14: What is remote sensing?

to uncover unseen features, processes, on the earth’s surface and the atmosphere, etc.

electromagnetic wave has an electric and magnetic wave field which are perpendicular to each other laser and radar waves are well defined, natural light has more irregular oscillation It travels at the speed of light frequency is the number of wave crests that pass in a second wavelength is the distance between crests

field model and raster data structure containing pixels in an image pixels are assigned one or more digital numbers relating to a particular band, representing reflectance values 0=black 255=white histogram plots the grey values and frequency of which they occur

• energy source or illumination emits radiation • some of the radiation is absorbed or scattered in the atmosphere • the radiation interacts with the earth’s surface and is either absorbed or reflected • the reflected radiation is recorded by the satellite sensor • the satellite communicates with other satellites/ground control for transmission, reception and processing of data • data is sent to computer for interpretation and analysis • data is sent to GIS for application

• the energy source • the atmosphere • energy-matter interactions at -the earth’s surface • the sensor • data processing and supply system • multiple data users

electromagnetic waves

Radio Microwave Infrared Visible Ultraviolet X-ray Gamma Ray Some can penetrate the atmosphere but not all Smaller wavelength, higher frequency, higher energy, higher temperatures

visible colour composite: red-green-blue (RGB) false colour composite: Near infrared => red Red => green green => blue

Rayleigh scatter - radiation interacts w/ atmospheric molecules and other tiny particles much smaller in diameter than wavelength of interacting radiation Atmospheric absorption - by water vapour, carbon dioxide and ozone

short waves are scattered more by molecules in the atmosphere than longer waves

Gamma rays, x-rays and ultraviolet light is blocked by the upper atmosphere Visible light observable from earth with some atmospheric distortion Most of the infrared spectrum is absorbed by the atmospheric gasses (best observed from space) at 10 micrometers thermal infrared escapes from the earth’s surface Radio waves are observable from Earth. Long-wavelength radio waves are blocked.

• Absorption • Transmission • Reflection we want to know about reflection

specular reflection - smooth surfaces (road) diffuse reflection - rough surfaces (tree canopy)

chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green and near infrared

reflects more of the blue and green light but not much red or infrared

look at the spectral signature/response curve - different features reflect and absorb different wavelengths Example: Green vegetation - green and near infrared Red edge - difference between red reflectance and near infrared reflectance - can help identify what is on the surface

check reflectance values using spectoradiometers on the ground, check what is actually there, important part of satellite remote sensing

a satellite will measure specific bands, each is a range of wavelengths. it will not measure continuously across all wavelengths of the EM spectrum images are a composite of these bands, combined they are a true colour image or false colour image (depending which bands are combined)

Spatial - pixel size Spectral - how many bands Temporal - how often Radiometric - how accurate

Landsat MSS 79m Landsat TM 30m SPOT 20m ASTER 15m IKONOS 0.8m

number of bands (wavelength ranges) detected across spectrum Red Green Blue Near infrared Used to construct spectral response curves

number of digital units used to store data collected by a sensor commonly expressed as the number of bits (binary digits) needed to store max value range • 6 bit • 8 bit • 12 bit

number of measurements in a given time span (repetitions of sensing over a given area) e.g. satellite overpass / aerial photography frequency allows to measure change patterns: short term (weather events) cyclic change (seasonal) sustained change (urban expansion) multidirectional change (drought stress and recovery)

spectral distribution of reflected sunlight and self-emitted energy is far from uniform solar level vary with time and location, different earth surface materials emit energy with varying degrees of efficiency