Explain Geometric correction.
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Geometric correction, also known as geometric rectification or image registration, is a process in remote sensing and GIS (Geographic Information System) that involves aligning and correcting satellite or aerial images to a specific map projection or coordinate system. The goal of geometric correction is to eliminate spatial distortions, inaccuracies, and misalignments present in raw or uncorrected images, ensuring that the imagery accurately represents the Earth's surface.
The Earth's surface is three-dimensional, while images are captured on a two-dimensional plane. As a result, distortions can occur due to variations in terrain, sensor position, and Earth's curvature. Geometric correction compensates for these distortions by applying mathematical transformations to the image, aligning it with known geographic coordinates.
The process typically involves the following steps:
Selection of Ground Control Points (GCPs): Identify distinct and easily identifiable features in both the image and a reference map with known geographic coordinates. These features, such as road intersections or prominent landmarks, serve as ground control points.
Collection of GCP Coordinates: Obtain the accurate geographic coordinates (latitude and longitude) of the selected ground control points from a reliable geodetic reference source, such as a topographic map or a GPS survey.
Transformation Model: Choose an appropriate transformation model based on the characteristics of the distortion present in the image. Common models include polynomial transformations or rubber-sheeting techniques.
Application of Transformation: Apply the selected transformation model to adjust the pixel locations in the image, aligning them with the corresponding ground control point coordinates. This process involves mathematical calculations to redistribute and reposition the pixels.
Resampling: Adjust the pixel values in the image to account for the changes made during the geometric correction process. Resampling ensures a smooth transition between pixels and maintains image quality.
Verification: Assess the accuracy of the geometric correction by comparing the corrected image to additional ground control points or reference data. This verification step helps ensure that the rectified image aligns accurately with the intended geographic coordinates.
Geometric correction is essential for various applications, including cartography, land cover mapping, change detection, and spatial analysis. Corrected images facilitate accurate measurements, overlaying with other spatial datasets, and integration into GIS workflows, ensuring that remote sensing data is spatially accurate and reliable for analysis and interpretation.