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Im a geomatics student,

During a geo referencing with QGIS, a .Points file is saved. I open it with Notepad++

It is written under this form :

mapX,mapY,pixelX,pixelY,enable
18.41330472103005533,-34.31455203862660852,18.41112659235668403,-34.30085589171973481,1

I would like to know what the different columns are?

The first coordinates mapX and mapY, are Daymark returned by the user.

Can anyone help me understand the meaning of these different columns?

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    GCP = Ground Control Point. It is a/the actual known location of an object in a map/aerial photograph/satellite image.
    – nmtoken
    Jul 4, 2015 at 12:54

2 Answers 2

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The following is basically quoted from the book: "Mastering QGIS" by Kurt Menke, Luigi Pirelli, Richard Smith Jr & John Van Hoesen on page 199:

  • mapX and mapY are the Destination Coordinates.
  • pixelX and pixelY are the Source Coordinates.
  • enable has a boolean value where 1 means it will be used in the transformation; 0 means it will not be used.
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The columns mean:

  • Geographical easting/longitude
  • Geographical northing/latitude
  • Location of the ground control point in image pixel space from left to right
  • Location of the ground control point in image pixel space from top to bottom
  • Boolean selection if the GCP should be used (1) or not (0) in computing

The anchor point in the image space is at top-left corner, and the top-left corner of the top-left pixel is at pixel coordinates (1,1).

The Ground Control Points are used as input values for GDAL exactly as when using the gdal_translate utility http://www.gdal.org/gdal_translate.html. However, notice that parameters are saved into the points file in different order. Gdal_translate takes pixel coordinates before georeferenced coordinates.

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