Urban Remote Sensing

Physical Principles of imaging: electro-magnetic spectrum

Electro-magnetic radiation

• All energy used in remote sensing is originally radiated from the sun

• E-M energy composed of many discrete units, called photons, (or quanta) whose energy is proportional to the frequency of the radiation

• Energy travels in waves at a particular frequency• The different types of energy differ by frequency

and wavelength• Generally frequency and wavelength are inversely

related

DefinitionsWavelength: distance from one wave peak to the next

Frequency: number of peaks passing a fixed point in space per unit time

Wave properties

• Characterised by wavelength () and frequency ()• Linked by the equation

c (i)

• Where c is the speed of propagation of electromagnetic energy in a vacuum (approximately 3x108 m/s)

Lillesand

and Keifer

a

b

c

Visible, Thermal and microwave energy

• In the reflective spectral region, we are more concerned about the reflective properties of an object. But in the thermal spectral region, we have to rely on the emittance of an object

• If we want to measure the temperature ie. sense the emittance from objects (at longer wavelength), we will have to either use very sensitive devices or use less sensitive device to view a larger area to get sufficient amount of energy.

• At wavelengths longer than the thermal infrared region, i.e. at the microwave region, the energy (radiation) level is very low. Therefore, we often use human-made energy source to illuminate the target (such as Radar) and to collect the backscatter from the target.

ALOS PALSAR Synthetic Aperture Radar image HH Polarisation

Advanced SpaceborneThermal Emission and Reflection Radiometer (ASTER) visible band

2(green) and thermal band13

November 2004

Units of measurement of EM spectrum

•micrometer – 1 X 10-6 m

•no clear dividing line

•divisions more to do with methods for remote sensing

Emission by illumination source

• The sun (surface temperature of 5800OK) has a peak emission with a wavelength of 0.5m (yellow) band and is the primary source for remote sensing radiation

• The earth has a surface temp of 300OK and radiates a peak (night time) wavelength of 10m (infrared)

Energy Emission

• The warmer an object, the more energy it will emit• A black body is an ideal radiator which totally

absorbes and re-emits all energy incident on it: actual objects only approach this ideal

• The range and amount of energy emitted by a black body object increases with temperature

• As temperature increases there is a shift to shorter wavelengths in the peak of blackbody radiation

• Emissivity also varies with the type of material eg. water, concrete, grass

Instrument basics

• Remote sensing instruments operate in all regions except those of radio waves, X and gamma rays

• Each instrument will only be able to detect a particular portion (band) of the spectrum (visible, infrared, microwave etc)

• Electronic detectors can detect a range of radiation approximately three times as large than the human eye and are called multispectral scanners

Relationship between visible wavelengths and other parts of the spectrum used in remote

sensing

Energy Recorded by the Sensing System

• The amount of energy recorded is a function of the energy of the photons in the band and the number of photons received by the detector which depends on the amount emitted and the amount absorbed by the intervening media (atmosphere or water)

Atmospheric absorption

• Atmosphere has effect on intensity and spectral composition of radiation available to sensing system, by absorption and scattering

• Path length: all radiation detected by remote sensors passes through some distance, or path length of the atmosphere

Atmospheric absorption- mainly affects long wavelelength thermal energy

• Certain wavelengths are more prone to absorption by the earth’s atmosphere than are others eg. longer wavelengths

• The amount of radiation at these wavelengths reaching the earth’s surface are much less

• Those wavelengths which do penetrate the atmosphere (atmospheric windows: refer to Fig. 1.5) are visible, near infrared, thermal infra-red and microwave bands

Atmospheric effects on short Visible wavelengths

Atmospheric scattering of visible wavlengths

‘The unpredictable diffusion of radiation by particles in the atmosphere’

• Rayleigh Scattering: occurs when radiation interacts with atmospheric particles that are much smaller in diameter than the wavelength of interacting radiation eg. atmospheric molecules and other tiny particles. Why is the sky blue?

• Mie Scattering: exists when diameter of atmospheric particles are equal to the wavelengths of energy being sensed eg. water vapour and dust

• Non-selective Scattering: when diameter of atmospheric particles are much larger than wavelengths of energy being sensed eg. water droplets

Effects of atmospheric scattering: LANDSAT ETM+ Band 1 (blue) and 4 (near infra-red)

Air photos – true colour and false colour