Levelling photographs using Photoshops ruler tool

This is a very quick and effective way to level up any photographs. It is particularly useful if you have photos of landscapes, and the horizon angle is incorrect. At a push, it may take 15 seconds per image - substantially less time then selecting the image, and rotating it around manually to the correct angle (that you would need to calculate..).

Keep a copy of the original image layer

As always, keep a copy of the original (tilted) image. Simply load the tilted image into Photoshop, and select the Ruler (measure) tool. This is an option available from the eyedropper icon on the tools bar.

Choose a reference line on your image to level

With the Ruler tool selected, decide which line you are going to level. In our example, we are choosing the horizon.
In images without a clear horizon, it will be necessary to choose some other line for reference - lines of pavements, roofs, skylines etc.

When the line has been chosen, simply draw it onto the horizon (keep left mouse down).

Once this has been done, simply select Image > Rotate Canvas > Arbitrary.

You will notice that Photoshop has prefilled the angle of rotation. After OK\'ing this, you will be left with a levelled image. However, this image will have some extra edges you may wish to remove.

Cleaning up the image using the crop tool

Removing these edges can be simply achieved with the crop tool. The crop tool is found, again, on the Tools bar.
With the crop tool selected, it is then possible to draw a rectangle marking out the area of the image you wish to keep. It is not necessary to be pixel perfect when drawing out the crop area - it wont cut anything away just yet, and the crop area can be adjusted by moving the "pull" boxes on the crop rectangle.

When the area to keep is completely marked out, simply press enter - or select the marquee tool on the Tools bar.