Assigning dates and identifying areas affected by fires in Portugal based on MODIS data

ABSTRACT An automated procedure is here presented that allows identifying and dating burned areas in Portugal using values of daily reflectance from near-infrared and middle-infrared bands, as obtained from the MODIS instrument. The algorithm detects persistent changes in monthly composites of the so-called (V,W) Burn-Sensitive Index and the day of maximum change in daily time series of W is in turn identified as the day of the burning event. The procedure is tested for 2005, the second worst fire season ever recorded in Portugal. Comparison between the obtained burned area map and the reference derived from Landsat imagery resulted in a Proportion Correct of 95.6%. Despite being applied only to the months of August and September, the algorithm is able to identify almost two-thirds of all scars that have occurred during the entire year of 2005. An assessment of the temporal accuracy of the dating procedure was also conducted, showing that 75% of estimated dates presented deviations between -5 and 5 days from dates of hotspots derived from the MODIS instrument. Information about location and date of burning events as provided by the proposed procedure may be viewed as complementary to the currently available official maps based on end-of-season Landsat imagery.

摘要 本研究提出一种自动化流程，可借助中分辨率成像光谱仪（MODIS）获取的近红外与中红外波段日反射率数据，识别葡萄牙境内的火烧区域并确定其发生时间。该算法可检测所谓(V,W)火烧敏感指数（Burn-Sensitive Index）月度合成产品中的持续性变化，并将W波段日时间序列中变化量最大的日期确定为火烧事件发生日期。本流程以2005年作为测试对象——该年度是葡萄牙有记录以来第二严重的林火季。将生成的火烧区域图与基于陆地卫星（Landsat）影像获取的参考数据进行对比，结果显示其正确比例（Proportion Correct）达95.6%。尽管该算法仅针对8月和9月进行部署，但其仍可识别出2005年全年近三分之二的火烧迹地。本研究同时对该定年流程的时间精度开展了评估，结果显示75%的估算日期与中分辨率成像光谱仪（MODIS）获取的热异常点日期偏差介于-5天至5天之间。本流程所提供的火烧事件位置与发生日期信息，可作为现有基于季末陆地卫星（Landsat）影像制作的官方火烧图件的补充数据。