Study Questions for Remote Sensing Midterm Exam 2024

**Review questions:** 1. **Define the following: Radiant flux, Radiance, Hemispherical reflectance, transmittance and absorptance, irradiance and exitance. Which of these does the sensor measure?**  **Radiant Flux (Φ):** Radiant flux is the total power of electromagnetic radiation emitte...

**Review questions:** 1. **Define the following: Radiant flux, Radiance, Hemispherical reflectance, transmittance and absorptance, irradiance and exitance. Which of these does the sensor measure?**  **Radiant Flux (Φ):** Radiant flux is the total power of electromagnetic radiation emitted, reflected, transmitted, or received, across all wavelengths, per unit time. It's measured in watts (W). Radiant flux quantifies the total energy that flows through a particular area.  **Radiance (L):** Radiance is the amount of radiant flux emitted, reflected, transmitted, or received by a surface, per unit area, in a specified direction, per unit solid angle. It's measured in watts per steradian per square meter (W/sr/m²). Radiance tells us the brightness or intensity of a surface as seen from a specific angle.  **Hemispherical Reflectance (ρ):** Hemispherical reflectance is the ratio of reflected radiant flux to incident radiant flux over a hemisphere above a surface. Reflectance ranges from 0 (no reflection) to 1 (complete reflection). It's essential in assessing surface properties in remote sensing.  **Transmittance (τ):** Transmittance is the fraction of radiant energy that passes through a substance or medium. It's also a value between 0 and 1. Higher transmittance means more energy passes through, which is crucial when studying atmospheric effects.  **Absorptance (α):** Absorptance is the fraction of incident radiant energy absorbed by a surface. It represents the energy that doesn't reflect or transmit through a material. Like reflectance and transmittance, it has values between 0 and 1 and is useful for understanding heat absorption and surface properties.  **Irradiance (E):** Irradiance is the radiant flux incident on a surface per unit area, measured in W/m². It represents how much energy hits a specific area, commonly used to describe incoming sunlight on Earth\'s surface.  **Exitance (M):** Exitance is the radiant flux emitted by a surface per unit area, measured in W/m². It indicates how much energy is leaving a surface, useful for understanding surface emissions and radiative cooling.  **Sensor Measurement**: Remote sensing sensors typically measure **radiance** because they capture the intensity of light emitted or reflected from surfaces at specific angles. 2. **Why does the clear sky look blue, and the sunset is a yellow reddish color?** The blue color of the sky and the reddish hue of sunsets are both due to Rayleigh scattering, which occurs when the atmosphere scatters shorter (blue) wavelengths of light more than longer (red) wavelengths. During the day, when the sun is high, blue light is scattered in all directions, making the sky appear blue. At sunset, the sun\'s light passes through a thicker layer of the atmosphere, scattering out most of the blue and green wavelengths and leaving red and orange hues. 3. **What is Lambertian surface? Describe the cosine law as mentioned in one of the lab lectures.** Lambertian Surface: A Lambertian surface is an idealized surface that reflects light uniformly in all directions, regardless of the viewing angle. In remote sensing, such a surface follows Lambert's Cosine Law, which states that the observed radiance decreases with the cosine of the angle between the incident light and the surface normal. This law helps model reflectance behavior in surface analysis. 4. **Emissions from the earth are in which part of the electro-magnetic spectrum?** Earth\'s emissions primarily fall within the thermal infrared spectrum (typically 8-14 µm). This is due to Earth's temperature, which, according to Planck's law, causes it to emit most energy at longer wavelengths compared to the sun. 5. **What is the solar constant? Does it vary with distance from the sun? Why?** The solar constant is the average amount of solar electromagnetic radiation received per unit area on a plane perpendicular to the Sun's rays outside Earth's atmosphere. It's approximately 1361 W/m² but can vary slightly with Earth's elliptical orbit around the Sun. At perihelion (closest approach), Earth receives more solar energy than at aphelion (farthest distance). 6. **What is the difference between digital numbers, radiance, apparent reflectance, and surface reflectance? Which one should I use for quantitative analysis?**  **Digital Numbers (DN):** Raw pixel values in a remote sensing image, representing the intensity of detected radiation.  **Radiance (L):** Physical measurement of light intensity per unit area, angle, and wavelength, derived by calibrating DN values.  **Apparent Reflectance:** The reflectance observed by a sensor without atmospheric corrections, typically affected by factors like atmospheric scattering.  **Surface Reflectance:** Corrected reflectance that removes atmospheric effects, offering a more accurate measure for quantitative analysis. 7. **Explain how the dark object method (DOS) of correcting for atmospheric absorption and scattering can be used to take reflectance on sensor (top-of-atmosphere) and compute surface reflectance.** DOS is used to correct atmospheric scattering and absorption by assuming that certain pixels in an image (typically water bodies or other dark objects) should have nearly zero reflectance. By subtracting these low values, it adjusts for haze and atmospheric interference, refining data from top-of-atmosphere (TOA) reflectance to approximate surface reflectance. 8. **AVIRIS was flown over the WTC after 9/11.  The temperatures of the fires were obtained from AVIRIS spectra.  How would you do this calculation?** To estimate temperatures from AVIRIS spectral data, you would apply Planck's Radiation Law or Wien's Displacement Law to the observed radiance, converting it to temperature based on spectral emission characteristics. 9. **Define four terms related with data resolution.**  **Spatial Resolution**: The size of the area represented by a pixel in an image.  **Spectral Resolution**: The width and number of spectral bands the sensor detects.  **Radiometric Resolution**: The sensor\'s ability to distinguish small differences in intensity, often defined by bit depth.  **Temporal Resolution**: The frequency at which the sensor revisits the same area on Earth. 10. **What are the three common types of image band interleaving? When would you use each?** **Band Sequential (BSQ)**: Each band is stored separately, useful for single-band processing. **Band Interleaved by Pixel (BIP)**: Bands are interleaved on a pixel-by-pixel basis, which is efficient for pixel-level analysis. **Band Interleaved by Line (BIL)**: Bands are interleaved line-by-line, providing a balance for row and spectral processing. 11. **What is a polar orbit? What is an equatorial orbit? What is a geostationary orbit? If you are the manager of the space program of Ecuador, a country located on the equator and rarely cloud free, which orbit would provide the most cloud free data? --- the least?** **Polar Orbits**: Cross over Earth\'s poles, covering the entire planet and providing cloud-free data more often. **Equatorial Orbits**: Remain above the equator, limiting coverage but suited for equatorial regions. **Geostationary Orbits**: Fixed position relative to Earth's surface, ideal for continuous monitoring but limited by cloud cover. 12. **Thermal infrared emitted from room temperature objects is maximum around what wavelength?** For room temperature objects, thermal infrared emissions peak around 10 µm, calculated using Wien's Displacement Law. 13. **When light interacts with matter, it may be reflected, (name two more).** Besides reflection, light may also be absorbed (energy retained by the surface) and transmitted (energy passing through the material). 14. **What is \'Ground Truth\' data?** Ground truth data are actual field measurements taken to validate and calibrate remote sensing data, ensuring accuracy by comparing observed satellite data with real conditions.