Are the boundaries between lava-flow hazard zones distinct lines?
No, the hazard zone boundaries are approximate and gradational. The degree of hazard from one zone to the next is gradual rather than abrupt, and the change can occur over the distance of a mile or more. In other words, the boundary between lava-flow hazard zones is not a sharp line that, in one step, you can cross from one zone into the next. Also, within a single hazard zone, the severity of hazard from one location to the next can vary on a scale too fine to map. For example, the hazards posed by lava flows decreases gradually as the distance from eruptive vents increases.
Due to local topography, however, there can be abrupt changes in the relative lava-flow hazard within a single zone. For example, the hills behind Nīnole stand high above the adjacent slopes of Mauna Loa and, consequently, are at a much lower risk from lava flows than the surrounding area.