interactions of savanna vegetation structure with c-band synthetic aperture radar (sar...
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Interactions of savanna vegetation structure with C-band
Synthetic Aperture Radar (SAR) at pixel level
V. Odipo1*, C. Berger, C. Schmullius
Intr
od
ucti
on
1 *,2,3Friedrich-Schiller-University JenaGrietgasse 6, 07743 Jena*[email protected]; [email protected],
[email protected]+49 3641 94 89 26/ +49 3641 94 88 80 / 94 88 82C
on
tact
Savanna vegetation structure especially aboveground biomass (AGB) and canopy cover (CC) are important for assessment of the
biome’s ability to provide ecosystem services under various disturbance scenarios. Studies aimed at tree-level structure mapping
provides the best way to monitor intrinsic changes when performed at resolutions permitting detection of forest degradation at localized
scales. ESA’s Sentinel-1 synthetic aperture radar (SAR) data is freely available at high temporal and spatial resolutions which can solve
problems of atmospheric interference experienced with optical data in the tropics. We assess feature detectability (cover classes) and
error propagation as a result of changing SAR resolution from high to low within a Lowveld Savanna using terrestrial laser scanner (TLS)
derived CC and AGB as reference data over a 9 sq.Km area in KNP. The study found that whereas changes are visible at higher
resolutions, the RMSE in savanna structure detectability reduced with downscaling SAR resolution, irrespective of polarization and
season.
Stu
dy A
rea
Data
sets
used
Pro
po
sed
Meth
od
olo
gy
Pre
lim
inary
An
aly
sis
Pre
lim
inary
Resu
lts
Co
nclu
sio
n&
Ou
tlo
ok
• High correlation on SAR backscatter and AGB in dry season
due to open canopy caused by „leaf-off“
• High correlation on SAR backscatter and CC in the wet
season due to closed canopy
• RMSE reduced while R-squared increased with
downgrading SAR resolution, and vise versa
• High AGB values – Inability to reconstruct trees with H<5 m
and DBH <10 cm (shrubs left our)
Refe
ren
ces • Odipo, V.O.; Nickless, A.; Berger, C.; Baade, J.; Urbazaev, M.; Walther, C.; Schmullius, C. Assessment of aboveground woody biomass dynamics using terrestrial laser scanner and L-band ALOS
PALSAR data in South African savanna. Forests 2016, 7, 294; doi:10.3390/f7120294
• Colgan, M.S.; Asner, G.P.; Levick, S.R.; Martin, R.E.; Chadwick, O.A. Topo-edaphic controls over woody plant biomass in South African savannas. Biogeosciences 2012, 9, 1809–1821. Doi: 10.5194/bg-
9-1809-2012
• Mathieu, R.; Naidoo, L.; Cho, M.A.; Leblon, B.; Main, R.; Wessels, K.; Asner, G.P.; Buckley, J.; van Aardt, J.; Erasmus, B.F.N.; Smit, I.P.J. Toward structural assessment of semi-arid African savannahs
and woodlands: The potential of multitemporal polarimetric RADARSAT-2 fine beam images. Rem. Sens. Of Environ. 2013,138: 215-231. Doi: 10.1016/j.rse.2013.07.011
• TLS tree H & DBH used in AGB (t/ha) calculation
AGB=0.109D(1.39+0.14In(D))H0.73ρ0.80 …..Colgan et al 2013 )
• LAStools: TLS derived CHM for CC (%)
• GAMMA: SAR processing: Rad. Calib.> Geocod.> Topo.
Normalization and Linear to σ0 (dB) conversion
DBH (cm) Height (m) Tree biomass CC (%)
tree plot tree plot kg/tree t/ha plot
Min 10 10 5.3 5.3 274.4 1.8 0
Median 31 41 10.7 10.7 6669.9 46.5 11.3
Mean 39.6 40.2 10.7 10.7 7793.4 54.3 15.6
Max 75 75 16.1 16.1 35285.2 226 90.2
• Heterogeneous
savanna in S.
Africa
• Mix of grass,
shrubs and trees.
• Disturbance &
heterogeneity
drivers: fire,
herbivory,
nutrient, climate
• Dry winter: May-
Sept, Wet
summer: Oct-
April
• MAP = 650mm,
MAT= 22°C
Fig.1: LC map of Skukuza study area, KNP,S. Africa
• Field data (Mar., 2015): 42 plots (30x30
m); H (>1m H); DBH at 1m H. Use:
validation of TLS measured H & DBH
• TLS data (Sept., 2015); Use: Tree H &
DBH, DTM, DSM & CHM; LC
classification in OBIA
Fig.3: Graphical summary of methodology used in the study
• Ground range detected (GRD) C-band
Sentinel-1A; IWS; Ascending pass;
• Period: Feb. 2015 - Mar. 2017 (VH &
VV)
Tab.1: Summary of
dependent variables:
AGB and CC for both tree-
and plot-level
Fig. 2: Datasets used in the study- tree DBH, H; TLS data; C-band data & soil
moisture
• AGB and CC vs. SAR
variables (VH/VV;
Wet/Dry) using RF („mlr“)
• Trimble RealWorks and
QuickTerrainReader
used for tree H & DBH
measurement from TLS
• High resolution- ease of change monitoring and life form
separation (visual)-pixel mixing at low resolutions
• C-band SAR is critical in savanna monitoring: high temporal
& spatial resolution; freely available
• TLS data – identification and measurement of tree H & DBH
• TLS data-lower vegetation not captured due to insufficient
point to reconstruct the tree trunk
• Error propagation-limited validation data (TLS area, few
plots with corse resolution)
• Possibility to assess effects of multi-temporal filters in a
heterogenous savanna
• Predicted AGB &
CC at different
resolutions: 10,
30, 70 and 75 m
• Average σ0 (dB)
per LCC, and
life-form
separability
• RMSE in AGB &
CC mapping
using C-band
SAR
Acknow
ledg
em
ent