Straighten a Horizon With GIMP

This tutorial also works for crooked vertical lines

GIMP straighten image with rotate tool

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.

    GIMP image guide added
  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.

  4. Check that Transform is set to Layer and change the Direction to Normal (Forward).

  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.

  6. Finally, set the Guides to No guides. It'll get too confusing with the line you already set.

    GIMP rotate tool set up
  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.

  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.

    GIMP straighten image with rotate tool
  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.

    GIMP crop to content
  11. The result you see after is roughly your final product. All that's left is to remove the guides. Select Image again from the top menu, and choose Guides and Remove all guides.

    GIMP straightened image cropped
  12. Once the guides are gone, you'll have your finished image. Save, export, or handle it however you choose from here.

    GIMP image horizon straightened