# Mapping Winter Wheat with Multi-Temporal SAR and Optical Images in an Urban Agricultural Region
## 4 Results and Discussion
### Analysis of Temporal Variables Used for Classification
#### 4.1.1 Temporal Variables Extracted from SAR Data  
Figure 2 shows the time series of mean backscatter (\(\sigma^{0}\)) values for each land cover class. The results show that the temporal variation of the backscatter of different classes is obviously different. The backscatter value of VH polarization compared to VV is relatively small. The average VH backscatter of wheat and rapeseed decreased from \(-13.2\) dB and \(-11.5\) dB on 22 November 2015, to \(-16.2\) dB and \(-12.8\) dB on 21 March 2016, respectively. The average VV backscatter of wheat and rapeseed were attenuated from \(-6.7\) dB and \(-4.2\) dB on 22 November 2015 to \(-11.9\) dB on 14 April 2016 and \(-7.4\) dB on 21 March 2016, respectively. With the growth of wheat and rapeseed, the backscatter values for VV and VH polarization both increased. The VV and VH backscatter of forest were first reduced and then increased to the maximum on 8 May 2016. The results showed that: (1) The VV polarization decay of winter wheat was higher than that of VH, because the wheat has obvious vertical structure by which the VV polarization was affected during the propagation process; (2) In the early stage of growth, soil played a leading role in radar backscatter [57; 58]. With the gradual growth of wheat, the leaf density and the rod density gradually increased, and the wheat was more uniformly covered on the ground surface. The dominant position of the surface scattering decreased gradually, and the backscatter value decreased with the increase of the leaf density trend; (3) With the maturation of wheat, the water content of the canopy gradually decreased, so that the scattering effect of the soil gradually increased, which eventually led the VH and VV polarization backscatter to increase [59].  
The backscatter values of the water body were very low due to the specular reflection of water, which causes less reflection towards the radar antenna [21; 60]. The backscatter values of the urban area were relatively high and remained constant.  
Figure 3 shows the time series of the mean coherence values for each land cover class. The results show that the coherence of the urban land cover was the highest (in the range of 0.45 to 0.57) and the coherence of the water body land cover was the smallest (in the range of 0.18 to 0.19) among these different land covers. Moreover, similar results have been observed in other studies [21]. This is because the water body surface was easily affected by natural factors, such as wind, and was thus extremely unstable, whereas the urban area was rarely affected by environmental factors [61]. Winter wheat had a maximum value of 0.44 generated from the 22 November 2015 and 9 January 2016 InSAR pair; the minimum value of 0.20 was generated from the 14 April 2016 and 8 May 2016 InSAR pair. [PERSON] and [PERSON]'s [62] research found that there was a strong correlation between winter wheat height and the coherence value. Thus, the degrees of change of the surfaces were different, and the coherence values of the land covers were different, which provided effective information for the classification of the land.  
Figure 2: Average backscatter values for each land cover class on six image acquisition dates.