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Remote Sensing Image Rectification and Restoration

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Remote Sensing Image Rectification and Restoration. Image Rectification and Restoration. Geometric correction Radiometric correction Geometric restoration. 1. Geometric Correction. For raw image rectification For multi-date images registration

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Remote Sensing Image Rectification and Restoration

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  1. Remote Sensing Image Rectification and Restoration

  2. Image Rectification and Restoration • Geometric correction • Radiometric correction • Geometric restoration

  3. 1. Geometric Correction • For raw image rectification • For multi-date images registration • For multi-resolution images or data layers registration • Systematic distortion vs. random distortion

  4. Skew Correction Coordinate transfer Pixel value resampling http://rst.gsfc.nasa.gov/Intro/Part2_15.html

  5. Ground Control Points (GCP) • Features with known locations on a map (X,Y coordinates). These are the “ground control points” • The same features can be accurately located on the images as well (column, row numbers) • The features must be well distributed on the map and the image • Highway intersections are commonly used ground control points

  6. Finding UTM coordinates on a map

  7. Coordinate Transform • Coordinate transform equations relate geometrically correct map coordinates to the distorted image coordinates x = a0 + a1X + a2Y y = b0 + b1X + b2Y x,y: column, row number X,Y: coordinates • Root Mean Square Error (RMSE) = √(dx)2 + (dy)2 Calculate RMSE for all control points

  8. Resampling • The purpose is to assign pixel values to the empty pixels in the rectified matrix output • Superimpose the rectified output matrix to the distorted image • The digital number (DN) of a pixel in the output matrix is assigned based on the DN of its surrounding pixels in the distorted image

  9. Re-sampling Methods • Nearest neighbor resampling • Bilinear interpolation • Cubic convolution resampling

  10. Nearest Neighbor Resampling • The DN of a pixel in the output matrix is assigned as the DN of the closest pixel in the distorted image • Advantages simple computation maintain the original values • Disadvantage spatial offset up to 1/2 pixel

  11. Bi-linear Interpolation • Distance-weighted average of DN values of the closest 4 pixels • Advantage output image is smoother than the nearest neighbor method • Disadvantage alters the original DN values

  12. Cubic Convolution Resampling • Uses DN values of the closest 16 pixels, adjusted by distance • Advantage smooth output image • Disadvantage alters the original DN values

  13. When to Rectify • Rectify before image classification • Rectify after image classification

  14. 2. Radiometric Corrections Radiometric responses differ by • dates • sensor types • images • Causes: - Illumination - Atmospheric conditions - View angle or geometry - Instrument response

  15. Radiometric Corrections • Sun elevation correction • Atmospheric correction • Conversion to absolute radiance

  16. Sun Elevation Correction DN • -------------------------------- Sin (Sun elevation angle) • Assuming the terrain is flat

  17. Satellite Summer Zenith Spring / Fall Solar Elevation Angles Winter Tangent plane Equator

  18. Atmospheric Correction • Haze compensation The DN value of an object (e.g., a deep clear water body) with 0 reflectance = Lp • Subtract the DN from the entire band

  19. Absolute Irradiance • Conversion of DN values to absolute radiance values • It is necessary when compare different sensors, or relate ground measurements to image data • L = (Lmax- Lmin)/255 * DN + Lmin

  20. 3. Geometric Restoration • Stripping • Line-drop • Bit errors

  21. Striping • Malfunction of a detector • Use gray scale adjustment to correct the strips

  22. Line Drop • using average of the above and below lines to fill the dropped line

  23. Bit Error • Salt and pepper effect due to random error • Use 3x3 or 5x5 moving window average to remove the noise

  24. Readings • Chapter 7

  25. Earth-Sun Distance Correction E0 Cosq0 • E = ------------ d2 • Irradiance is inversely related to the square of the earth-sun distance • E - normalized solar irradiance • E0 - solar irradiance at the mean Earth-sun distance • q0 - sun angle from the zenith • d - Earth-sun distance

  26. Atmospheric Correction rET • Ltot = --------- + Lp p r - reflection of target E - irradiance on the target T - transmission of atmosphere Lp - scattered path radiation

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