Module 8 - Remote Sensing - Thermal and Multispectral Analysis

Thermal and Multispectral Analysis to Identify a Geographic Feature

This week, we added Thermal Infrared, or TM band 6, to our multispectral analysis toolbox.
Unlike the near-infrared and mid-infrared wavelengths, which are reflected from the surface of the Earth, back to the sensors, thermal infrared energy is actually emitted from the body of the Earth itself.  When shorter-wavelength energy hits the earth, some of it is converted to heat energy, and that is what is emitted back to the thermal infrared sensors on platforms such as LANDSAT.

Sources of heat, such as fires, can be identified by the thermal infrared band, but cool features can be as well, from their anomalously low emission of thermal infrared energy: they are cool.  Water bodies are examples of features such as this.  In the map above, a band combination of Near Infrared (band 4: red), Visible Green (band 2: green) and Thermal Infrared (band 6: blue) were used, along with several other band combinations, to identify the dark green grid-like feature above as a collection of ponds or flooded fields.  In the image pictured here, it can be seen that the ponds reflect varying degrees of visible green light, depending on their depth.  The fact that they are a fairly dark green compared to the river to the west show that they are filled with relatively clear water.  Whether they are dark or light green also depends upon their depth.  Almost all Near Infrared wavelengths are absorbed by the water, and they emit very little thermal infrared energy.  This is shown by the very low amounts of blue and red displayed by the ponds in the image.  The area to the east of the ponds emits considerably more thermal infrared energy, as shown by its bright blue color, while the vegetated areas shown by pink and red around the ponds reflect much more Near Infrared energy.