Geometric Corrections, the Advantage of Geometric Corrections, Non-Systematic Distortions (Especially for GATE-Geospatial 2022)

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Geometric Corrections

  • Paw digital images often contain serious geometrical distortions that arise from earth curvature, platform motion, relief displacement, non-linearities in a scanning motion. The distortions involved are of two types:
    • Non-systematic Distortion
    • Systematic Distortions
  • Rectification is the process of projecting image data onto a plane and making it conform to a map projection system. Registration is the process of making image data conform to another image. A map coordinate system is not necessarily involved. However, rectification involves rearrangement of the input pixels onto a new grid which conforms to the desired map projection and coordinate system. Rectification and Registration, therefore, involve similar sets of procedures for both the distortions.

The Advantage of Geometric Corrections

The advantage of geometrically correcting an image prior to further analysis and interpretation is that it would then allow proper measurements of distances and areas to be made from features in the image. This may be particularly useful in different applications where true measurements are necessary, such as in urban mapping applications. Also, the geographic locations of features could be determined. Once an image is geometrically registered to a known geographic base, it can be combined with other mapped data in a digital environment for further analysis.

Non-Systematic Distortions

These distortions are caused due to variations in spacecraft variables. These distortions can be evaluated as follow:

Distortion Evaluated from Tracking Data

  • The amount of earth rotation during 26 sec required to scan image results in distortion.
  • The correction for this distortion can be done by scanning 16 successive groups of lines, offset towards the west to compensate for the earth rotation, which causes the parallelogram outline of the restored image.
Geometric Corrections

Distortion Evaluated from Ground Control Caused during the spacecraft scan of the ground.

Altitude Variation

If the sensor platform departs from its normal altitude or the terrain increases in elevation, which results in scale change?

Attitude Variation

Pitch, roll & yaw. When the sensor systems axis departs from its maintained attitude (parallel to spacecraftีšs direction of travel and normal to earthีšs surface) results in distortions.

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