Recipe: IDL program to read data from downloaded file: Difference between revisions

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 ==== Modifications to read a subset of the data ====
-The above python code can be easily modified to read a subset of the data instead. For example, to read temperature data inside the box bounded by latitudes 25<sup>o</sup>N, 50<sup>o</sup>N and longitudes -130<sup>o</sup>N, -65<sup>o</sup>N, we will replace line 15 in the code above by
+The above IDL script can be easily modified to read a subset of the data instead. The modified code, to read temperature data inside the box bounded by latitudes 25<sup>o</sup>N, 50<sup>o</sup>N and longitudes -130<sup>o</sup>W, -65<sup>o</sup>W, is
 <syntaxhighlight lang="python">
-T = rootgrp.variables['T'][0,:,229:280,99:230]
+; file
+file = 'c1440_NR.inst01hr_3d_T_Cv.20060918_0900z.nc4'
+; read data from file
+ncid = ncdf_open(file)
+; bounding box
+;   lons = -130:0.5:-65
+;   lats =   25:0.5:50
+imin = round((-130. + 180.)/0.5) - 1
+imax = round(( -65. + 180.)/0.5) - 1
+jmin = round((  25 +  90.)/0.5) - 1
+jmax = round((  50 +  90.)/0.5) - 1
+; corresponding array sizes
+im = imax-imin+1
+jm = jmax-jmin+1
+lm = 72 ; read all 72 levels
+; start and count vectors
+offset = [imin, jmin, 0, 0]
+count = [im, jm, lm, 1]
+ncdf_varget, ncid, 'T', T, count=count, offset=offset
+ncdf_close, ncid
+; max/min of T
+print, 'max(T):', max(T)
+print, 'min(T):', min(T)
+; plot T at the surface
+lons = findgen(im)*0.5-130.
+lats = findgen(jm)*0.5+25.
+basemap = map('Cylindrical Equal Area', limit=[25., -130., 50., -65.])
+cntrplot = contour( $
+           T[*,*,71], $ ;; level=71 => surface
+           lons, $
+           lats, $
+           grid_units=2, $ ;; degrees
+           n_levels=20, $
+           rgb_table=34, $
+           ;; /fill, $
+           /overplot, $
+           title='surface air temperature' $
+                  )
+m1 = mapcontinents(/countries)
+END
 </syntaxhighlight>
-The conversion of lat/lon co-ordinates to indices is left as an exercise for the user.
+Running the modified script, produces the text output
+ max(T):      305.605
+ min(T):      191.696
-Running the modified script, produces the text output
+and the plot
- rootgrp.variables['T'].shape (1, 72, 361, 720)
- Reading T... done.
- T.shape: (72, 51, 131)
- min(T): 191.6956
- max(T): 305.6048
 == See Also ==