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The two longer wavelength Landsat TM bands are imaged on this page. Band 7 depicts reflected infrared radiation beyond the narrow spectral interval where vegetation is highly reflective. For some materials, the amount of reflected radiation (and hence its tonal representation) is quite distinctive in these intervals. Band 6 produces an emitted radiation image in the Thermal IR; in general, light tones mean higher temperatures (surface has become warmer owing usually to greater radiative absorption) and darker tones denote a cooler surface. During the daytime, water is almost always cooler than most land surfaces.


 

TM Band 7 image of Morro Bay, CaliforniaNow look at TM Band 7 which is centered near 2.2 µm in the reflected IR band. The image has some similarities to bands 1 and 4 but there are several obvious differences. All water is now completely dark (no signs of reflecting sediments). Only faint traces of the wave breakers remain. The beach sand is somewhat brighter than in Band 4. The sun-facing slopes in the hills seem to be moderately brighter than in the other two bands. A bit more detail emerges in the partially shadowed slopes. Some areas, such as crop fields, that were quite bright in 4, are again dark as in 1. The mystery area near s shows some internal pattern or structure, including elongate strips of lighter and darker that provides another clue to its identity. The towns have a more diffuse street pattern and are generally darker overall. The extraction pits so prominent in Band 1 are almost impossible to pick out in Band 7.

TM Band 6 image of Morro Bay, CaliforniaNext, we will examine Band 6 - the emitted thermal band (10.5 - 12.5 µm). Remember that the spatial resolution for this band is 4 times coarser - 120 m compared with 30 m - than the other bands. The effect of this reduced resolution is immediately evident in viewing the subscene in that many details discernible in the other bands tend to be "smeared" out in this one. Yet the main elements you have come to know from bands already scanned are still readily recognized.

In fact, the hilly terrain is even more like a 3-D rendition because of the sharp contrast between the hotter sun-facing slopes and the shadowed slopes that are cooler and appear now as fairly uniform, dark, shadow-patterns. This contrast gives a superior sense of topographic relief to this part of the scene and also highlights the valley regions.

Some medium gray tones, as at v, represent the cooling effects of grass ground cover. An extremely bright, but isolated, spot at h coincides with Morro Rock which, having a bare rock surface with steep slopes, absorbs solar radiation and heats up so that it emits thermal energy. The towns, labeled in yellow, have moderate gray levels, indicating they are somewhat cooler than the natural hillsides, but there are several lighter tones within the downtown areas which are customarily warmer than residential areas (that have lawns and more trees).

1-8 Speculate on why the upper right corner area of TM 6 is notably darker. ANSWER

Water, which has a notably higher heat capacity than most dry materials on land, tends to appear cooler in daytime thermal IR images than most land surfaces that receive direct sunlight. (Section 9 deals with thermal imagery and reviews the principles governing the tonal displays of features at different radiant temperatures.) That heat capacity of water is evident in this image in which the ocean and the bay have moderately-dark gray levels. If this image had been taken at night, the water would be expressed in much lighter tones than the neighboring land, because it retained much of its internal heat. The land, on the other hand, would appear darker, because it radiated its absorbed solar energy, thereby cooling to radiant temperatures lower than the water.

An uneven pattern of areas of slightly lighter gray levels, shown in the Pacific Ocean is not real here, but is a noise-like artifact associated with the sensor characteristics. There is one pattern that is anomalous and meaningful. In the water just north and west of Morro Rock is an irregular pattern with a lighter tone that jets out like a plume of sediment entering the ocean from the shore. This is precisely what we expect from the nearby power plant (at t) which dumps its heated waste water into a nearby body of standing water.

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Primary Author: Nicholas M. Short, Sr. email: nmshort@ptd.net