<<< Part 1. Active-Fire Measures                                                  >>> Pre-Fire Measures    

The simplest measure of post-fire effects is the extent or area burned. Numerous methods have been developed over the years to measure this extent and this is considered one of the more accurate measure that can be obtained from remote sensing imagery.

Most methods rely on the assumption that pre-fire surfaces are dominated by photo-synthetically active material (i.e. green tree leaves) and that post-fire surface are dominated by surfaces that have removed that material; whether by charcoal or by having more soil exposed.

As such, most methods try and detect 3 main changes:

• a considerable drop in visible reflectance, due to the replacement of vegetation by char or soil
• a considerable drop in NIR (0.7-1.2 microns) reflectance, due to the loss of photo-synthetically active material
• a rise in long IR and surface temperature due to the replacement of cool vegetation by warmer soils and char (i.e. drop in evapotranspiration)

The simplest approach to measure the area burned is to conduct a supervised classification. Manually select image regions (ideally from knowledge of field conditions) that represent the range of burned and unburned conditions and run an algorithm like maximum likelihood in a standard GIS of image processing software package. This will produce a map that is often accurate to within 95% of the ground assessments.

Note: You should always use "2 separate methods" to (a) assess the extent of the area burned and (b) analyze the range of variations within the burned area (aka the severity). This is because when we want to measure the area burned we are trying to find methods that most clearly separate the burned and unburned surfaces. These methods will seek to minimize the variability within each "class" of burned area, in order to make it easy to distinguish from unburned. In contrast, if we want to assess post-fire effects we want methods that capture the widest range of variability of the values within those areas Verstraete and Pinty 1996; Pereira 1999). Therefore, if using DNBR for post-fire effects, use another method to first segment the area as burned. 

For More Information read:

    Pereira JMC (1999) A comparative evaluation of NOAA/AVHRR vegetation indexes for burned surface detection and mapping. IEEE Transactions on Geoscience and Remote Sensing 37, 1, 217-226.

    Verstraete MM, Pinty B (1996) Designing optimal spectral indices for remote sensing applications, IEEE Transactions in Geosciences and Remote Sensing 34, 5, 1254-1265.