When I do swales by hand I have typically used an A-frame with a 3ft level attached to the cross member. This will give me roughly 6ft span between each point which allows for pretty quick lines. I'll use some twigs to temp in the swale spots to see how the contour flows. This way if it ends up crossing something like a tree or an area not desired, I can easily move up or down slope a bit to accommodate. Since we have pretty heavy clay soil here in Oklahoma, I've found it much easier to borrow or rent a walk behind tiller to run the span of the swale just behind the twig markers. The loose soil can then be easily scooped with a square tip shovel and placed on the downhill berm side and smooths the swale out at the same time.
