CoastWatch Utilities: Exercise 2a- Opening a Level-1 dataset and generating an RGB composite

This exercise will use a S-NPP VIIRS Level-1 dataset containing emissive bands. A Level-1 product contains geolocations and raw values.  The emissive bands used are top-of-atmosphere and do not have corrections applied that would be present in a Level-2 product.  These data are available through https://coastwatch.noaa.gov/cw_html/cwViewer.html Links to an external site. and used as inputs to CoastWatch mapped Level-3 True Color products.

Goal:  Generate an RGB composite from multi-banded data

Start CDAT:  From a command window,  type ‘cdat’<enter>  or use your OS Desktop to locate the software and run it.

From the tool bar,  select the ‘Open’ icon:

Navigate to the “SampleData2022/SatelliteData” directory and select the Level-2 RGB HDF dataset : 

VRSLCW.B2022266.205808.hdf

Select three emissive visible bands (M5,M4,M3) by holding the ‘Ctrl’ key down while selecting variables.  Bands M5,M4,M3 will be used as the Red-Green-Blue channels in our image.

Upon the image loading,  note the edges of the image.  Lines appear increasingly missing as you approach the edge of the image.  This is referred to as the ‘bow-tie’ effect and is used to reduce data size onboard the satellite.  The further from nadir,  each pixel covers more area. 

As a self-guided exercise,  the CoastWatch Utility command ‘cwregister2’  can quickly map the data (by default to orthographic) and appropriately fill the data deletions. 

If you wish to use the orthographic image, load the M5,M4, and M3 bands from the VRSLCW_quickmap.hdf located in the “SampleData2022/exercise_outputs”  directory.

Select the ‘RGB Color Composite’ control tab and assign the RGB components to the bands as follows  (M5=>R,  M4=> G,  M3=>B) and then check the Perform color composite box:

Switch to the ‘Enhancement’ control tab and adjust the enhancement minimum and maximum for each band.  You can start by normalizing each band and then further refine the image as necessary.  Note,  to enhance water,  clouds will often become over saturated in top-of-atmosphere imagery.

Using ‘Normalization’ and the Gamma Function with a maximum of 10.0 for each band yields the following:

Locate an area of interest and refine the image.  Use the Overlay control tab to add predefined layers, graticules, coastlines, etc.  When complete, export the image to PNG: