Rotation of Image to True Vertical or Horizontal.

You need to rotate an image to bring it to true horizontal or vertical, (Fig 1)

Select the Measure Tool (on Eyedropper icon – press key I) and, holding the mouse button own, draw a line parallel to the vertical (or horizontal) part of the image you want to correct to, for example:

You do this as follows:

(the line should be as long as possible to give the best accuracy)

Once drawn, you can adjust the line by moving either end, and/or moving the whole line if you grab it in the middle. (Fig 2)

Then, from the Main Menu, select Image, Rotate canvas, Arbitrary.  You will see that it has already calculated the degree and direction of rotation needed.  Just click OK  (Fig 3)

If the correct result is not achieved, step back in the History Panel and repeat.

You will need to crop the image after this to square-up the edges. (Fig 4)

    Fig 1                                                                     Fig 2

    Fig 3                                                                        Fig 4