Straighten a Horizon With GIMP

This tutorial also works for crooked vertical lines

GIMP is suitable for a wide range of digital photo editing uses, from the simple through to quite advanced digital photo editing. A common problem that often needs correcting in digital photos is straightening a crooked or skewed horizon. This can be achieved very easily using GIMP, as demonstrated in this tutorial.

Straighten Your Photo

  1. For this tutorial, you'll obviously need a digital photo with a crooked horizon. To open the picture in GIMP, go to File > Open and navigate to the photo and click Open.

    GIMP image open
  2. Drag a guide line down from the top guide above your image. Place it where your horizon will be, when it's straightened.

    A screenshot of GIMP with an arrow indicating dragging a guide down
  3. Now you can set up the Rotate Tool in preparation for correcting the horizon.
    Click on the Rotate Tool in the Toolbox and you'll see the Rotate options appear in the palette below the Toolbox.

    A screenshot of GIMP with the Rotate Tool highlighted
  4. Check that Transform is set to Layer and change the Direction to Normal (Forward).

    A screenshot of GIMP with the Transform and Direction options for the Rotate tool highlighted
  5. Use the Cubic setting for Interpolation as this produces a good quality image. Change the Clipping option to Crop to result as this will produce an image that has vertical and horizontal edges and makes the resulting image as large as possible.

    A screenshot of GIMP with the Clipping settings highlighted
  6. Finally, set the Guides to No guides. It'll get too confusing with the line you already set.

    A screenshot of GIMP with the Guides option in the Rotate tool highlighted
  7. Select the image itself with the Rotate Tool active to bring up its controls. You can either click and drag to rotate the image manually, or you can use the floating dialog to enter degree of rotation directly.

    A screenshot of the Rotate window in GIMP with the Angle box highlighted
  8. When you have everything set, and your horizon lines up with the guide, press Rotate in the floating dialog to make the change final.

    A screenshot of the Rotate window in GIMP with the Rotate button highlighted
  9. The image will shrink a bit, leaving a border of empty space around the edges. As long as you're happy that it's straight, you can move forward.

    GIMP image rotated
  10. Open up the Image menu at the top of the screen. Then, choose Crop to Content.

    A screenshot of GIMP with the Crop to Content command highlighted
  11. Save, export, or handle it however you choose from here.

    GIMP image horizon straightened