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GEOG 380 > Topic 9: Introduction to Remote Sensing > Flashcards

Topic 9: Introduction to Remote Sensing Flashcards

1
Q

Explain the wave and particle theory of EMR

A

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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

A

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5
Q

Remote sensing: definition

A

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

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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”
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7
Q

Wavelength

A
  • Distance between crests (or troughs) of wave form

- measured in micrometers or nanometers

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8
Q

Frequency

A
  • # of crests that pass a point per unit time (one second)

- Usually measured in megahertz or gigahertz

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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

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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
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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

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12
Q

Blackbody Emission Spectra

A

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

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13
Q

Radiant flux

A
  • Radiant energy per unit time (Joules/ second)

- Watts hitting surface

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14
Q

Irradiance

A
  • Radiant flux that is incident on a surface

- Watts/m^2

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15
Q

Radiant emittance or exitance

A
  • Amount of energy leaving a surface
  • Thermal
  • Watts/m^2
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16
Q

Radiance

A

Radiant flux leaving a surface within a given solid angle

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17
Q

RS Instruments and EMR

A

Remote sensing instruments collect data across specific wavelengths, depending on the instrument’s purpose, platform, and technology

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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

A
  • 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
Q

Atmospheric windows

A
  • 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
Q

Reflectance

A
  • 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)
27
Q

Specular Reflection

A

Incoming radiation is reflected in a single direction

- Mirror-like reflectance from a ‘smooth’ surface

28
Q

Diffuse Reflection

A

Incoming radiation is reflected across many angles
- ‘Rough’ surface consisting of many specular planes

Lambertian Surface: an ideal diffuse reflector

29
Q

Wavelength Dependence

A
  • 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
Q

“Active vs “Passive”

A

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
Q

REFLECTANCE CURVES

A
  • Materials interact with EMR in different ways

- An object’s pattern of reflectance across different wavelengths is called its spectral signature

32
Q

Reflectance curve for water?

A
  • Relatively low
  • Wavelength: .4-.7
  • percentage of radiation: 5-7%
  • water absorbs longer wavelengths and reflects shorter wavelengths
33
Q

Reflectance curve for vegetation?

A
  • 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
34
Q

Reflectance curve for dry bare soil?

A
  • 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
35
Q

Leaf structure and Reflectance

A
  • Blue and red are largely aborbed
  • Photographic IR reflects off different cells
  • How much is reflected is dependent on he wavelength
36
Q

TM bands and Spectral Reflectance

A

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
Q

Types of resolution and their definitions?

A

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)

38
Q

Resolution in Remote Sensing

A

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

39
Q

Spectral Resolution

A

Multi spectral: more than one
Hyperspectral: might have hundreds
Panchromatic: one band records and entire part of spectrum

40
Q

The Decimal System of Numbers

A

How many different levels = bit depth

think of hundredths table

41
Q

The Binary System of Numbers

A

Same concept as decimal system of numbers, but the base is 2

- each column = one bit

42
Q

Common Raster Bit Depth

A

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
Q

“Spaces” in image analysis

A

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
Q

How we display imagery

A

Basic colour theory

  • Additive
  • Subtractive

Colour “composites” and multiband images

GEOG 380 (7 decks)

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