Transcript ppt

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
Overview
 Fire scars are mappable due to ecological changes
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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 Cband 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.
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.
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.
Introduction
 Mapping and monitoring fire scars is important for
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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.
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.
Objectives
 Goal of study was to develop technique for mapping
and monitoring fire-disturbed boreal forests on a
global basis with the following objectives:
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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.
Background on Fire Mapping with SAR
 Benefit of using C-band SAR for fire scar mapping is its ability to
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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.
Radarsat VS Landsat
Clouds
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.
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 northsouth direction, a sequence of three to eight
adjacent north-south images were obtained
from each study swath.
Study Areas
Climate and Fire Data
Fire Scar Detection and Mapping in Canada
 Procedure:
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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.
Results of Fire Scar Analysis
Results of Fire Scar Analysis
Results of Fire Scar Analysis
Results of Fire Scar Analysis
Discussion and Conclusions
 Fire scars are mappable in in boreal ecosystems using C-band SAR
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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.
Potential Applications
 Fire scar mapping in deciduous forests.
 Fire scar mapping in chaparral.
 Fire scar mapping in grasslands.
 Forest reduction in Amazon rainforests.