7th Workshop on Forest Fire Management - EARSeL, European ...

2.2 - Active fire identification
Early warning system for fires in Mexico and Central America 183
Immediately after each satellite overpass, Level-0 data is preprocessed to
Level-1B, including geolocation, calibration, and bowtie correction. In the
case of MODIS, a suite of standard products is derived automatically including
MOD14. The extracted fire information is projected to a Lambert
Conformal Conic projection and exported to a generic binary format for further
processing. Then, the fires are attributed using additional GIS information
such as geographic coordinates, states and districts, type of vegetation,
slope, and information on the proximity to NPAs. Contiguous fires
are grouped and defined as a fire complex. Besides tabular information, the
following suite of visual products is created: 1) a quick look to provide rapid
clarification for satellite coverage and cloud cover; 2) information about
cloud cover and invalid data percentages; 3) Enhanced Vegetation Index
(EVI) with superimposed fires; 4) 10-day FPI composite; 5) shaded relief of
a DEM (200 m) with superimposed fires.
2.3 - Burnt area identification
Many burnt area products use a set of thresholds together with a time series
analysis to detect rapid and dramatic land cover changes.
For instance, the on-going Global Burnt Areas 2000-2007 (L3JRC; Tansey et
al., 2007) or the MODIS burnt area product (MCD45A1) (Roy et al., 2002,
2005). All products are available on a monthly or annual basis. In contrast,
this study uses two spectral indices for daily assessment, which have been
deemed useful for burnt area detection (Walz et al., 2007). For this study,
daily surface reflectance data of the MODIS-Aqua instrument with 500 m
spatial resolution (MYD09GA) have been employed. In order to utilize only
high quality data pixels, the quality assurance data set (QA) provided with
the MODIS granules has been analyzed. Only pixels which fulfilled the
strong specifications of quality and with a sensor zenith angle lower than
45° were preserved.
Two indices have been used for burnt area detection in this study. The
NDVI; Eq. (1) indicates vegetation with higher photosynthetic activity and
has been used for burnt area mapping in boreal forests and the Normalized
Burn Ratio (NBR; Eq. (2)), indicates burnt areas by low values (Walz et al.,
2007). While the reflectance in the near infrared and therefore also in the
NDVI decreases due to the loss of biomass, the reflectance in the shortwave
infrared increases because of the change in water content and soil
exposure.
NDVI = ρNIR – ρRED / ρNIR + ρRED eq (1)
NBR = ρNIR – ρSWIR / ρNIR + ρSWIR eq (2)