Assessing the capabilities of geospatial data to map built structures and evaluate their bushfire threat

Bushfire threat was evaluated for built structures for three areas in Victoria (Australia) that had been impacted by the devastating 16 February 1983 Ash Wednesday fires. Structures were mapped for 1982 and 2006 using human interpretation of high-resolution (0.35-m pixels) digital orthophotographs. Damage to structures from the 1983 Ash Wednesday fires was also evaluated using human interpretation of the digital orthophotographs. Approximately 25% of the structures present were not mapped due to either interpreter error or overhanging vegetation. The majority of unmapped structures were sheds and garages. The error of omission for houses was between 7 and 10% with the error of commission for houses being less than 0.5%. Bushfire threat was modelled using information about topographic slope and aspect, forest vegetation, and prevailing wind direction during days of high fire danger. The method detected a substantial change in bushfire threat from 1982 to 2006 for one of the three study sites whereas no change in overall bushfire threat was observed for the other two. Considering the location of structures built since 1982, these results appeared reasonable. However, the 1982 bushfire threat was not related to actual structure damage sustained during the 1983 Ash Wednesday fires. Estimating bushfire threat using this methodology cost AU$6 per structure or AU$4.60 per property.