14: What is remote sensing? Flashcards
Remote sensing
to uncover unseen features, processes, on the earth’s surface and the atmosphere, etc.
WHAT IS ELECTROMAGNETIC RADIATION
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
LIST BASIC CHARACTERISTICS OF REMOTE SENSING IMAGES
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
STEPS IN THE REMOTE SENSING PROCESS
- 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
ISSUES IN REMOTE SENSING
- the energy source
- the atmosphere
- energy-matter interactions at -the earth’s surface
- the sensor
- data processing and supply system
- multiple data users
Basic measurements unit
electromagnetic waves
wavelengths of EMR
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
Remote sensing images
visible colour composite: red-green-blue (RGB)
false colour composite:
Near infrared => red
Red => green
green => blue
Energy-atmosphere interactions
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
Why is the sky blue?
short waves are scattered more by molecules in the atmosphere than longer waves
Atmospheric windows
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.
3 interactions when energy strikes the earth’s surface
- Absorption
- Transmission
- Reflection
we want to know about reflection
two types of reflection
specular reflection - smooth surfaces (road)
diffuse reflection - rough surfaces (tree canopy)
reflection from vegetation
chlorophyll strongly absorbs radiation in the red and blue wavelengths but reflects green and near infrared
reflection from water
reflects more of the blue and green light but not much red or infrared
how can we identify features from space?
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
ground truthing
check reflectance values using spectoradiometers on the ground, check what is actually there, important part of satellite remote sensing
bands
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)
image data resolution (4)
Spatial - pixel size
Spectral - how many bands
Temporal - how often
Radiometric - how accurate
Spatial resolution
Landsat MSS 79m Landsat TM 30m SPOT 20m ASTER 15m IKONOS 0.8m
Spectral resolution
number of bands (wavelength ranges) detected across spectrum Red Green Blue Near infrared
Used to construct spectral response curves
Radiometric resolution
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
Temporal resolution
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)
issue: energy source
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
issue: atmosphere
atmosphere restricts ‘where we can look’ spectrally and its effects vary with wavelength, time and place. The importance of these is a function of wavelengths involved, sensor used, and sensing application at hand
issue: energy-matter interactions at the earth’s surface
spectral world is full of ambiguity, radically different material types can have great spectral similarity, hindering differentiation. Need of energy-matter interaction understanding for earth surface features at the elementary level for some materials
issue: the sensor
no single sensor is sensitive to all wavelengths. all have fixed limits in spatial and spectral sensitivity
there is tradeoff in the radiometric resolution, photographic systems have good spatial resolution but lack broad spectral sensitivity
issue: data processing and supply system
processing sensor data for machine readable and human interpretable form requires planning, hardware, time, experience and additional data
issue: multiple data users
no single data acquisition mission, application and analysis procedure will satisfy the needs of all data users
