This one always takes me a while to find when a flerf claims water is always flat so I'm putting it here to save time
"The rails must be aligned vertically to follow the curvature of the earth horizontally and in roll (in Southampton, the sagitta for an east to west orientated tank of 138m is about 0.6mm)"
https://www.southampton.ac.uk/~assets/doc/The-Boldrewood-Towing-Tank.pdf
