Transformation with an arbitrary grid

Given a skewed image with at least four points with known coordinates, a perspective transformed version can be computed using the steps described which follows.

Original image as loaded in AirPhotoSE:

Thanks to Jean Roc Morreale, Centre départemental d'Archéologie of the Conseil Général du Pas-de-Calais, France for permission to use this image.

Turn on the quadrilateral in the control point menu and move each corner to the four initial known points in the image:

Hold down the Shift key and the left mouse key to drag a corner.

On the File menu, click on Clone New or click on the New button on the main toolbar:

This will generate a new blank image with the same size as the original:

Make sure that the new image has the focus. On the control menu, click on Load Control:

In the Load Control Points dialogue, choose the control point file from the original image and load it:

This will display the control point quadrilateral from the original on the new image. It is perspectively distorted:

From the control point menu click on AddGrid or click on the Grid button of the main Toolbar:

You now see the control point quadrilateral overlain by a grid:

Expand the image to full scale using the magnifier button of the Navigation window (first press F2 if it's not visible) and drag each corner so that the quadrilateral is aligned with the grid:

Verify this with a click on the magnifier again to view the whole image again:

Turn off the grid using the Grid button or the AddGrid entry of the control point menu:

If you wish, turn on the otherwise invisible control point green tracker magnifier from the control point menu and drag the black magnifier to each point in turn.  The green magnifier should be now almost exactly over the control point in the original image.  If not, adjust the position of the quadrilateral in the new blank image. 

Click on the Transform menu or on the little blue camera button:

The image will be transformed so that it is now rectangular with the control points corresponding to your selected grid positions::