After reviewing some landscape photos from a recent trip, I noticed that several had tilted horizons and needed straightening.

When I opened them in GIMP, it wasn't immediately clear how to do this.

Eventually, I found that the Measure Tool (default shortcut: Shift+M) is the right tool for the job.

For my workflow, these settings worked best:

  • Orientation: "Auto"
  • Transform: "Layer"
  • Interpolation: "NoHalo"
  • Clipping: "Crop to result"

The GIMP documentation explains the available interpolation methods. I chose "NoHalo" because it's described as:

"This method performs a high-quality interpolation. Use the NoHalo method when you downscale an image to less than half of its original size." — GIMP documentation

To straighten an image, use the Measure Tool to draw a line along the horizon, then click "Straighten" in the tool options. This automatically rotates the image to level the horizon.