Mapping Fire Scars in Global Boreal Forests Using Imaging Radar Data

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Mapping Fire Scars in Global Boreal Forests Using
Imaging Radar Data

• Written By L.L. Bourgeau-Chavez, E.S. Kasischke,
  S. Brunzell, J.P. Mudd, and M. Tukman
• Reviewed By Daniel C. Dunning

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Overview

• Fire scars are mappable due to ecological changes
  that occur post burn including increased soil
  moisture.
• High soil moisture causes an enhanced backscatter
  signal to be received from burned forests.
• Regional ecological differences can affect post
  fire changes thus impacting appearance of scars
  in C-band SAR imagery.
• C-band European Remote Sensing Satellite (ERS)
  and Radarsat SAR data was used in this study to
  detect, map, and monitor boreal forests globally.
• Study sites include four areas in Canada and an
  area in central Russia.

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Overview Continued

• Fire boundaries were mapped from ERS SAR data
  without prior knowledge of fire scar locations.
• Maps were validated utilizing fire service
  records and ground-truthing.
• C-band SAR data has a high potential for
  detecting and mapping fire scars globally.

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Review

• SAR Synthetic Aperture Radar
• Uses a 1-2 meter fixed length antenna and
  synthesizes a much larger antenna that can be
  hundreds of meters in length and has improved
  resolving power which achieves very fine
  resolution from great distances.
• C-band Radar Wavelength of 5.6 cm
• Experiences some surface scattering and volume
  scattering in the heart of a tree stand, does not
  reach the ground if trees are present. Also has
  the ability to penetrate clouds.

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Introduction

• Mapping and monitoring fire scars is important
  for resource and land management.
• ERS SAR images of Alaska fire scars were 3-6 dB
  brighter that adjacent unburned forests.
• Burned areas were determined to be detectable due
  to ecological changes occurring post burn.
• Changes included removal of tree canopy, exposure
  of rough surfaces, and increased ground moisture.
• Research revealed phenomenon only occurred when
  burned areas were wet such as early spring, early
  autumn, or after rain events.
• The enhanced brightness allowed fire scars to be
  detected with moderate precision.

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Introduction Continued

• Increased ground moisture is due to reduced
  surface albedo and the melting of the permafrost
  layer as well as reduced evapotranspiration.
• Study was conducted using C-band SAR imagery
  collected over four Canadian boreal regions and
  an area in central Russia.

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Objectives

• Goal of study was to develop technique for
  mapping and monitoring fire-disturbed boreal
  forests on a global basis with the following
  objectives
• Determine if fire scars can be detected and
  mapped in varying ecological conditions using
  C-band SAR data
• Determine if mapping fire scars with SAR imagery
  alone is feasible and
• Identify any geophysical, ecological, or temporal
  conditions which may affect fire scar detection
  and area estimation in ecologically different
  boreal regions.

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Background on Fire Mapping with SAR

• Benefit of using C-band SAR for fire scar mapping
  is its ability to penetrate ground cover.
• ERS SAR sensor is a C-band, 5.6 cm wavelength
  imaging radar with vertical transmit and receive
  polarization (C-VV). It has a resolution of 30 m
  and a footprint of 100 km by 100 km. ERS-1 was
  launched in 1991 and ERS-2 (still in operation)
  was launched in1995.
• Radarsat launched in 1995, and is also a C-band
  system. It also has a resolution of 30 m and a
  footprint of 100 km by 100 km.
• Research indicates that SAR C-band data has the
  potential to be used in conjunction with Landsat
  TM data for high accuracy fire scar mapping and
  monitoring.
• Image on following slide indicates a before and
  after of a fire occurring in Alaska. Imagery
  indicates that both Radarsat and Landsat can be
  used to map burn extent, but cloud obscurity is a
  problem with the Landsat TM imagery.

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Radarsat VS Landsat
Clouds
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Ecology of Canada Study Areas

• The North American boreal forest extends from New
  Foundland to Alaska with the northern limit
  ranging from 68 N Latitude in the Brooks Range
  in Alaska to 58 N Latitude at the western edge
  of Hudson Bay. The southern limit is less
  distinct and is dependant on precipitation and
  soil moisture.
• Climate ranges from dry with extreme annual
  temperature variations in the west to relatively
  warmer, wetter, maritime climate of eastern
  Canada.
• Fire is more frequent in the drier regions of
  western Canada and Alaska than in the eastern
  regions. Variations in fire frequency are tied
  to variations in climate.

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Study Areas

• To determine if fire scars can be detected and
  mapped in ecologically varying boreal ecozones of
  Canada three ERS study areas were chosen to
  capture the west to east differences.
• The study areas selected were the Northwest
  Territories, Ontario, and Quebec.
• To capture ecological variation in the
  north-south direction, a sequence of three to
  eight adjacent north-south images were obtained
  from each study swath.

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Study Areas
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Climate and Fire Data
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Fire Scar Detection and Mapping in Canada

• Procedure
• SAR data acquisition
• Visual evaluation and rating of data
• Selection of best rated images to be
  georeferenced and mosaicked and
• Digitization of potential fire scar boundaries.

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Results of Fire Scar Analysis
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Results of Fire Scar Analysis
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Results of Fire Scar Analysis
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Results of Fire Scar Analysis
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Discussion and Conclusions

• Fire scars are mappable in in boreal ecosystems
  using C-band SAR imagery.
• For Ontario and NWT study sites, fire detection
  is feasible with SAR. For NWT, SAR data
  indicates more area burned than was mapped by
  CFS.
• Data availability for Quebec was limited and
  analysis inconclusive. Further evaluation of SAR
  data collected over Quebec is necessary for any
  conclusions to be drawn.
• Seasonal variations in fire scar visibility occur
  globally, with the best viewing season being
  either spring or autumn.
• Major problem distinguishing fires form wetlands.
• For boreal regions in Russia the SAR data was
  limited to only two years over a geographic
  region, and the areas were not well mapped by
  fire service agencies.
• Manual interpretation was used for this study,
  but technology exists to automate the process.
• An improved method for fire scar mapping and
  monitoring might use a combination of SAR and
  multi-spectral data.

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Potential Applications

• Fire scar mapping in deciduous forests.
• Fire scar mapping in chaparral.
• Fire scar mapping in grasslands.
• Forest reduction in Amazon rainforests.