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Geographic Information Science > 14: What is remote sensing? > Flashcards

14: What is remote sensing? Flashcards

1
Q

Remote sensing

A

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

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2
Q

WHAT IS ELECTROMAGNETIC RADIATION

A

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

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3
Q

LIST BASIC CHARACTERISTICS OF REMOTE SENSING IMAGES

A

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

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4
Q

STEPS IN THE REMOTE SENSING PROCESS

A
  1. energy source or illumination emits radiation
  2. some of the radiation is absorbed or scattered in the atmosphere
  3. the radiation interacts with the earth’s surface and is either absorbed or reflected
  4. the reflected radiation is recorded by the satellite sensor
  5. the satellite communicates with other satellites/ground control for transmission, reception and processing of data
  6. data is sent to computer for interpretation and analysis
  7. data is sent to GIS for application
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5
Q

ISSUES IN REMOTE SENSING

A
  • the energy source
  • the atmosphere
  • energy-matter interactions at -the earth’s surface
  • the sensor
  • data processing and supply system
  • multiple data users
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6
Q

Basic measurements unit

A

electromagnetic waves

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7
Q

wavelengths of EMR

A 
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

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8
Q

Remote sensing images

A

visible colour composite: red-green-blue (RGB)

false colour composite:
Near infrared => red
Red => green
green => blue

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9
Q

Energy-atmosphere interactions

A

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

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10
Q

Why is the sky blue?

A

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

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11
Q

Atmospheric windows

A

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.

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12
Q

3 interactions when energy strikes the earth’s surface

A
  1. Absorption
  2. Transmission
  3. Reflection

we want to know about reflection

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13
Q

two types of reflection

A

specular reflection - smooth surfaces (road)

diffuse reflection - rough surfaces (tree canopy)

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14
Q

reflection from vegetation

A

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

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15
Q

reflection from water

A

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

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16
Q

how can we identify features from space?

A

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

17
Q

ground truthing

A

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

18
Q

bands

A

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)

19
Q

image data resolution (4)

A

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

20
Q

Spatial resolution

A 
Landsat MSS 79m
Landsat TM 30m
SPOT 20m
ASTER 15m
IKONOS 0.8m
21
Q

Spectral resolution

A 
number of bands (wavelength ranges) detected across spectrum
Red
Green
Blue
Near infrared

Used to construct spectral response curves

22
Q

Radiometric resolution

A

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
23
Q

Temporal resolution

A

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)
24
Q

issue: energy source

A

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

25
Q

issue: atmosphere

A

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

26
Q

issue: energy-matter interactions at the earth’s surface

A

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

27
Q

issue: the sensor

A

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

28
Q

issue: data processing and supply system

A

processing sensor data for machine readable and human interpretable form requires planning, hardware, time, experience and additional data

29
Q

issue: multiple data users

A

no single data acquisition mission, application and analysis procedure will satisfy the needs of all data users

Geographic Information Science (20 decks)

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