|Quoting MPDPilot (Reply 4):|
The runway in the background or the cracks in the foreground. or something else.
..or a flat roof/vertical wall in the background. Something in the middle of the image is going to be less affected by any optical distortion due to viewing angle or lens focal length - assuming that the image hasn't been cropped off-centre.
I wouldn't necessarily use the cracks in the foreground because there's not always going to be one that's parallel/perpendicular to the line of sight to the centre of the image. Checking the horizontal tar lines on the photo, there are some that are horizontal and some that are rotated cw slightly.....