Topic 9: Introduction to Remote Sensing Flashcards
Explain the wave and particle theory of EMR
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Explain the different types of energy-matter interactions
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Explain spectral reflectance curves
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Explain how band selection and assignment in a RGB model produces different colour images
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Remote sensing: definition
the science and art of obtaining information about an object, area, or phenomenon through analysis of data acquired by a device that is not in contact with the object, area, or phenomenon under investigation
How is EMR generated?
- EMR from the sun is generated during thermonuclear fusion
- EMR is absorbed by an atom in the form of potential energy stored in the excited state of electrons
- ## EMR is given off when the electron “looses its excited state”
Wavelength
- Distance between crests (or troughs) of wave form
- measured in micrometers or nanometers
Frequency
- # of crests that pass a point per unit time (one second)
- Usually measured in megahertz or gigahertz
Wave theory
- EM waves are perpendicular to the direction of travel
c = vl
l = wavelength v = frequency c = speed of light (3 x 10^8 m/s)
Wavelength and frequency are inversely proportioned
Particle theory
- EMR is composed of photons
- Energy of a photon is given by:
Q = hv
Q = energy of a photon, Joules (J) h = Planck's constant (6.626 x 10^-34 J.s v = frequency
Black bodies
A theoretical object that completely absorbs all incident radiation, and emits the absorbed energy at the maximum possible rate as given by the Stefan Boltzmann law
object is a perfect radiator
on earth, the closest black body is water
Blackbody Emission Spectra
Peak blackbody emittance given by Wien’s displacement law
- tells the wavelength of maximum radiation given off by a body
hotter the object = shorter the wavelength
colder the object = longer the wavelength
Radiant flux
- Radiant energy per unit time (Joules/ second)
- Watts hitting surface
Irradiance
- Radiant flux that is incident on a surface
- Watts/m^2
Radiant emittance or exitance
- Amount of energy leaving a surface
- Thermal
- Watts/m^2
Radiance
Radiant flux leaving a surface within a given solid angle
RS Instruments and EMR
Remote sensing instruments collect data across specific wavelengths, depending on the instrument’s purpose, platform, and technology
Material interactions
Comes from sun and hits atmosphere
- EMR encounters ozone in stratosphere - aborbs shortwave lengths (UV)
- what is left enters lower level of of atmosphere - troposphere (where all the weather takes place, pollutants, particular matter, water) - some of the radiation scatters off of different things. Depending what is there affects how much scattering there is
- transmitted - some makes it to the ground (some is absorbed, some is reflected, some is scattered
- EMR is also being emitted from the ground
Transmission & refraction
- “Bending” of light
- Occurs when EMR is transmitted through matter
- Index of refraction
1. 0 vacuum
1. 002926 atmosphere
1. 33 water - wavelength dependent
- When EMR passes through the atmosphere it slows down because of that it refracts
- part of the reason it is difficult to get high resolution from space
Atmospheric scattering
- Similar to reflection, but unpredictable
- operates through absorption and re-radiation by atoms or molecules
- when scattering occurs in a volume (as in the atmosphere), we specify three types: Mie, Rayleigh, Non-selective
Rayleigh scattering
- Occurs when the particles are smaller (usually < 0.1 times) the wavelength
- Caused mainly by gases in the upper atmosphere
- eg., why the sky is blue
Mie scattering
- Occurs when particles are approximately the same size as wavelength
- Caused by dust, smoke, particulates in lower atmosphere
- eg., think sunsets - prettier colours when it is dusty
Non-selective scattering
- Occurs with particles many times greater in size than wavelength
- Caused by water droplets, ice crystals in lower atmosphere
- Non-selective with respect to visible wavelengths
Absorption
- Occurs when EMR is absorbed by material and converted into other forms of energy (water vapour, CO2, oxygen, ozone, chlorophyll, minerals)
- Wavelength dependent: those not greatly affected called ‘atmospheric windows’
Atmospheric windows
- The atmosphere absorbs most of the shorter wavelengths
- By 400 nm there is visible light that passes through
- Atmospheric windows = where EMR reaches Earth’s surface
- Blocked = Absorption band
- Not blocked = atmospheric window - need window for terrestrial remote sensing
Reflectance
- Re-radiation of photons in unison, in a layer approximately 1/2 wavelength deep (bouncing off a surface)
- Measured as a ratio of the amount of radiation reflected to the amount received by the surface (usually specified by wavelength)
Specular Reflection
Incoming radiation is reflected in a single direction
- Mirror-like reflectance from a ‘smooth’ surface
Diffuse Reflection
Incoming radiation is reflected across many angles
- ‘Rough’ surface consisting of many specular planes
Lambertian Surface: an ideal diffuse reflector
Wavelength Dependence
- A single surface can act ‘rough’ at one wavelength and ‘smooth’ at another
- it is dependent on the relative size of the wavelength in question and the size of the ‘bumps’ on the surface
- at one wavelength a surface may diffuse another wavelength may be specular
“Active vs “Passive”
Passive sensors have an no on-board source of EMR
- Usually operate in the naturally-abundant visible and infrared portions of the spectrum
Active sensors carry their own source of EMR
- Usually operate in low-energy or naturally-scarce regions of the spectrum
REFLECTANCE CURVES
- Materials interact with EMR in different ways
- An object’s pattern of reflectance across different wavelengths is called its spectral signature
Reflectance curve for water?
