• Goal: estimate sub-pixel woody shrub fractional cover at landscape scales
• Approach: evaluate the Simple Geometric Model (GM) against the 631 nm directional signal from CHRIS; adjust against CHRIS multi-angle data
• Major Challenge: to obtaining adequately accurate estimates of background reflectance anisotropy at CHRIS/Proba acquisition angles
• Solution: estimate background reflectance anisotropy from the isotropic, geometric, and volume scattering weights of Li-Ross model kernels
• Results: mapped distributions of woody shrub cover are good with reference to estimates made using IKONOS 1 m panchromatic imagery (mean absolute RMSE = 0.06, N = 38,934).
Woody Shrub Cover from Multi-Angle Imaging with CHRIS
CHRIS/Proba Jornada Experiment: Advances in Multi-Angle Imaging
M. Chopping et al. January 2006Page 1
Arid and semi-arid lands, including desert, scrubland, grassland and savanna, cover about
40%, or an estimated 58.5 million km2, of the terrestrial surface. Vast areas, including a large proportion of the southwestern USA, have experienced a dramatic increase in the abundance of woody shrub vegetation over the last century, replacing the former continuous cover of grasses. This has resulted in profound changes to hydrological and biogeochemical cycles as well as severe impacts on the ecology and economic value of the land at local to regional scales, and on the surface radiation budget at regional to global scales. The large extent, limited accessibility and surface heterogeneity of desert vegetation prohibits the use of ground survey in monitoring: remote sensing is the only means of obtaining geographically and temporally comprehensive measurements. These slides present recent work using data from the Sira Electro-optics Ltd. Compact High Resolution Imaging Spectroradiometer (CHRIS) flown on the European Space Agency’s Proba-1 platform, together with a geometric-optical canopy reflectance model, to map shrub cover in the CHRIS/Proba Core Site at the USDA, Agricultural Research Service Jornada Experimental Range near Las Cruces, New Mexico, USA.
CHRIS/Proba Jornada Experiment: Advances in Multi-Angle Imaging
M. Chopping et al. January 2006Page 2
Estimated surface bidirectional
reflectance vs. surface bidirectional
reflectance at 631 nm modeled using the
Simple Geometric Model (SGM) when
driven with IKONOS-derived shrub
statistics and backgrounds estimated a
priori from isotropic-LiSparse-RossThin
LiSK model kernel weights (represented
by the Walthall model). N= 38,934.
M. Chopping et al. January 2006Page 3
Reflectance at 631 nm
CHRIS/Proba Jornada Experiment: Advances in Multi-Angle Imaging
M. Chopping et al. January 2006Page 4
y = 0.8746x + 0.0055
R2 = 0.9987
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Measured Fractional Shrub Cover
Modeled Fractional Shrub Cover
1:1 line
Modeled vs. measured woody shrub fractional cover, the
latter estimated from IKONOS 1 m panchromatic
imagery, for calibration of the spatially-dynamic
background signal (N = 11). These results were obtained
by adjusting the Walthall model coefficients (representing
the signal from the combined soil-understory background)
so that the lowest absolute RMSE was obtained between
the Simple Geometric Model (SGM) and the
corresponding CHRIS/Proba 631 nm multi-angle
reflectance data. The SGM was supplied with measured
shrub number density and mean radius values and mean
mid-crown height and crown shape ratios were set to 2.0
(low) and 0.2 (oblate), respectively. Inversions must
estimate the Walthall model a priori, so error in retrieving
shrub cover using this method is almost entirely owing to
inaccuracy in estimating the background signal.
CHRIS/Proba Jornada Experiment: Advances in Multi-Angle Imaging
Multi-angle false colour composites
from CHRIS/Proba:
(a) RGB = band 421 composite (b) RGB = band 421 simulation (c) RGB = band 431 composite (d) RGB = band 431 simulation
Note: scaling is 2 standard deviations on a per-band basis and is not consistent between images. These composites show strong agreement between modeled and observed CHRIS/Proba data in different views and also provide an indication of the information content of the multi-angle imagery.
(a)
(d)(c)
(b)
M. Chopping et al. January 2006Page 5
CHRIS/Proba Jornada Experiment: Advances in Multi-Angle Imaging
Frequency distributions of
absolute deviation between
retrieved and measured
fractional woody shrub cover
(retrieved using SGM with
calibrated Walthall soil-
understory response). Almost
90% of estimates are
associated with a deviation
from the IKONOS-estimated
value of less than 0.08.
M. Chopping et al. January 2006Page 6
0
100
200
300
400
500
600
700
0.001 0.076 0.151 0.226 0.301 0.376 0.451
Absolute deviation
Frequency
.0%
20.0%
40.0%
60.0%
80.0%
100.0%
120.0%
Frequency
Cumulative %
CHRIS/Proba Jornada Experiment: Advances in Multi-Angle Imaging
Mapped values of retrieved fractional woody shrub cover closely match the distribution of the IKONOS-estimated values. Note that our method allows retrieval of shrub cover over dense as well as sparse understories. (a) fractional woody shrub cover estimated using IKONOS 1 m panchromatic imagery (b) fractional woody shrub cover retrieved by adjusting the SGM against CHRIS/Proba multi-angle data.
500 m
M. Chopping et al. January 2006Page 7
Estimated with
IKONOS
Estimatedwith
CHRIS/SGM
CHRIS/Proba Jornada Experiment: Advances in Multi-Angle Imaging
Conclusions: The multi-angle remote sensing signal from CHRIS/Proba at 631 nm can be explained in terms of a combined soil-understory background response and woody shrub cover; and exploited to map this important structural attribute of desert grasslands at landscape scales with good accuracy. It is important to note that our modeling method allows retrieval of shrub cover over both dense understories with an important understory component as well as over sparse understories with a high proportion of exposed soil.
Publications: Chopping, M., Su, L., Laliberte, A., Rango, A., Peters, D.P.C., and Kollikkathara, N., 2006. Mapping shrub abundance in desert grasslands using geometric-optical modeling and multiangle remote sensing with CHRIS/Proba (submitted January 2006). Similar results have been obtained using the NASA/JPL Multi-Angle Imaging Spectro-Radiometer on the Terra satellite: Chopping, M., Su, L., Rango, A., Martonchik, J.V., Peters, D.P.C., and Laliberte, A., 2006. Remote sensing of woody shrub cover in desert grasslands using MISR with a geometric-optical canopy reflectance model (accepted for publication in Remote Sensing of Environment).
M. Chopping et al. January 2006Page 8
CHRIS/Proba Jornada Experiment: Advances in Multi-Angle Imaging
Acknowledgments: The data presented in this presentation are derived from the CHRIS instrument, developed by Sira Technology Ltd (formerly Sira Electro-Optics Ltd), mounted on board the European Space Agency’s PROBA-1 platform. The work described herein has been significantly enabled by NASA grant NNG04GK91G to M. Chopping (EOS project EOS/03-0183-0465 “Quantifying Changes in Carbon Pools with Shrub Invasion of Desert Grasslands using Multi-Angular Data from EOS Terra and Aqua”.