One of the problems with all landscape photography is a level horizon and
whilst it is always better to solve problems ‘in camera’ Photoshop
does allow correction after the event. However, most of the methods are
crude relying in eye judgement, which in my case is the reason I’ve had
the problem in the first place.
The following technique sorts this out to tenths of a degree. First select the measure tool it can be found hiding behind the eyedropper.
Select the tool and drag it along what should be a horizontal or vertical line in the image.
The go to the menu option Image>Rotate Canvas>Arbitrary and you get a small dialog box with the required rotation and direction to get the dragged line horizontal or vertical.
This can be down to the hundredth of a degree.
So click OK and away you go, perfect level horizons.