
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162 | (NB The NCFIELDS assignment can be typed as one long line, but is |
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163 | split below for readability.) |
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164 | |
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165 | ncfile='!my.nc' |
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166 | angle_attr={units:'degrees'} |
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167 | wind_attr={units:'m s-1'} |
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168 | press_attr={units:'pascals',missing_value:1e10} |
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169 | g_attr={units:'m s-2'} |
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170 | globattr={source:'My program',version:2} |
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171 | |
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172 | ncfields = 'pressure[longitude,latitude,time]=p:press_attr; ' $ |
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173 | + 'longitude[]=lons:angle_attr; ' $ |
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174 | + 'latitude[]=lats:angle_attr; ' $ |
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175 | + 'ubar[latitude,time]:wind_attr; ' $ |
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176 | + 'year[*time]=yr; ' $ |
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177 | + 'g=9.8:g_attr @ globattr' |
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178 | |
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179 | @ncdf_quickwrite |
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180 | |
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181 | => This makes "my.nc", overwriting any existing my.nc. |
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182 | "ncdump -h my.nc" reports the following: |
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183 | |
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184 | netcdf my { |
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185 | dimensions: |
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186 | longitude = 96 ; |
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187 | latitude = 73 ; |
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188 | time = UNLIMITED ; // (100 currently) |
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189 | variables: |
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190 | double pressure(time, latitude, longitude) ; |
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191 | pressure:units = "pascals" ; |
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192 | pressure:missing_value = 1.e+10f ; |
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193 | float longitude(longitude) ; |
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194 | longitude:units = "degrees" ; |
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195 | float latitude(latitude) ; |
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196 | latitude:units = "degrees" ; |
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197 | double ubar(time, latitude) ; |
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198 | ubar:units = "m s-1" ; |
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199 | int year(time) ; |
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200 | float g ; |
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201 | g:units = "m s-2" ; |
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202 | |
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203 | // global attributes: |
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204 | :source = "My program" ; |
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205 | :version = 2s ; |
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206 | } |
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207 | |
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208 | (Note that in this example you have created a NetCDF file |
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209 | with verbose variable names even though the IDL variables |
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210 | have terse names.) |
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211 | |
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212 | BUGS: |
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213 | |
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214 | => Writing string variables may fail. |
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215 | |
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216 | |
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217 | LIMITATIONS: |
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218 | |
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219 | => An expression following an "=" sign should not contain ":" or ";" signs |
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220 | because these will be treated as separators. |
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221 | |
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222 | => An expression following an "=" sign gets evaluated twice. If this is |
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223 | computationally expensive, store it in a variable and reference the |
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224 | variable instead. |
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225 | |
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226 | => Attribute names are case-insensitive because they are structure tags. |
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227 | In fact, they are stored as lower case. |
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228 | |
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229 | |
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230 | AUTHOR: |
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231 | |
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232 | Alan Iwi <A.M.Iwi@rl.ac.uk> |
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233 | |
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234 | |
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235 | COPYRIGHT: |
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236 | |
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237 | I am putting these routines in the public domain. |
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238 | |
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239 | But the IDL program itself is subject to copyright restrictions. |
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240 | |
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