Image Processing Reference
In-Depth Information
increase of high spatial resolution satellite imagery during the last couple of years,
textural and image segmentation approaches will play important roles in urban land
cover classification.
14.4
Urban Thermal Mapping
14.4.1
Principles of Thermal Radiation
A basic understanding of thermal radiation is necessary for the interpretation of
thermal imagery acquired by scanners. The region beyond 3 µm of the electro-
magnetic spectrum is characterized by emitted electromagnetic energy from
natural materials which can be related to temperature according to the Stefan-
Boltzmann and Wien's laws. The total energy emitted by a blackbody is a func-
tion of the surface temperature as expressed by Stefan-Boltzmann law, which
states that:
MT
=
4
s
(14.3)
where
M
is total radiant emittance energy from the radiant surface of a material;
watts (W) m
−2
;
s
is Stefan-Boltzmann constant, 5.6697 × 10
−8
W m
−2
K
−4
; and
T
is the absolute temperature in K of the emitting material. Wien's displacement law
establishes the relations between the wavelength at which a blackbody radiation
reaches a maximum and its temperature in the following equation,
2898
T
λ =
m
(14.4)
where l
m
is the dominant wavelength in micrometers,
T
is temperature in K. Both
laws (Eqs.
14.3
and
14.4
) are related to the ideal behavior of a blackbody; however,
natural materials are not perfect and only approximate blackbodies.
Due to the Earth's absorption of certain wavelength by gases, or the so-called
absorption window, remote sensing of the thermal infrared region is limited to 3-5 mm
and/or 8-14 mm of the electromagnetic spectrum. The maximum Earth's energy at
300 K is emitted at 9.7 mm. Radiometers and scanners can be used to measure
thermal energy from the Earth's surface at anytime of the day or night. All objects
above 0 K, or −273°C, emit energy whose spectral composition and intensity are
characteristic of the composition and kinetic temperature of the target. The digital
numbers in a thermal image are measures of the radiant energy which is a function of
the emissivity of the different targets. A blackbody has an emissivity of 1 whereas
all natural materials have values between 0 and 1. Consequently, the temperature
derived from the thermal infrared using the Wien's displacement law and Stefan-
Boltzmann law is less than the true surface temperature. The ability to derive the 'true'
