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Unit 3. Remote Sensing Basics

Unit 3. Remote Sensing Basics. From data to information Reflection of energy Earth’s atmosphere and atmospheric windows Emission of energy Multispectral information Orbits. DATA TO INFORMATION. Complex reality of data processing. Data levels. Level 0 is raw data

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Unit 3. Remote Sensing Basics

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  1. Unit 3. Remote Sensing Basics • From data to information • Reflection of energy • Earth’s atmosphere and atmospheric windows • Emission of energy • Multispectral information • Orbits

  2. DATA TO INFORMATION

  3. Complex reality of data processing

  4. Data levels • Level 0 is raw data • Level 1 adds in ancillary data such as atmospheric water vapor to make corrections • Level 2 • Level 3 is a final product such as a map of vegetation, they involve some type of interpretation (could be on any media) • Current issue: Having products that are compatible with the requirements and analysis software belong to the end user (e.g. oil industry and icebergs)

  5. Maps needed by the offshore oil industry? • How do they want their information • What are their time requirements?

  6. REFLECTED AND EMITTED ENERGY • Two types of energy that can be recorded by remote sensors: • Solar energy reflected by an object on Earth’s surface OR • Energy emitted from an object on Earth’s surface

  7. Reflected energy Sun’s energy (shortwave) hits the earth, some is absorbed (turned into heat) and some shortwave is reflected. We, and remote sensing instruments, see reflected light but only after it has passed through the atmosphere.

  8. Albedo Albedo is the percentage of solar energy (shortwave radiation) reflected from the Earth back into space. Average earth albedo is 30-35% The Albedo depends on incoming frequency (usually numbers given are for visible light)

  9. Reflection by earthly objects • What a sensor ‘sees’ depends on what happens when the suns energy hits it. • Specular vs diffuse reflection (usually objects are neither one or the other) • Also ‘smooth and specular depends on the wavelength

  10. Electromagnetic Energy and the Atmosphere • Atmosphere as being between the sun and the ground and between the ground and the remote sensor • Within the atmosphere, energy is • Transmitted • Absorbed and re-radiated • Scattered • Absorbed and scattered light reduce the amount of energy the remote sensor receives (particularly if there is a lot of atmosphere between the ground and the instrument)

  11. Atmospheric Scattering • Scatterers: Dust, smoke, large molecules such as water vapor (or droplets) • Atmospheric scattering causes blue sky .. Short (blue) waves are scattered more than long wavelengths, we are looking at scattered light. • A reminder that we can see in shadows because of scattered light

  12. Effect of scattering on energy recorded by a remote sensor • Atmospheric brightness (scattering) masks variations on the ground • Details are lost .. Like driving into the sun on a hazy day

  13. Atmospheric Absorption • Energy absorbed is reradiated at longer wavelengths, usually on thermal infared (ie heat!) • Gases that absorb shortwave radiation and reradiate heat are greenhouse gases … largely water vapor, carbon dioxide, methane, ozone. • Location of these gases?

  14. Atmospheric Windows • Some wavelengths are transmitted through Earth’s atmosphere with little absorption • Ranges of wavelengths that can get through the atmosphere (be transmitted) are known as atmospheric windows • Important windows: • UV-Visible-near IR 0.3 – 1.2μm • Bands in the mid-IR • Microwave

  15. Electromagnetic (EM) radiation emission • All objects with temperatures above absolute zero (-459.7°F) emit EM radiation • Quantity of EM emitted depends on: temperature and emissivity • Emissivity: how effectively a material radiates (a ‘blackbody’ has a perfect emissivity of 1) • Most emitted energy is 3-14 μm in the thermal infrared • Also some emitted energy in the microwave portion of the spectrum • Quantities of emitted energy are low, hence low resolution

  16. Passive microwave Image of Sea Ice

  17. bands of data Bands are when we artificially divide the returning energy into segments (In reality there are no sharp divisions; the spectrum is a continuum)

  18. Continuous spectrum to bands

  19. Why bands: Spectral properties of objects • Surfaces interact with the electromagnetic spectrum • Vegetation looks green because it is mainly the green portion of the spectrum that is reflected • Each material has a spectral signature

  20. Spectral reflectance curves Segments of the spectrum (bands) are selected such that people can differentiate between two crops or minerals. Hence the development and use of multispectral imagery

  21. Spectral Bands in common use • Generally bands in the visible, near-infrared and mid infrared • Also UV (absorbed and scattered by atmosphere so little used, and not from satellites).

  22. Vegetation classification ‘… almost everything is green in the true color image on the left (although subtle variations in the color are noticeable). In the false color image on the right, it is easier to see vegetation with higher reflectance.’ This enables vegetation classification. http://www.ctahr.hawaii.edu/miuralab/projects/makaha/intro_RS.html

  23. Applications of bands • Visible blue: greatest water penetration, also vegetation, geology, culture • Visible green: vegetation, geology, culture, water quality • Visible red: vegetation health, geology, culture • Panchromatic (wide range of wavelengths • Near-infrared: vegetation health • Thermal Infrared: detecting heat in buildings, soil moisture

  24. Orbit • Many remote sensing platforms are designed to follow a north-south orbit which, in conjunction with the Earth's west-east rotation, allows coverage of most of the Earth's surface over a certain period of time.

  25. Swaths In polar orbits (of which there are a large number of types), coverage of earth is built up swath by swath The orbit passes near both geographic poles

  26. Ascending swaths The orbits overlap the least at the equator and the most near the poles.

  27. Ascending and descending swaths

  28. Geosynchronous Orbit • Orbit at the same speed as the earth, so is at the same location relative to the earth • About 35,786 km above earth • Uses: Communications, weather

  29. GEOS (Geostationary Operational Environmental Satellite) • Provides weather data (NOAA) • Latest image: http://www.goes.noaa.gov/

  30. Remote sensing outside the electromagnetic spectrum • Acoustic energy - sound • Sonar systems • Seismic surveys And since I have room on this slide: Units Kilometers (km): 1000 meters Meter (m) = 3.3 ft (more accurately 3.2808399 ft) Centimeter (cm): 0.01m Millimeter (mm): 0.001m Micrometer (μm):0.000001m (AKA micron)

  31. Wavelength and Frequency • Speed of light (c) = frequency (f) wavelength (λ) • Speed of light is a constant at 3 x 108 meters/sec Frequency is the number of cycles of a wave to pass some point in a second. The units of frequency are thus cycles per second, or Hertz (Hz). The shorter the wavelength the higher the frequency and the greater the energy, high frequency microwaves can pass through our soft tissues

  32. Red Sunsets? • Light must travel farther through the atmosphere before it gets to you. • More of the light is reflected and scattered. • Only the longer wavelengths (reds)are left in the direct beam that reaches your eyes.

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