Remote Sensing: Module 8 - Thermal Imagery

     In continuing with digital image processing, thermal imagery is introduced. Thermal remote sensing is unlike other forms of remote sensing in that sensors record emitted energy as opposed to reflected energy. A general rule of thumb when examining thermal data is that good absorbers are good emitters of thermal radiation while good reflectors are poor emitters of thermal radiation. Thermal characteristics will also vary by time of day and season which is taken into consideration when planning a study or recording data for a site.

     Thermal imagery has a coarser spatial resolution than other bands because it has a larger instantaneous field of view. This came into play during the lab as there were two choices of imagery for analysis. I initially chose a different image than I ultimately used because I thought it had obvious features (like recently irrigated fields) to identify using the thermal layer (which was the goal of the assignment). The lab, however, wanted the feature highlighted in the map and when I attempted to make the map the irrigated field was too pixelated for useful visual analysis. Thus, the map deliverable below. It shows an image with a better spatial resolution (and I chose a large feature to eliminate scale issues altogether). On the left side of the map is the feature visualized in a band combination (4, 5, 1) that highlights its extent. On the right side of the map is the thermal layer (Landsat ETM+, Band 6) displayed using a red, yellow, and blue gradient. 

     The thermal image shows a distinct spot near the coast that is much warmer than the surrounding water (also note the red areas as evidence of cultural activity/urbanization). When it is visualized in the multispectral image it shows as a bright red swath along the coast. The red is a result of reflected near infrared energy. This could indicate some photosynthetic activity or agricultural runoff. A combination of histrogram analysis/manipulation, viewing the image in greyscale for each band, and the inquire tool (in Imagine) were used to analyze this image.

Map 1: An Exercise in Thermal Image Interpretation