I have created public altars in CA and Berlin and I'm currently working on one for NYC, and each time I have found the experience to be quite unique. Unlike creating and maintaining a permanent altar, where the energy is sustained over time, creating a public altar, an altar that you will then leave to the elements, animals, and people, is a powerfully intense experience wherein the energy explodes in one act, one moment, and then dissipates gradually over time, feeding the land itself. It is a new way of honoring that which is holy within the framework of one's city or town, of bringing an awareness of the sacred into the most mundane and temporal part of your life and of making a little doorway through the creation of the altars, by which the sacred may be experienced. Since materials for the altar are drawn from your daily travels, it also has the side effect of causing you to be more aware of where you're going, what you're seeing and what's right under your nose!
In closing, I think the main thing to remember when learning to work with the spirit of your city is that no matter where you stand, no matter where you are, you're standing on sacred ground. You're standing on the body of a God or a Goddess (Ymir, Geb, Gaea, Erda, depending on your religion). Wherever you walk is holy.
This article was adapted from an article that first appeared in BBI Media's "NewWitch."