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It's always a question about "horizontal." Should photographers go for actual level, or perceived level?
Have you tried leveling the image (i.e. lowering the right side), using the far edge of the valley as your guide? Especially the first image.
Charlie, thank you for the critique. I agree, it is sometimes hard to establish "horizontal".
In this particular case, I used trees as verticals (only in the center, to avoid the influence of distortion towards the edges, and I used hi-res version). The terrain in Phoenix is noticeably higher towards north-east (even the temperatures are ~ 10 degrees lower).