Draw GCPoints

An alternative method of registering images is when we know the coordinates of specific ground points that can be easily identified on an image such as house corners, cross roads, etc… In such cases the registering processes is achieved by plotting the GCPs on the image and assigning the coordinates to them. This is what this option allows. When selected the cursor changes into a cross and every left-click adds one more vertex of a polyline. The inserting step ends with a right-click. Another right-lick on the line or any of the inserted points displays a menu with these options:

  1. Set reference Points
  2. Show GCPs residuals
  3. Register Image
  4. Change registration method
  5. Delete GCPs
  6. Show GCPs numbers
  7. Hide connection line
  8. Show connection line

The next step is to assign the ground coordinates to points. Selecting option 1 opens a table like this one

The first two columns contain the image pixel coordinates (non-referenced coordinates). Columns 3 and 4 must now be filled with the georeferenced coordinates. Once that is done to all points we can check the goodness of the referencing process by using option 2 - Show GCPs residuals. Analyzing the residuals one can decide that some points need a position adjustment or even deletion. The first case is done in the normal way of double-click the target point to pass to edition mode and than click-and-drag. To delete a point, while in edition mode click on it one to make it the active point and that hit the keyboard “r” to remove that point. One satisfied use the Register Image to do it says.

The option 4 - Change registration method opens a small window where one can select among the different warping methods used in the registration. See references at the end of Activate Image-to-Image/Map GCP Tool to get further information on those methods.

The meaning of points 5 to 8 is obvious and does not need any further explanation.

Copyright © <2010>, <Joaquim Luis>

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