Smooth out an oblique horizon with Gimp
It is very easy to smooth out an horistont with Gimp if you accidentally hold the camera angle. In the picture here you can see the horizon clearly is oblique. We choose measurement tool and it is the tool I am 
pointing to the left and the right is the rotation tool, which we shall use later. After selecting the measurement tool i drag out a line along the horizon. You can see down to the bottom edge to see how many degrees to the horizon leans.
Now select the rotation tool and get a menu where you can fill in how many degrees you should rotate with. You choose of course, the angle you’ve measured up. Once you have rotated the image or layer so you get a problem with empty spaces in the corners that you can 
remove either by cropping the image or clone from which i take some from the surrounding image. I did a little of each. The grass in the lower left corner cloned I get there a little grass from above the empty field.
Original Tutorial: Open Source Photography, where you’ll find more tutorials for the Gimp.
