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/ 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. 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 () of zero and by varying the orientation angle () 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 = 45° and = 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. Productivity zones generated from an unsupervised classification of images for HH, VV, HV, RR, RL and linear polarizations with = 45° and = 135° for wheat (1), canola (2) and peas (3), June 28, 2000 .

The three growth zones representing high to low biomass for the wheat fields were used as a mask to extract the H/ parameters of Cloude and Pottier. The H/ 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 () vs Entropy (H) - Density Histogram

A = Multiple Scattering B = Volume Scattering C = Surface Scattering
D = Low Entropy E = Medium Entropy F = High Entropy

	Entropy	Anisotropy
Average	0.81	0.43	42.04	17.51
Standard Deviation	0.03	0.07	2.65	2.89

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

Area 48: wheat Line 0 Pass 0 0
Cloude Alpha () vs Entropy (H) - Density Histogram

A = Multiple Scattering B = Volume Scattering C = Surface Scattering
D = Low Entropy E = Medium Entropy F = High Entropy

	Entropy	Anisotropy
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/ plane for a medium biomass area within the spring wheat field.

Area 13: wheat Line 0 Pass 0 0
Cloude Alpha () vs Entropy (H) - Density Histogram

A = Multiple Scattering B = Volume Scattering C = Surface Scattering
D = Low Entropy E = Medium Entropy F = High Entropy

	Entropy	Anisotropy
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/ plane for a low biomass area within the spring wheat field.