Remote Sensing Part 2Review : Color TheoryReview: Imagery / Raster Data ModelReview: Spatial Resolution & Image FootprintReview: Temporal ResolutionReview: Spectral ResolutionAtmospheric InteractionsInteractions with the AtmosphereScatteringSlide 10Rayleigh ScatteringSlide 12Slide 13Mie ScatteringAbsorptionAbsorption (Cont.)Atmospheric WindowsSolar Output, Atmospheric Windows, & Sensor Spectral ResolutionEnergy Measured At SensorImage Pre-ProcessingRadiometric Correction: Atmospheric CorrectionGeometric CorrectionSlide 23Image Spatial Enhancement: FilteringFilter ExamplesSlide 26Image Spectral Enhancement: StretchingImage Enhancement Example:Contrast Enhancement example:Image DisplaySlide 31Slide 32Remote Sensing Part 2Atmospheric Interactions & Pre-processingReview : Color Theoryred+green=yellowgreen+blue=cyanred+blue=magenta•red(255) + green (255) + blue (255) = white•red(0) + green (95) + blue (0) = dark green•red(255) + green (0) + blue (255) = purple•red(0) + green (255) + blue (255) = cyan•red(170) + green (170) + blue (170) = gray•red(0) + green (0) + blue (0) = blackReview: Imagery / Raster Data Model101030255301030301. Space is covered continuously with cells2. Each cell has one number PER SPECTRAL BAND indicating the amount of energy received from the cell3. The cell is called pixel (picture element)4. The size of the pixel is the spatial resolution sensorReview: Spatial Resolution & Image Footprint185 kmswath175kmsceneLandsatSatellite ground track705kmSpatial ResolutionPixel size=(30x30m)Review: Temporal ResolutionReview: Spectral ResolutionAtmospheric Interactions•Key Concepts–Scattering–Absorption–Atmospheric WindowsInteractions with the Atmosphere Particles and gases in the atmosphere can affect the incoming electromagnetic radiation. These effects are caused by the mechanisms of scattering and absorption.ScatteringScattering occurs when particles or gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path. sunScattering•Scattering is affected by: –The wavelength of the radiation–The size of the particles or gas molecules that scatter the energy–The abundance of particles or gas molecules that scatter the energy–The distance the radiation travels through the atmosphere•There are 2 major types of scattering: –Rayleigh scattering–Mie scatteringRayleigh Scattering•Occurs when radiation interacts with atmospheric molecules that are much smaller in diameter than the wavelength of the interacting radiation–Atmosphere composed mostly of N2 (78 %) and O2 (21%)–O2 molecule diameter ~0.000316 * μm (μm = 10-6 m)–Wavelength of green light is ~ 0.5 μm•Shorter wavelengths are more readily scattered•Responsible for blue skies and red sunsets•Rayleigh scattering occurs mostly in the upper regions of the atmosphere (above the troposphere)Rayleigh Scattering•Short wavelengths are scattered by this mechanism more than long wavelengthshttp://en.wikipedia.org/wiki/Image:Rayleigh_sunlight_scattering.pngThe sky is blue because of Rayleigh Scattering. Shorter wavelengths (i.e. blue) of the visible spectrum are scattered more than longer visible wavelengths. So why isn’t the sky indigo or violet?Rayleigh ScatteringMie Scattering•Mie scattering exists when atmospheric particle diameters essentially equal to or larger than the wavelengths of the energy•Caused by water vapor, dust, pollen, smoke, etc.•Affects all wavelengths of light•Responsible for white clouds•Mie scattering occurs mostly in the lower portions of the atmosphere where larger particles are more abundantAbsorptionAbsorption is the other main mechanism at work when electromagnetic radiation interacts with the atmosphere. In contrast to scattering, this phenomenon causes molecules in the atmosphere to absorb energy at various wavelengths. Ozone, carbon dioxide, and water vapor are the three main atmospheric constituents which absorb radiation.Absorption (Cont.)•Ozone serves to absorb the harmful (to most living things) ultraviolet radiation from the sun. Without this protective layer in the atmosphere our skin would burn when exposed to sunlight.•Carbon dioxide is referred to as a greenhouse gas. It tends to absorb radiation strongly in the far infrared (heat) portion of the spectrum - the area associated with thermal heating. •Water vapor absorbs much of the incoming long wave infrared and shortwave microwave radiation (between 22mm and 1m). •O2 and N2 absorb very short wavelengths like X-Rays and Gamma RaysAtmospheric WindowsThose areas of the spectrum which are not severely influenced by atmospheric absorption and thus, are useful to remote sensors, are called atmospheric windows.Solar Output, Atmospheric Windows, & Sensor Spectral ResolutionEnergy Measured At SensorImage Pre-Processing•Radiometric Corrections–Changing the image data BVs to correct for errors or distortions•Atmospheric effects (scattering and absorption)•Sensor errors•Geometric Corrections (a.k.a. georectification)–Changing the geometric/spatial properties of the image data –Done to link pixels with locations on the ground using a coordinate system–Accuracy is determined using the Root Mean Standard Error (RMSE)•Topographic Correction–Geometric correction that makes use of a 3-D surface (e.g., a DEM)•Image Enhancement–Spatial Enhancement: Filtering–Spectral Enhancement: StretchingRadiometric Correction: Atmospheric Correction•Scattering conditions are not constant, so reflectance values recorded by a sensor might vary despite an unchanged surface and identical energy input (e.g., anniversary date imagery)•Various algorithms are used to correct for each type of scattering.–Most common type = dark object subtraction•Classifications can be made without correction. Often no correction is used. •Atmospheric correction is especially important when the spectral characteristics of two images are being compared directly (i.e., not post-classification).Geometric Correction•Four Basic Steps of Rectification1. Collect ground control points (GCPs)2. “Tie” points on the image to GCPs.3. Transform all image pixel coordinates using mathematical functions that allow “tied” points to stay correctly mapped to GCPs.4. Resample the pixel values (BVs) from the input image to put values in the newly georeferenced image5. The differences between the user-defined tie point locations and