We think of sidewalks as level, until the first time you drop something round or cylindrical and watch it roll towards the curb. Sidewalks are of course graded to send rainwater into the gutters. Just how sharply they are graded can be seen anywhere there is scaffolding. Look at the height difference between these two parallel uprights, which by necessity must have level bases.
The base of the outermost uprights are always, at least in New York, raised up on a stack of 2x12 Douglas Fir cut-offs.
The cut-offs are toe-nailed together.
Here you can see a couple of nails are driven in the usual manner through holes in the base of the upright, and then the clinched nail technique is employed.
You might think that's not a very secure way to hold a scaffold upright in place. But in fact, the nails are not there to hold the uprights in place. They are there to prevent the 2x12 cut-offs from sliding around.
The weight of the scaffold, which consists of many members, is such that simple gravity will hold it securely in place. The cross-members prevent racking. It reminds me of those Chinese earthquake-proof temples, where the uprights are not even fastened to the stone pilings they rest on.