- Relatively low
- Wavelength: .4-.7
- percentage of radiation: 5-7%
- water absorbs longer wavelengths and reflects shorter wavelengths
Reflectance curve for vegetation?
- Peaks in green visible light - absorbs blue an red preferentially
- peaks dramatically in near infrared - most sensors are focused on this area, primary spot for vegetation
Reflectance curve for dry bare soil?
- in infrared: vegetation is brighter than soil, and soil is much brighter than water
- in middle infrared: soil is going to be brighter than vegetation and water
Leaf structure and Reflectance
- Blue and red are largely aborbed
- Photographic IR reflects off different cells
- How much is reflected is dependent on he wavelength
TM bands and Spectral Reflectance
TM - thematic mapper
- old sensor
- been around in different iterations for decades: around 1985
- standard for terrestial data
- Created a sensor roughly equivalent to what we see as blue, green, and red and then near infrared
- RGB colour - we can combine 3 bands to create an image (Almost always assign LONG TO RED, and SHORT TO BLUE
- choose bands to give colour rendition we want
Types of resolution and their definitions?
Spatial:
- How narrowly defined is a representation (in terms of remote sensing/ raster = size of pixel)
Temporal
- How often observations are taken
- How long are the observations taken for
Attribute:
- How well defined is the attribute (eg., precision of preciptation measurement, number of brightness levels in an image)
Resolution in Remote Sensing
Spatial:
- spatial resolution: size of the smallest recording unit OR smallest size of feature that can be mapped or measured
- Roughly analogous to pixel size in a raster database
Temporal
- How often are observations taken
- How long are the observations taken for - important factor
Radiometric
- The precision the measurement - determines the bit depth
Spectral
- Number of portions of the EM Spectrum that are differentiated
Spectral Resolution
Multi spectral: more than one
Hyperspectral: might have hundreds
Panchromatic: one band records and entire part of spectrum
The Decimal System of Numbers
How many different levels = bit depth
think of hundredths table
The Binary System of Numbers
Same concept as decimal system of numbers, but the base is 2
- each column = one bit
Common Raster Bit Depth
8-bit integer = 1 Byte
- Stores pixel values ranging from 0-255
- Common for satellite imagery
Unsigned 16-bit integer
- Stores values ranging from 0-65535
- Common for optical and radar imagery
Signed 16-bit integer - one bit is used for sign
- Stores values ranging from -32767 - 32767
- Common for digital elevation models
32 and 64 bit real - has fraction component
- Often referred to as single and double precision
- Digital elevation models
“Spaces” in image analysis
Observation Space:
- Spatial arrangement
- Object based image analysis/ classification (homogenous areas within imagery in terms of their brightness)
Data space:
- Bivariate graph of where the combination of reflections are for pixels
- pixel based
- not incorporating spatial arrangement
Histogram:
- Brightness values within a particular image for a particular band
- Enhancing or transforming pixel values to new values
- More for visual looks
How we display imagery
Basic colour theory
- Additive
- Subtractive
Colour “composites” and multiband images
