INTRO & FUNDAMENTALS I (Ch. 1 & 2)

Question 1

What is remote sensing?

Answer 1

“The science of deriving information about an object fri measurements at a distance from the object.” - Landgrebe “The use of instruments or sensors to ‘capture’ the spectral and spatial relations of objects and materials observable at a distance - typically from above them.” - NASA Remote sensing characterizes different targets based on the electromagnetic energy emitted or reflected. RS requires understanding of EM and how it interacts with targets and the atmosphere.

Question 2

What are two determinants in the origins and early development of Remote Sensing?

Answer 2

1) Aerial photography 2) Military applications

Question 3

What is remote sensing data made up of?

Answer 3

Remotely sensed data is electromagnetic (EM) radiation reflected or emitted from the earth’s surface. Examples are visible light, x-rays, radio waves etc.

Question 4

What are Passive & Active Remote Sensing?

Answer 4

1) Remote Sensing requires a source of energy to characterize different targets or land cover types. Active RS: The sensor emits energy pulses and records how long it takes for them to come back (ex. radar, sonar, lidar, etc.). Passive RS: The sensor records the solar energy reflected or emitted back from the ground or atmosphere (ex. Landsat, SPOT).

Question 5

Passive Remote Sensing

Answer 5

1) Remote Sensing requires a source of energy to characterize different targets or land cover types. Passive RS: The sensor records the solar energy reflected or emitted back from the ground or atmosphere (ex. Landsat, SPOT). Passive sensors detect natural energy this is reflected or emitted

Question 6

Electromagnetic Energy - GENERAL

Answer 6

• Energy that travels as waves along straight paths at the speed of light - ex. solar energy. - EM energy is created a number of ways including nuclear reactions within the sun - ex. visible light, x-rays. - All objects emit, reflect, or absorb EM radiation, with the exception of things at absolute zero. - Different targets interact with EM differently -> we can obtain information about the characteristics of an object by analyzing the EM radiation that it either emits or reflects.

Question 7

Electromagnetic Radiation - PROPERTIES

Answer 7

WAVELENGTH, FREQUENCY, & VELOCITY - EM radiation propagates as a wave motion at the speed of light. The parameters that characterize a wave motion are wavelength, frequency, and velocity. - Wavelength, frequency, and velocity are used to characterize the EM measured at the sensor and thus the type of targets being observed. Wavelength: it is the distance between two crests or peaks. Frequency: it is the number of crests passing a fixed point per unit of time. * C=&v C (velocity) is constant so higher frequencies are associated with shorter wavelengths (and lower frequencies with longer wavelengths.

Question 8

What is Wavelength as it refers to properties of Electromagnetic Radiation?

Answer 8

• EM radiation propagates as a wave motion at the speed of light. The parameters that characterize a wave motion are wavelength, frequency, and velocity. Wavelength: it is the distance between two crests or peaks. Wavelength is measured in length units: meters or some factor of meters

Question 9

Electromagnetic Spectrum (EMS)

Answer 9

• Targets reflect/emit energy as a wide range of electromagnetic waves. - A continuum of EM energy ranging from the shorter wavelengths (x-rays) to the longer wavelengths (radio waves). - Visible light is a very small portion of the EMS and can be divided up into different colors. - EMS is divided into different regions with similar characteristics according to wavelength range (visible is 0.4 - 0.7 um [micrometers]). - The color of a target is determined by the color of the light it reflects (does not absorb).

Question 10

Spectral Signature

Answer 10

The spectral signature is the key to identifying targets. - Spectral signatures are generated based on the degree to which energy is reflected/emitted from different regions of the spectrum. - Targets reflect energy differently across the EMS spectrum (e.g., soil vs. forest). - Reflectance: portion of the incoming energy that is reflected back to the sensor. - Spectral signature is the pattern of reflectance values across the EMS. Different objects interact and reflect EM radiation differently across the EM spectrum (which creates a spectral signature). - The spectral signature is important because different materials/surfaces can be distinguished by comparing their spectral signatures. Ex. Vegetation is green because it reflects on the green portion of the EM spectrum. However it reflects the most on the near infrared.

Question 11

Digital Number

Answer 11

• Satellite sensors measure the energy emitted or reflected from each pixel on the ground within a particular band (portion of the EMS) and store it is as number (DN). - Most sensors record multiple images of the same area (one image per band).

Question 12

BANDS

Answer 12

LANDSAT Band 1 - Blue 0.4 - 0.5 Band 2 - Green 0.5 - 0.6 Band 3 - Red 0.64 - 0.7 Band 4 - NIR 0.78 - 0.9

Question 13

Terrestrial Remote Sensing

Answer 13

Focuses on the measurement and analysis of EM radiation and how it interacts with the atmosphere and Earth’s surface.

Question 14

Using EM to get information on Target

Answer 14

You can characterize several attributes (composition, structure, etc.) of a target by interpreting and analyzing the EM reflected by it.

