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I am a beginner in geomatics.

I am trying to georeference some images in QGIS.

I would like to understand what is happening behind the different transformation types (linear, polynomial, Helmert...). What I don't understand is the red lines that appear after inserting a GCP. These lines take as a starting point the point the user put, but I don't understand their orientation.

Does this show a gap between the coordinates given by the user and the coordinates of the map?

Example : http://hpics.li/dbe04f9

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The following are quoted from the QGIS docs regarding the Available Transformation algorithms:

  • The Linear algorithm is used to create a world file and is different from the other algorithms, as it does not actually transform the raster. This algorithm likely won’t be sufficient if you are dealing with scanned material.
  • The Helmert transformation performs simple scaling and rotation transformations.
  • The Polynomial algorithms 1-3 are among the most widely used algorithms introduced to match source and destination ground control points. The most widely used polynomial algorithm is the second-order polynomial transformation, which allows some curvature. First-order polynomial transformation (affine) preserves collinearity and allows scaling, translation and rotation only.
  • The Thin Plate Spline (TPS) algorithm is a more modern georeferencing method, which is able to introduce local deformations in the data. This algorithm is useful when very low quality originals are being georeferenced.
  • The Projective transformation is a linear rotation and translation of coordinates.

In terms of the red lines , these are likely be georeferencing errors which can show the residual difference between various sets of map control points.

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