Nature Photography journal
The horizon line is the only decision that matters
Where you put the horizon determines what the picture is about. Everything else is adjustment.
Most landscape advice is about light. Light is easier to talk about because it changes and you can wait for it. The horizon is fixed by where you stand and it decides more.
What each choice says
Low horizon, big sky. The picture is about weather, scale and atmosphere. The land becomes a base for something happening above it. This is the choice that survives a wide crop and the choice that leaves space for type, which is why it dominates commercial landscape work.
High horizon, big foreground. The picture is about the ground: texture, pattern, a path, the way terrain folds. It is more intimate and much harder to use in a layout, because the busy area is where a designer needs space.
Centred. Usually read as a mistake, and occasionally exactly right. A centred horizon says the two halves are equal, which is true for a reflection and rarely true for anything else. Used deliberately with a symmetrical subject it is strong; used by default it looks like nobody chose.
Against the rule of thirds
The rule of thirds is a placeholder for judgement. It gives an acceptable answer when you have not thought about the question, which is genuinely useful when you are learning and a limitation afterwards.
The real question is what the picture is about. If it is about a storm, put the horizon at the bottom and give the storm four-fifths of the frame; a third of the frame is not enough sky for weather to do anything. If it is about a hillside's texture, the horizon may not need to be in the frame at all.
Level it
A tilted horizon in a landscape reads as an error rather than as dynamism, because viewers know water and distance are level. Half a degree is visible; a degree is glaring.
This matters more in commercial use than in a print, because a tilted horizon next to a horizontal type baseline is obvious in a way it never is on its own.
Think about the crop
If a frame will end up in more than one aspect ratio, the horizon decides which crops survive.
A low horizon crops beautifully to a wide format, because the crop takes sky you had plenty of. A high horizon crops badly to anything wide, because the crop takes the foreground the picture was about.
Deciding this at the moment of exposure is worth more than any amount of fixing later.