Image Processing Reference
temperature of an object depends on the applying the approximate emissivity values
from the blackbody laws. Materials that have high emissivity absorb and emit
relatively larger proportions of incident energy.
Retrieval of Thermal Data from Satellite Imagery
Brightness temperatures (also referred to as blackbody temperatures) can be derived
from satellites' thermal infrared measurements through Planck's law (Flynn et al.
2001 ; Dash et al. 2002 ). The digital numbers of Landsat TM and ETM+ thermal
infrared band 6 (10.4-12.5 µm) are converted into radiance using the equation:
gain DN offset
where L l is at sensor radiance, DN is the digital number of a pixel, gain is slope of
the radiance/ DN conversion function in Wm −2 sr −1 mm −1 , offset is the rescaled bias
which is the intersection of the radiance/DN conversion function in Wm −2 sr −1 mm −1
(Landsat Project Science Office 2004 ). Each Landsat TM scene is accompanied by
gain and offset values as part of the metadata. The TM band 6 spectral radiance values
are subsequently transformed to surface temperature values using the relationship:
where T the radiant surface temperature in K and K are thermal calibration con-
stants in Wm −2 sr −1 mm −1 supplied by the Landsat Project Science Office ( 2004 ), and
L l is spectral radiance of thermal band pixels in Wm −2 sr −1 mm −1 . For Landsat 7, K1
is 666.09 and K2 is 12822.71, and for Landsat 5 K1 is 607.76 and K2 is 1260.56.
The composite emissivity values for urban mosaics are rarely known, but usually
assumed to be near 1. The typical emissivity values for man-made surfaces such as
concrete and asphalt range from 0.95 to 0.97 (Buettner and Kern 1965 ). Here, it is
important to acknowledge the ambiguity introduced in urban surface temperature
distribution by incomplete knowledge of surface emissivity.
Urban Heat Island
The phenomenon commonly referred to as urban heat island, results from the inad-
vertent urban climate modification from anthropogenic activities (Oke 1987 ). Urban
materials such as construction material, roofs, asphalt, concrete and roads absorb
more heat from the sun. The subsequent release of the energy causes urban areas
to be warmer compared to the surrounding non-urban areas giving rise to the