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Image Preprocessing and Information Extraction

Image Preprocessing and Information Extraction. Ruiliang Pu Ecosystem Science Division Department of ESPM University of California, Berkeley, USA February 24, 2004. Outline. Atmospheric correction Geometric correction Topographic correction Image enhancement Information extraction

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Image Preprocessing and Information Extraction

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  1. Image Preprocessing and Information Extraction Ruiliang Pu Ecosystem Science Division Department of ESPM University of California, Berkeley, USA February 24, 2004

  2. Outline • Atmospheric correction • Geometric correction • Topographic correction • Image enhancement • Information extraction • Summary

  3. Atmospheric correction

  4. Atmospheric correction Sun Irradiation from sun E0 Sensor Total radiance from different sources Atmospheric path radiance Transmittance Transmittance Atmosphere layer Surface reflection • A diagram showing atmosphere how to affect on RS data

  5. Atmospheric correction Interaction with the Atmosphere Scattering Absorption Scattering occurs when particles or large gas molecules present in the atmosphere interact with and cause the electromagnetic radiation to be redirected from its original path. Absorption 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.

  6. Atmospheric correction Mie scattering

  7. Atmospheric correction Rayleigh scattering Rayleigh scattering occurs when particles are very small compared to the wavelength of the radiation.

  8. Atmospheric correction Mie scattering Mie scattering occurs when the particles are just about the same size as the wavelength of the radiation

  9. Atmospheric correction Nonselective scattering Nonselective scattering. This occurs when the particles are much larger than the wavelength of the radiation

  10. Atmospheric correction Atmospheric window: It refers to the relatively transparent wavelength regions of the atmosphere Those areas of the spectrum which are not severely influenced by atmospheric absorption and thus, are useful to remote sensors, are called atmospheric windows.

  11. Atmospheric correction • Atmospheric correction of ALI imagery (Beijing, China, May 3, 2001) After Before After Before SL Liang, University of Maryland, College Park

  12. Atmospheric correction • MODIS imagery of Chinese northeastern coast, (False color composite, May 7, 2000) (After AC) (Before AC) SL Liang, University of Maryland, College Park

  13. Atmospheric correction • MODIS imagery of Chinese northeastern coast, (Natural color composite, May 7, 2000) (After AC) (Before AC)

  14. Geometric correction

  15. Geometric correction Satellite Characteristics: Orbits and Swaths • Geostationary • Sun-synchronous Geostationary: Satellites at very high altitudes (e.g., 36,000 km), which view the same portion of the Earth's surface at all times have geostationary orbits. Sun-synchronous: Satellite orbits cover each area of the world at a constant local time of day called local sun time. At any given latitude, the position of the sun in the sky as the satellite passes overhead will be the same within the same season. Orbit:The path followed by a satellite.

  16. Geometric correction Swath Swath: As a satellite revolves around the Earth, the sensor "sees" a certain portion of the Earth's surface. The area imaged on the surface, is referred to as the swath

  17. Geometric correction Geometric distortion sources • Sensing principles • Central Perspective • Cross-track • Along-track

  18. Geometric correction Orthographic &Central Perspective

  19. Geometric correction Geometric distortion sources • Platform Status • Airborne: altitude variation, velocity variation, attitude (pitch, roll, yaw) • Satellite: earth rotation, earth curvature

  20. Geometric correction • Platform Attitude http://www.cnr.berkeley.edu/~gong/textbook/

  21. Geometric correction Geometric distortion sources • Map distortion • deformation of map sheets • map projection differences • map scale variation • map generalization

  22. Geometric correction • Image georeferencing 1) Geometric rectification and image rectification recovers the imaging geometry 2) Image-to-image registration refers to transforming one image coordinate system into another image coordinating system 3) Image-to-map registration refers to transformation of one image coordinate system to a map coordinate system resulted from a particular map projection. Georeferencing generally covers 1) and 3).

  23. Geometric correction Georeferencing (geometric correction) • A process of transforming from one coordinate system (X, Y) (e.g., image), which might be distorted due to various factors, to another coordinate system (U, V) (e.g., map) with its underlying map projection.

