Using the pixel inspector to apply a threshold

Sometimes we wish to apply a threshold to an image to isolate areas that meet certain criteria. In this example we are using Sentinel 1 imagery to create a new map layer that shows areas where there is surface water present, indicating flooding.

In radar imagery such as Sentinel 1, surface water appears very dark. As we know that these dark pixels are surface water, we can use this to apply a threshold. The images below show a Sentinel 1 image of the Nederrijn river before and after flooding.  

Before flooding:

After flooding:

To determine the value to use as a threshold for our mask, we can use the pixel inspector. Right click on a pixel in the image where there is water/flooding, and extract the value for the relevant band (in this case we are using vv_dB). 

We can then use the Classify image using thresholds block to set the “flood” class using the value we got from the pixel inspector:

The threshold assigns a classification of “flooded” if the S1 vv_db pixel is less than -14.7. When applied across the area of interest we have a map of the flooded area as shown below.