Sampling Locations (cwsample)

It's often necessary to compare data from a satellite sensor to data from in-situ measurements. The cwsample tool helps performs this function by taking as input a geographic (latitude, longitude) point or set of points and extracting the data values found at those points in the file.

Taking a look at the VIIRS chlorophyll data file example_viirs_chlor_sep_2022.nc in the Gulf of Mexico, suppose that there are six stations with in-situ sample data as shown here:

The cwsample tool can extract data at these stations with the following command:

cwsample --samples gom_stations.txt example_viirs_chlor_sep_2022.nc gom_samples.txt

The gom_stations.txt file contains the input station locations as lines of latitude / longitude values:

28 -96
29 -93
29.5 -86
28.5 -83
26 -82.5
24 -97

The gom_samples.txt file contains the output sample values and locations:

28 -96 0.3442
29 -93 1.0987
29.5 -86 0.2949
28.5 -83 1.9696
26 -82.5 NaN
24 -97 NaN

With a few more options to cwsample, the output rows can be imported into a spreadsheet with column headers and comma-separated values (CSV):

cwsample --samples gom_stations.txt --header --delimit "," example_viirs_chlor_sep_2022.nc gom_samples.txt

This command produces to following output file:

latitude,longitude,chl_oci
28,-96,0.3442
29,-93,1.0987
29.5,-86,0.2949
28.5,-83,1.9696
26,-82.5,NaN
24,-97,NaN

Bonus exercises:

• Read the cwsample Unix man page or user's guide section.  
• Add an option to the cwsample command line to mark the invalid samples (marked with NaN to indicate no data) as -999 instead.  
• Add an option to also output the image (row, column) coordinates.
• Run the same cwsample command line, but on the example_altim_surface_curr_feb_2023.nc data file instead.  See how the output is different — notice how many columns there are now.