Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 112

2.5.4 Setting the Image and
Determining the Angle - Measuring
Figure 2.41
Using the Measure Tool to
determine the image angles.
Obviously, the image needs to be straightened out. To specify the rotation
angle, drag a guide on the image. Click the top ruler, and while holding down
the left mouse button, drag it downward. Position the guide on the upper-left
corner of the image. You now have a horizontal check mark.
Using the Measure Tool , you can, for example, measure the angle of
the tilt so you can use the same amount to rotate the image upright. Select
the Measure Tool in the Toolbox. Check the Use Info Window option to open
a window that will show the values obtained by the Measure Tool. For your
convenience, the measured values are also displayed in the status bar at the
bottom of the image window.
The cursor should have changed into a crosshair marker or sighting
reference. Click on the upper-left corner of the image while holding down
the left mouse button and drag it to the upper-right corner of the photo.
The status bar (and the info window if you opened it) now shows an angle
of approximately 1 degree, the exact amount to rotate the image in order to
undo the tilt.