Activate Image-to-Image/Map GCP Tool

This tool is used to georegister or georeferencer an image by registering an image to a previously registered Master Image and creating a new "georeferenced" image. It works in two steps:

  1. Load the reference (registered) image and select this option on Image Menu. Inspecting the Image menu, you will see a check mark on it.
  2. Load the image that is going to be registered (the Slave Image) and the Mirone window transmutates into something like the example below.

We are now going to zoom in on the Master Image, where we previously plotted a chunk of full definition coastlines and draw some ground control points (GCPs). Adding GCP is done on the following way:

  1. Hit the button with the square in the cross icon (the fourth button) and the cursor will turn into a cross
  2. Select distinctive common features on both the Slave and Master Images and click on the Slave Image (left image) followed by a click on Master Image (right image) over the same feature. This will add a pair of yellow dots, one on each image.
  3. Repeat this process until you have a “reasonable” number of GCP pairs. To stop, double-click somewhere on one of the images

Next figure shows an example of what may look a referencing session

Right click over one of the yellow dots pops up a menu where we can select to show/hide the GCP pair numbers, delete a chosen pair of GCPs, to save them and, specially, to show a table with the points position and the error residue. Scroll this table to identify potentially bad located GCP pairs. If you have such a situation you can either kill the pair or simple adjust their position by double-clicking the specific point and click-and-drag to move.

When you are satisfied hit the R button, wait a bit and… that is it. You now have a georeferenced image.

To control the algorithm used in the image registration and how to interpolate the user can select between several options available in two popups located at the figure top center (which display Affine & bilinear in current example). Full documentation of those options is a bit long and interested people may consult it by reading the doc of MATLABs cpt2tform and imtransform, available for example at

www.mathworks.com/access/helpdesk/help/toolbox/images/cp2tform.html

www.mathworks.com/access/helpdesk/help/toolbox/images/imtransform.html

On version 1.4 it was added also the possibility to do the registration using a MEX of GDAL program gdalwarp (Matlab solutions, inexplicably, just fail too much). A final word to the “cherry button”. Its tooltip says Automatic Registration and in fact that is what it tries to do. If you have two photos of the same area, one registered (for example an ortophoto) and the other not there are pretty good chances that this option automagically references your target image. The thing is done using the fantastic SIFT algorithm.

Copyright © <2010>, <Joaquim Luis>

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