Leveling Horizons

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Tutorial: Leveling a Horizon

This tutorial was last tested for the Gimp 2.2.13

Overview

Humans are very good at picking a horizon that is not level. However when we are taking photographs we don't always get it right. This tutorial is a simple method of correcting that. The before and after views:

Original	Altered

I will be using the original photo on the left than can be downloaded here.

Prerequisites

None

Steps

1. Select the "Measure distances and angles" tool.
   
2. Click and hold a point right on the horizon, drag to another point on the horizon.
3. GIMP will draw a line right on the horizon between the first click point and the release point.
4. At the bottom of the window it will give you two pieces of information, the distance between the points and the angle, we are interested in the angle.
   
5. Go to <Image> / Layer / Transform / Arbitrary Rotation (or use Shift + R)
6. Rotate the image using the Angle field by the amount shown, a positive number will rotate the photo clockwise, a negative number will turn it the other way.
   
7. The problem now is that the image has transparent corners.
   
8. To remove them you have several choices:
   1. Clone nearby image into them.
   2. Crop the photo (my preference is to crop).

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