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Geometric Correction


Geometric Correction:


- Geometric correction is a critical process in remote sensing and digital image processing. It involves adjusting and aligning an image so that it accurately represents the Earth's surface in terms of scale, orientation, and spatial accuracy. This correction compensates for various geometric distortions and errors introduced during image acquisition and sensor characteristics, ensuring that the image can be used for precise geospatial analysis and mapping.


Source of Geometric Error:


- Geometric errors in remote sensing arise from various sources, including inaccuracies in sensor characteristics, platform movement, Earth's curvature, terrain relief, atmospheric conditions, and other factors. These errors can lead to distortions, misalignments, and inaccuracies in the positioning and representation of objects within an image.


Types of Geometric Error:


- Geometric errors can manifest in different ways, including:

  1. Scale Error: Inaccurate representation of distances in the image.

  2. Positional Error: Errors in the location of objects within the image.

  3. Angular Error: Errors in the orientation or rotation of objects.

  4. Distortion: Misrepresentation of object shapes or sizes.

  5. Parallax Error: Discrepancies in object positions due to elevation differences.

  6. Relief Displacement: Displacements of objects due to variations in terrain elevation.

  7. Atmospheric Refraction: Errors due to the bending of light in the atmosphere.

  8. Satellite Ephemeris Errors: Errors in satellite position data.

  9. DEM Errors: Inaccuracies in the Digital Elevation Model used for terrain correction.

  10. Time-Dependent Errors: Errors that change over time.

  11. Resampling Error: Errors introduced during pixel value interpolation.

  12. Control Point Error: Errors in the accuracy of ground control points.


Types of Geometric Correction:


- Geometric correction techniques are used to rectify or mitigate these errors. Common types include:

  1. Image-to-Map Transformation: Matching control points to align the image with a map.

  2. Rubber Sheet Transformation: Non-linear correction using polynomial functions.

  3. Affine Transformation: Linear correction for basic distortions.

  4. Projective Transformation (Homography): Correcting complex distortions, including perspective.

  5. Orthorectification: Comprehensive correction accounting for terrain and Earth's curvature.

  6. Bundle Adjustment: Simultaneous adjustment of multiple images for 3D mapping.

  7. Sensor Model-Based Correction: Using detailed sensor models for correction.

  8. Resampling: Interpolating pixel values after correction.


Each type of geometric correction is chosen based on the specific nature of the errors in the imagery and the desired level of accuracy for the application at hand.

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