Topic 9: Introduction to Remote Sensing Flashcards
(44 cards)
1
Q
Explain the wave and particle theory of EMR
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2
Q
Explain the different types of energy-matter interactions
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3
Q
Explain spectral reflectance curves
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4
Q
Explain how band selection and assignment in a RGB model produces different colour images
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5
Q
Remote sensing: definition
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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
6
Q
How is EMR generated?
A
- 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”
7
Q
Wavelength
A
- Distance between crests (or troughs) of wave form
- measured in micrometers or nanometers
8
Q
Frequency
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- # of crests that pass a point per unit time (one second)
- Usually measured in megahertz or gigahertz
9
Q
Wave theory
A
- 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
10
Q
Particle theory
A
- 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
11
Q
Black bodies
A
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
12
Q
Blackbody Emission Spectra
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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
13
Q
Radiant flux
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- Radiant energy per unit time (Joules/ second)
- Watts hitting surface
14
Q
Irradiance
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- Radiant flux that is incident on a surface
- Watts/m^2
15
Q
Radiant emittance or exitance
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- Amount of energy leaving a surface
- Thermal
- Watts/m^2
16
Q
Radiance
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Radiant flux leaving a surface within a given solid angle
17
Q
RS Instruments and EMR
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Remote sensing instruments collect data across specific wavelengths, depending on the instrument’s purpose, platform, and technology
18
Q
Material interactions
A
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
19
Q
Transmission & refraction
A
- “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
20
Q
Atmospheric scattering
A
- 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
21
Q
Rayleigh scattering
A
- 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
22
Q
Mie scattering
A
- 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
23
Q
Non-selective scattering
A
- 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
24
Q
Absorption
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- 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’
25
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
26
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)
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Specular Reflection
Incoming radiation is reflected in a single direction
| - Mirror-like reflectance from a 'smooth' surface
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Diffuse Reflection
Incoming radiation is reflected across many angles
- 'Rough' surface consisting of many specular planes
Lambertian Surface: an ideal diffuse reflector
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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
30
"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
31
REFLECTANCE CURVES
- Materials interact with EMR in different ways
| - An object's pattern of reflectance across different wavelengths is called its spectral signature
32
Reflectance curve for water?
- Relatively low
- Wavelength: .4-.7
- percentage of radiation: 5-7%
- water absorbs longer wavelengths and reflects shorter wavelengths
33
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
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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
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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
36
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
37
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)
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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
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Spectral Resolution
Multi spectral: more than one
Hyperspectral: might have hundreds
Panchromatic: one band records and entire part of spectrum
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The Decimal System of Numbers
How many different levels = bit depth
| think of hundredths table
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The Binary System of Numbers
Same concept as decimal system of numbers, but the base is 2
| - each column = one bit
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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
43
"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
44
How we display imagery
Basic colour theory
- Additive
- Subtractive
Colour "composites" and multiband images
