Somerville's zoning laws refer to "open space" without any subcategories. This vagueness in the definition of open space allow some truly bizarre equivalencies to be created. "Open space" includes grass fields, but it also includes artificial turf fields. It also includes concrete plazas and asphalt parking lots.
Somervision requires new open space to be created in Somerville, but new parking lots would meet this goal. This would be a perversion of the intent of Somervision. Part of the rationale for having open space is for the emotional and physical well-being of the residents the city, but natural environments heal us far better than concrete open spaces. The climate (both local and global) needs to be part of the discussion as well. Grass cools; plastic heats.
The definition of "open space" in Somerville must take into account the desirability of some types of surfaces rather than others. For example, grass is better than artificial turf, while artificial turf is better than asphalt. A definition which acknowledges this might allow an artificial turf field to be installed over an asphalt roof, but would ensure grass would never be sacrificed for plastic.