  24. Geometric correction Geometric Transformation – more general approach • A first order transformation from image coordinate system (x, y) to ground (map) coordinate system (U, V, Z) • In this case, Z is not considered. To estimate series of A and B, we need to find at least 3 linearly independent points with known (x, y) and (U, V) coordinates, such points are called ground control points (GCPs). • An explicit geometric transformation would require ground points’ elevation Z (derived above) to be known

  25. Geometric correction Geometric Transformation • If the number of GCPs is, i.e., n=3, we get a full rank transformation matrix • U=MA  A=M-1U • V=MB  B=M-1V

  26. Geometric correction Geometric Transformation • If n>3 Using least squares method to solve the overdetermined equations U=MA  A=(MTM)-1MTU

  27. Geometric correction u v Resampling Image Resampling • Nearest Neighbor • Bilinear • Cubic

  28. Geometric correction Georeferencing (geometric correction) Before After

  29. Geometric correction Image to Image Registration • We can also do Image-to-image registration, it refers to transforming one image coordinate system into another image coordinating system • We will introduce it in lab

  30. Topographic correction Topographic correction

  31. Topographic correction sz i Representation of the sun’s angle of incidence i and the solar zenith angle sz

  32. Topographic correction Aim • An ideal slope-aspect correction removes all topographically induced illumination variation so that two objects having the same reflectance properties show the same DN despite their different orientation to the sun’s position.

  33. Topographic correction =radiance observed for horizontal surface; = radiance observed over slope terrain; =average of for forest pixels or specific cover type or whole image. =sun incidence angle in relation to the normal on a pixel =slope of the regression line and b= intercept of regression line. i m Correction Methods (1) 1. Statistic-empirical method, used for correcting single scene image After Before

  34. Topographic correction Correction Methods 2. Cosine correction, used for correcting multitemporal scenes • Applied in flat terrain to equalize illumination differences due to different sun positions in multitemporal scenes (2) sz = sun’s zenith angle Note: The cosine correction only models the direct part of the irradiance Before After

  35. Topographic correction Correction Methods 3. Minnaert correction, used for correcting multitemporal or multisensor dataset. Named from the Belgian astrophysicist Marell G. J. Minnaert (1941) k = Minnaert constant [0, 1] (3) Note: When cos(i) near 0, k increases the denominator and prevents a division by small values. Thus one can counteract an overcorrection in Eq (2) Eq. (3) Eq. (2)

  36. Topographic correction Correction Methods 4. C-correction, used for correcting multitemporal dataset. (4) c = correction parameter Note: Mathematically, the effect of c is similar to that of Minnaert constant (k). It increases the denominator and weakens the overcorrection of faintly illuminated pixels. Before After

  37. Image enhancement

  38. Image enhancement Histogram Image plane 2 Histogram Image plane 1 Histogram Image plane 3 Histogram

  39. Image enhancement Image stretch and compression

  40. Image enhancement g’ g’ g’ g’ g’ g g g g g More transform functions (to single band image) Lower Medium Higher Exponential Logarithm

  41. Image enhancement Histogram equalization Original composite (NIR,R,G) Linear stretch

  42. Image enhancement 1.0 0.5 f 1.0 0.5 f Spatial filtering LP • Spatial frequency • Filtering procedure • Low-pass filter • High-pass filter HP Spatial filtering encompasses another set of digital processing functions which are used to enhance the appearance of an image. Spatial frequency refers to the frequency of the variations in tone that appear in an image. filtering procedure involves moving a 'window' of a few pixels in dimension over each pixel in the image, applying a mathematical calculation using the pixel values under that window, and replacing the central pixel with the new value

  43. Information extraction Information extraction

  44. Information extraction Through elements of visual interpretation

  45. Information extraction Through image transformations • Image subtraction • Spectral ratios • Vegetation index (VI) • Normalized difference vegetation index (NDVI) • NDVI.N • Linear transformations • PCA (principal component analysis) • K-T (Kauth-Thomas) – Tasseled cap transform

  46. Information extraction Vegetation indices

  47. Information extraction Red NIR NDVI

  48. Information extraction Principal component analysis

  49. Information extraction PCA transformation …… Original bands 1 -20 PC1 PC2 PC3

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