# Monitoring of Sugarcane Harvest in Brazil Based on Optical and SAR Data
## 5 Discussion
### Analysis of the Time Series  
Consider the causes of the errors on the examples of time series for several cells. Lengthy cloudiness impedes the construction of a regular _NDVI_ time series during the harvest period. For the cell shown in Figure 12 and 13 (cid is cell id), the April harvest date (18 April 2018) is determined correctly by both algorithms. The December harvest date (4 April 2018) is preceded by dense clouds (Figure 13a,b), resulting in a false response of the optical algorithm in November. The real harvest date is omitted due to the lack of the _NDVI_ decrease required by the algorithm. According to the SAR data, harvest was in early November-early December. Harvest date cannot be determined more accurately as there is no SAR image in December. However, the behaviour of time series of the SAR data holds out the hope that the date can be identified in case of the available image. Figure 12: Example of false response of the optical algorithm due to cloudiness in November (cid = 450). Actual harvest dates (red lines): 18 April 2018 and 4 December 2018; harvest dates according to the optical algorithm (blue lines): 18 April 2018 and 14 November 2018; harvest date according to the SAR algorithm (green line): 24 April 2018. \begin{table}
\begin{tabular}{c c} \hline \hline
**Method of Calculation** & **Area, ha** \\ \hline Visual interpretation & 3522.22 \\ Optical algorithm & 3421.40 \\ SAR + Optical algorithm & 3157.59 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The areas harvested within a year. Cloudiness results in the _NDVI_ drop within late March (29 March 2018) while harvest takes place actually in early April (Figures 14 and 15). The date of the harvest completion in April (13 April 2018), determined according to the optical data, may be corrected according to the SAR data as the date of 12 April 2018 shows high coherence. Apparently, false response of the SAR algorithm in August is connected with some agotechnical operations within the field. The November harvest date is omitted by the SAR algorithm due to the image lack. The optical algorithm responds early due to dense clouds. As before, the early response of the optical algorithm is caused by clouds (Figures 16 and 17). SAR algorithm determines the harvest date correctly with all its possible accuracy: an interval between its determined harvest date 23 June 2018 and real harvest date 12 June 2018 does not exceed the interval between surveys. During late June-early August, certain agotechnical works are carried out within the field resulting in the coherence decrease. Figure 14: Example of false response of the optical algorithm due to cloudiness in March and false response of the SAR algorithm due to field works in August (\(\text{cid}=27\)). Actual harvest dates (red lines): 8 April 2018, 13 April 2018, and 14 November 2018; harvest dates according to the optical algorithm (blue lines): 29 March 2018, 13 April 2018, and 9 November 2018; harvest dates according to the SAR algorithm (green line): 12 April 2018 and 22 August 2018. Figure 13: (**a–c**) Example of the cloudiness effect during the harvest period (November). In the case of minor cloudiness, the time delay of the harvest dates determined from the optical data is not more than 5 days relative to the actual dates, which is the characteristic interval between the Sentinal-2 survey within that area. A long-term cloudiness period may result in two errors: preliminary false response and omission of the real harvest date. Clouds cause rapid _NDVI_ drop; during the following harvest, _NDVI_ experiences no decrease as it already has low values. Note that less than 40% cloud cover (Figure 10a) provides a rather accurate evaluation of the harvested areas. In this case, the harvest dates cannot be determined according to the SAR data during the maximum cloudiness period due to the lack of SAR image in November. Nevertheless, the SAR data allow the ability to clarify the harvest dates obtained by the optical algorithm.