Question 15

Colors

Answer 15

White light: combination of all colors of the visible spectrum at 100% saturation. Black: absence of visible light (wavelengths from that part of the spectrum) Gray: Colors are mixed in equal proportions but low %. Grayscale is a common way to represent single bands in RS.

Question 16

RS Components

Answer 16

Question 17

Radiance vs. Reflectance

Answer 17

Reflectance: - Dimensionless (0 to 1 or %) - Proportion of energy reflected from a surface relative to the energy incident upon it.

Radiance:
– Expressed using radiance units (W m-2sr-1) or (W m-2μmsr-1) -Radiance at sensor:what the sensor is measuring

Advantage: radiance changes over time due to changes in incident energy, but reflectance accounts for changes in incident energy

Question 18

Interactions with the atmosphere: ATMOSPHERE

Answer 18

The atmosphere interacts with the solar energy affecting the quality of the data measured by the sensors and the images obtained. - Aerosols introduce “haze” in the imagery, reducing contrast

Question 19

Interactions with the atmosphere: 3 INTERACTION PROCESSES

Answer 19

Atmospheric gasses (e.g., oxygen, carbon dioxide) and particles modify solar radiation due to two main physical processes: Absorption and Scattering Absorption: Scattering: Transmission: radiation passes through the atmosphere without significant attenuation (reduction).

Question 20

Interactions with the atmosphere: ABSORPTION

Answer 20

• The atmosphere is a selective filter for different wavelengths (attenuates radiation). - RS based observations are less effective (or not possible) in spectral regions (wavelength ranges) where the EMR is attenuated or blocked by the atmosphere). - Atmospheric absorbers: oxygen, carbon dioxide, water vapor, ozone absorb energy at specific wavelengths.

Question 21

Interactions with the atmosphere: ATMOSPHERIC WINDOW

Answer 21

• Regions of the spectrum with relatively high transmission and minimal atmospheric effect.. - Energy is severally attenuated outside the atmospheric windows and not useful in RS. - Transmittance is high (ability to transmit energy). Transmission does not effect EMR.

Question 22

Interactions with the atmosphere: SCATTERING

Answer 22

• Redistribution of the incident beam of Electromagnetic radiation in all directions due to particle or gas molecules in the atmosphere. - Result: attenuation and change in the original path of EM radiation. The radiation is redirected back to space, the sensor, and/or the Earth’s surface. - Amount of scattering depends on: the relationship between the particle diameter (pd) and wavelength of incident radiation (wi).

Question 23

Interactions with the atmosphere: 3 TYPES OF SCATTERING

Answer 23

1) Rayleigh Scattering (clear sky scattering): - pd (particle diameter) <<< wi (incident wavelength). - Causes the shorter wavelengths (blue) to scatter more than longer ones (red) and haze in imagery. - Primary scatterer in the upper atmosphere and is why the sky is blue. 2) Mie Scattering: - pd ~ wi (e.g., dust, pollen, smoke). Particles are larger than in Rayleigh - not as wave dependent as Raleigh (produces white light from mist and fog). 3) Non-selective Scattering: - pd >>> wi (water droplets, dust) - all visible wavelengths are equally scattered (fog and clouds appear white - red + green + blue light = white light)

Question 24

Interactions with the surface (targets): 3 TYPES OF INTERACTIONS

Answer 24

Solar radiation, after passing through the atmosphere, interacts in 3 ways with the surface: 1) Absorbed: absorbed and transformed (e.g., photosynthesis, warming). 2) Transmitted: passes through the target. 3) Reflected: reflected and redirected (bounces off target). The % of energy reflected, absorbed, or transmitted depends on: - Characteristics of the target - Incident wavelength - Angle of illumination

Question 25

Interactions with the surface (targets): TYPES OF REFLECTANCE

Answer 25

How a target reflects depends mainly on the surface texture: - Specular: smooth surfaces, energy directed away in a single direction. - Diffuse: rough surfaces, energy directed away in all directions.

Question 26

Variability in Spectral Signatures in RS

Answer 26

Variability due to: - Sun position and time of day - Atmospheric conditions & scattering - Viewing angle of the sensor relative to the surface - Sensor design (like spectral resolution) - Condition of the target: phenology or season, water turbidity

Question 27

Spatial Resolution

Answer 27

Spatial resolution influences spectral signature. - pure pixels are rare and will most likely be mixed. - a larger pixel has more chances of being mixed than a smaller one.

Question 28

What are Passive & Active Remote Sensing?

Answer 28

1) Remote Sensing requires a source of energy to characterize different targets or land cover types. Active RS: The sensor emits energy pulses and records how long it takes for them to come back (ex. radar, sonar, lidar, etc.). Passive RS: The sensor records the solar energy reflected or emitted back from the ground or atmosphere (ex. Landsat, SPOT).