• 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”.