Exercise 2: Hot spot and plume detection - continued
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|  | MODIS: Bands 31 and 32 | |
MODIS Bands 31 and 32 are used to best detect a volcanic plume and distinguish it from meteorological clouds. The bands range from 10.780 - 11.280 μm (31) and 11.770 - 12.270 μm (32), and correspond to the thermal IR.
1. How can you differentiate between a volcanic plume and a meteorological cloud?
We have to use a simple band maths operation to solve this problem. If you subtract the bands from each other (band 31 minus band 32), the difference between the brightness temperatures usually results in negative values for volcanic plumes containing ash, and positive values for atmospheric clouds.
In the following image you can see that the plume is very dark (negative values) and the clouds appear light (positive values).
MODIS band subtraction for ash detection - 24 July 2001
To perform the subtraction, open the two previous images again but do not stretch them:
File > Open > etna_mod_2001-205_bd31
File > Open > etna_mod_2001-205_bd32
Go to Image > Arithmetic
For image #1 take band 31 (etna_mod_2001-205_bd31)
For image #2 take band 32 (etna_mod_2001-205_bd32)
Looking at the result you will see that you still have to stretch the image to reduce it from 65536 colour shades to 256 colour shades (8 bit). Go to > Enhance > Interactive Stretching.
You see that the plume now appears dark, and the clouds white.
Remember that due to stretching, the original values are lost.
| | MODIS band subtraction for ash detection - 23 July 2001 | |
Here is another example of an RGB image and a band subtraction (band 31 – band 32) from 23 July 2001, one day earlier.
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