It’s best that before you take a photograph always look at the horizon on two points:
a)Is the horizon straight? It is easier to have your horizon straight in the camera viewfinder before pressing the shutter release, saving time to straighten the image later in editing software's.
b)Where to place the horizon? The ‘correct’ natural spot for a horizon is on either the top third line or the bottom line of the ‘rule of thirds’, rather than completely in the middle. In saying this, rules can be broken if you are completely sure that your image with the horizon is very striking.
(See diagram below for where to place the horizon)
Example of photograph showing the horizon on the 'Top Third' line.
Example of photograph showing the horizon on the 'Bottom Third' line.