The Atlas makes two kinds of things visible: city ground and city nature. These core concepts are related but not the same. Both involve breaking down established binary categories of thought that create blindness.
The city is made up of vertical and horizontal surfaces: buildings and grounds. The surfaces of buildings are conceived of in terms of how they look, and what they contribute to their environment. The surface of the ground—streets, sidewalks, curbs, tree pits, crosswalks—is widely conceived in utilitarian terms, or as background, when in reality the ground must perform many roles, resisting more forces than the buildings. Cars and trucks drive on it; people walk on it, water stands on it, snow accumulates, corrosive salts are spread on it to melt ice and snow.