A system and method is shown for determining the spatial boundary of certain measurable phenomena, such as broadcast signals, at various locations within geographical areas covered by the phenomenon. Data representing relative signal quality at various locations within the geographical area is created within a mobile device, such as a cell phone capable of receiving the phenomenon. The data is stored and refined within the device so as to define weak signal quality areas within at least a portion of the geographical area traveled by the mobile device. The refinement, which in one embodiment comprises coalescing and splitting stored data locations in the device allows for long storage periods by reducing memory requirements. By devoting the majority of memory to locations in the vicinity of the good/bad boundary, the most detailed picture possible of the boundary is achieved for a given amount of memory. By collecting such data from a plurality of such devices, a central system can map the signal strength over the entire geographical area.