I love sweeping views, with a distant horizon and plenty to take your eye around the middle and foreground of the landscape. The problem is, if you make the horizon a focus of your picture, the viewer will expect it to be perfectly horizontal! Even a fraction of a degree will be obvious.
So short of setting up a tripod with a spirit level, (I do have one but I still manage to get the horizon off by a few degrees frequently) you will have to resort to rotating the image. Looking at the main Photo Editing packages this is not such an easy task, because they don't have any nice referencing tools. Most just have a little panel that you type in the desired rotational angle. Some packages might let you copy the image to a separate layer then rotate it by freehand, which means by mousehand! If you've tried either you know why I'm suggesting they are not so easy to get right.
So here is a little trick I worked out. Most editors have a rectangular masking tool that stretched out perfectly horizontally. Just point to one end of the horizon and stretch it out. It will give you a good idea of which way and by how much to adjust.
Some editors automatically select the area inside your mask, in which case only inside the mask will be rotated so remove it before rotating. I then prefer yo use the rotational dialogue. Say starting with 3 degree clockwise. I then check with the rectangular mask. Ok that was to much I do an Undo and then rotate it by only two and a half degrees.
Check with the rectangular mask again. Perfect!
Now just a quick crop
Actually when you get up high enough in an aeroplane the horizon becomes a curve.