# The Suitability of PlanetScope Imagery for Mapping Rubber Plantations
## 4 Discussion
### Research Limitations and Prospects  
(1) In studies of rubber remote sensing mapping, in addition to the vegetation index with visible bands, some vegetation indices containing strong water absorption bands, such as the land surface water index (LSWI) [16; 50; 51] or normalized difference moisture index (NDMI) [9; 16], which consider shortwave-infrared band 1 (1550-1750 nm); the normalized burn ratio (NBR) [13], which considers shortwave infrared band 2 (2080-2350 nm) of the Landsat TM and ETM+; and OLI sensors, were introduced to monitor rubber plantations. Vegetation indices that consider the red-edge band have also displayed excellent estimation abilities for rubber plantation assessments [24], and they include the normalized difference RE1 (NDRE1) index and the red-edge spectral index (RESI), both of which use data collected  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **RF (6)** & **RF (19)** & **RF (26)** & **SVM (6)** & **SVM (19)** & **SVM (26)** \\ \hline Overall accuracy & 90.42\% & 90.80\% & 90.04\% & 88.12\% & 90.42\% & 92.34\% \\ KIA & 0.88 & 0.88 & 0.87 & 0.85 & 0.88 & 0.90 \\ Producer’s accuracy & 87.95\% & 87.95\% & 87.95\% & 87.95\% & 87.95\% & 91.57\% \\ User’s accuracy & 91.25\% & 91.25\% & 91.25\% & 91.25\% & 93.59\% & 93.83\% \\ \hline \hline \end{tabular}
\end{table}
Table 6: The response of the pixel-based RF/SVM classifiers to the number of feature variables.  
by Sentinel-2 sensors. In comparison with these Land-sat/Sentinel-2 data, PlanetScope dataset contain fewer spectral bands, which may cause issues when selecting and using images; however, PlanetScope data have obvious advantages regarding their spatial and temporal resolutions.  
(2) In this study, PlanetScope image features taken during the key phenological periods of rubber trees were comprehensively utilized to construct an object-oriented classification method for monitoring rubber plantations; good results were achieved, and it was particularly important to determine the appropriate monitoring time for the defoliation and leaf-extraction stages. Based on MODIS-NDVI time series changes, the optimal time window for monitoring rubber plantations in the study area using remotely sensed imagery was determined to be from the 49 th day to the 69 th day of 2019; thus, we selected the best-quality PlanetScope images taken during this period for this research. However, due to differences in temperature and precipitation conditions among different regions, the defoliation and leaf-extraction periods of rubber forests were inconsistent. Therefore, at the provincial scale or for larger spatial scopes, it is necessary to determine the best monitoring period for different regions individually.  
(3) In addition, as latex prices have decreased, many rubber farmers have begun to utilize understorey planting to reduce their economic losses; these activities have a certain impact on the accuracy of identification for remotely sensed rubber plantations. Determining how to remove or reduce the influence of understorey green vegetation will be the focus of future research.