comparison of l and p band radar time series for the monitoring of sahelian area
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Comparison of L and P band radar time series for the monitoring of Sahelian area. P.-L. Frison, G. Mercier, E. Mougin, P. Hiernaux. Context : Better understanding Sahelian surface processes and their interaction with monsoon variability - PowerPoint PPT PresentationTRANSCRIPT
Comparison of L and P band radar time series for the monitoring of Sahelian area
P.-L. Frison, G. Mercier, E. Mougin, P. Hiernaux
Context:• Better understanding Sahelian surface processes and their interaction with monsoon variability
• Improve our understanding and documentation of long term trend in vegetation in response to climate change
• radar data: 2 key parameters: soil moisture and vegetation
Goal:• Comparison of L band PALSAR and C band ASAR data
for Sahelian surface monitoring. Relation between radar vs surface parameter temporal evolution
Outline:
• Study site
• PALSAR and ASAR data
• change detection method
• Results and discussion
The Sahel
Semi-arid area
herbaceous layer (0-50 %) (annual grasses)
Dry season (Nov. – Apr.)
bare soil
Rainy season (May – Oct.)
shrub (0-20 %)
trees (1-5 %)+
Region of study: the Gourma - Mali
Seno
PALSAR acquisitions: L band (Jan. 2007 - Apr. 2009)
DATASET
Mode # Polarization Resolution Incidence angle
Pass
Fine Beam 7 HH 15 m 40° Ascending
Fine Beam 5 HH / HV 15 m 40° Ascending
Wide Swath 6 HH 100 m 30° Descending
ASAR acquisitions: C band (Jul. - Dec. 2005)
Mode # Polarization Resolution Incidence angle
Pass
Wide Swath 41 HH 150 m 18-35° Ascending+
Descending
Gourma Region (MALI)ASAR –Wide Swath - HH
20th Dec 2005
GOURMA Region (MALI)PALSAR–WIDE BEAM- HH
1st Jan 2008
Gourma Region (MALI)ASARC-band
PALSARL-band
C-band (ASAR):Shallow sand and silt soils
L -band (PALSAR):Better discrimination of geological features
Remnant of alluvial systems and lacustrine depressions
Gourma Region (MALI)ASARC-band
PALSARL-band
C-band (ASAR):Shallow sand and silt soils
L -band (PALSAR):Better discrimination of geological features
Remnant of alluvial systems and lacustrine depressions
PALSAR Fine Beam – HH polarizationTemporal color composite image
GOURMA - MALI17 Jan. 200720 Oct. 200722 Jan.2009
Water ponds
Hombori mounts
Low-land (accacia forest)
Change detection method
Constraints:
Large dynamic range (high differences over bright patterns)
Even after multi-looking, presence of noise (speckle)
absolute or relative differences, ratios, rms,….. not
significant
Time series color composite image Relative differences
Temporally stable regions gray
Change detection colored areas
Case of 3 channels
Change detection method
Constraints:
Large dynamic range (high differences over bright patterns)
Even after multi-looking, presence of noise (speckle)
absolute or relative differences, ratio, rms,….. not
significant
Temporally stable areas gray areas (no saturation)
Change detection colored areas (saturation)
RGB space R
B
G0
HSV space
Case of 3 channels
Value
Saturation
Hue
Change detection method
Constraints:
Large dynamic range (high differences over bright patterns)
Even after multi-looking, presence of noise (speckle)
absolute or relative differences, ratio, rms,….. not
significant
17 Jan. 200720 Oct. 200722 Jan.2009
RGB Space
Value
Hue
Saturation
HSV Space
Areas that have changed
Color composite image Saturation image
17 Jan. 200720 Oct. 200722 Jan.2009
Change detection for a 3-date color composite image
PALSAR Fine BeamHH polarization
Change detection method
Case of N channels (N>3):
P iterations:
1) random draw of 3 among the N available
channels
2) Compute the saturation channel from HSV
space
Average of the P saturation channels
Example: 12 Finebeam acquisitions at HH pol.
N=12 12! / (9! * 3!) = 220 possible random
draws
P =50 (arbitrary)
Color composite image Saturation image
17 Jan. 200720 Oct. 200722 Jan.2009
Change detection for a 3-date color composite image
PALSAR Fine BeamHH polarization
Color composite image Saturation image
Change detection for 12 FineBeam acquisitions (HH polarization)
17 Jan. 200720 Oct. 200722 Jan.2009
PALSAR Fine BeamHH polarization
Jan. 2007 – Apr. 2009
Temporal changes detected over 12 Fine Beam acquisitions
PALSAR dataHH polarisationJan. 2007 – Mar. 2009
Water ponds
Fields (millet) dep. Orientation!
Significant penetration depth over sandy soils
Changes in dry seasonChanges in rainy season
Temporal changes detected over 12 Fine Beam acquisitions
Water pondspermanent
PALSAR dataHH polarisationJan. 2007 – Mar. 2009
Fields (millet) dep. Orientation!
ALOS/PALSAR – FBD6th June 2008
HH HV
Sandy soils
Shallow soils
INFLUENCE OF POLARISATION
Change detection between HH and HV
PALSAR DUAL POLARIZATION
Shallow soils+
Water ponds
ALOS/PALSAR – WB1: HH Polarization
Water ponds discrimination
2008 dry season:1 Jan16 Feb2 Apr
3-date color composite image
Change detection (dry season)
ALOS/PALSAR – WB1: HH Polarization
Water ponds discrimination
• Main water resource • Hydrological indicator
surface runoffareas increase since begining of drought
period (50’s)
ASAR –Wide Swath – HH polarisation
41 acquisitions22 acquisitions in ascending pass
5 acquisitions same incidence angle (35°) – Jul. –Dec. 2005
ASAR –Wide Swath - HH polarisation
2nd Sept. 2005 change detection (5 dates)
low penetration over sandy soil upper surface changes
sandy soils
ASAR –Wide Swath - HH polarisation
2nd Sept. 2005 change detection (5 dates)
low penetration over sandy soil upper surface changes
sandy soils, water ponds
change detection (5 dates)PALSAR (L-band)
low penetration over sandy soil upper surface changes
sandy soils, water ponds
High penetration dpeth over sandy soilswater ponds millet fields
Comparison P band / L band temporal change detection
Conclusion
• RGB HSV simple but performant for change detection!
• Temporal change detection: penetration depth illustration
histogram must be the same across the hole image!
C band (low penetration over sandy soils)
mix of upper surface changessandy soils (soil moisture + vegetation)water ponds
more difficult te discriminate special features
• L band: some variation over sandy soils (soil moisture?) Cross over with SMOS mission
L band (high penetration over sandy soils)Water pondsMillet fields
Thank you for your attention!