Scattering Mechanisms and Within-Field Variation

Relating backscatter values to crop condition is not always straight forward. An area of high biomass within a wheat field for example, may have lower backscatter or a higher backscatter relative to an area of lower biomass depending on the phenological stage of a crop and/or environmental conditions (including background soil moisture) at the time of image acquisition. For example, in a wheat field with high volumetric soil moisture, low biomass areas will have higher backscatter than the high biomass area. The opposite can be true given low volumetric soil moisture.

Polarimetric classification algorithms such as those based on Cloude and Pottier's H/A/Greek small letter alpha can be used to identify the scattering mechanism and thus help in the interpretation of the observed backscatter.

Polarimetric C-SAR data were collected near Indianhead, SK using the CV-580 platform. Several polarizations were synthesized from the complex data. The polarizations used in the unsupervised classification including HH, VV, HV, RR, RL and linear polarizations with 45° and 135° orientations. The resulting map created 6 productivity zones over three crop types (Figure 9-9).

In one study performed by McNairn et al. McNairn et al.,2002c polarimetric C-SAR data were collected near Indian Head, SK in June 2000. Several polarizations were synthesized from the complex data, including two circular polarizations (RR and RL). Linear polarizations were synthesized by choosing an ellipticity angle (Greek small letter chi) of zero and by varying the orientation angle (Greek small letter phi) at 45° increments from 0° to 180°. Incidence angles over the test site were between 42° and 46°. The SAR image was classified into sixteen clusters using seven of these polarizations as input and a K-Means algorithm (Figure 9.9). The polarizations used in the unsupervised classification included HH, VV, HV, RR, RL and linear polarizations with Greek small letter phi = 45° and Greek small letter phi = 135°. These seven images were used as input to classify the wheat crops into six classes representing three growth zones: very healthy growth (zone 1); average growth (zone 2); poor growth (zone 3).

Figure 9-9
Figure 9-9. Productivity zones generated from an unsupervised classification of images for HH, VV, HV, RR, RL and linear polarizations with Greek small letter phi = 45° and Greek small letter phi = 135° for wheat (1), canola (2) and peas (3), June 28, 2000 McNairn et al 2002c.

The three growth zones representing high to low biomass for the wheat fields were used as a mask to extract the H/Greek small letter alpha parameters of Cloude and Pottier. The H/Greek small letter alpha plane for a high biomass site is presented in Figure 9-10. In general, regions 1, 4, and 7 identify regions dominated by multiple scattering, regions 2, 5,and 8 dominated by volume scattering, and regions 3, 6, and 9 by surface scattering.

Figure 9-10 suggests that the backscatter from high biomass wheat is dominated by volume scattering. Figure 9-11 was derived from the zone of average growth and again, volume scattering by the canopy is the dominant mechanism. In Figure 9-12 the zone of poor growth, where crop biomass is low, is dominated by surface scattering [McNairn, Personal Communication].

Area 24: wheat Line 0 Pass 0 0
Cloude Alpha (Greek small letter alpha) vs Entropy (H) - Density Histogram
Figure 9-10
A = Multiple Scattering B = Volume Scattering C = Surface Scattering
D = Low Entropy E = Medium Entropy F = High Entropy

Entropy Anisotropy Greek small letter alpha Greek small letter beta
Average 0.81 0.43 42.04 17.51
Standard Deviation 0.03 0.07 2.65 2.89

Figure 9-10. H / Greek small letter alpha plane with bounds and partitioning showing a high biomass area within the wheat field

Area 48: wheat Line 0 Pass 0 0
Cloude Alpha (Greek small letter alpha) vs Entropy (H) - Density Histogram
Figure 9-11
A = Multiple Scattering B = Volume Scattering C = Surface Scattering
D = Low Entropy E = Medium Entropy F = High Entropy

Entropy Anisotropy Greek small letter alpha Greek small letter beta
Average 0.75 0.53 41.34 16.39
Standard Deviation 0.03 0.06 1.76 3.19

Figure 9-11. Scattering as shown in the H/Greek small letter alpha plane for a medium biomass area within the spring wheat field.

Area 13: wheat Line 0 Pass 0 0
Cloude Alpha (Greek small letter alpha) vs Entropy (H) - Density Histogram
Figure 9-12
A = Multiple Scattering B = Volume Scattering C = Surface Scattering
D = Low Entropy E = Medium Entropy F = High Entropy

Entropy Anisotropy Greek small letter alpha Greek small letter beta
Average 0.74 0.44 35.52 15.27
Standard Deviation 0.04 0.06 3.45 2.99

Figure 9-12. Scattering as shown in the H/Greek small letter alpha plane for a low biomass area within the spring wheat field.