## 6 Discussion  
### Discussion of the Results  
The methodology presented in Section 4.2 for the classification of ash and tephra deposits obtained good results when applied to the case studies of Agung I (Figure 3) and Krakatau II (Figure 4). Due to cloud cover, only a partial area of the S flank of Agung could be analyzed. Despite this, the PlanetScope scenes used were suitable for analyzing ash and tephra deposition due to the temporal proximity to the eruptions of Agung I. It is reported that the ash deposition happened over a wide area outside of the PlanetScope imagery extent [36; 37]. Therefore, the area derived from the _dNDVI_ analysis (25.78 km\({}^{2}\)) (Section 5.1) massively underestimates the total ash deposit area. However, the spatial distribution of ash deposits in the near surroundings of Agung is comparable to reports from the GVP [36], which shows increased deposition on the SW flank. Here, ash deposition increases with proximity to the crater. The comparatively higher _dNDVI_ values in the close vicinity of the crater can be explained with the unvegetated crater environment, which does not reveal any changes in the NDVI signal [18]. Within the analyses of the Krakatau II eruption, much of the negative _dNDVI_ values on neighboring Krakatau Kecil Island can be explained in terms of ash and tephra deposits created by the powerful eruption of Anak Krakatau. However, it can be assumed that a  
Figure 8: Classification result of lava flows on the S flank of Krakatau (I). (**a**) PlanetScope pre-scene (14 March 2018) of Anak Krakatau (I). (**b**) Classified lava flow area with the post-scene (17 December 2018) in the background. large area of change can be attributed to the destruction of vegetation. The surrounding islands of Rakata, Sertung, and Kecil were all massively impacted by the tsunami [9; 42]. Modelling of the tsunami shows a wave run-up of 10-23 m on the S, W, and NW coasts of Kecil [9], indicating the destruction of vegetation there and explaining the significantly lower _dNDVI_ values in this area. Additionally, in the E of Anak Krakatau, the sharp decrease in the _NDVI_ signal can be explained by the complete destruction of vegetation due to explosive eruptive activity [19]. The presented OBIA change detection method for mapping lava flows (Section 4.3) using VHR PlanetScope data also achieved promising results. For the Karangetang I eruption event, based on the _dNDVI_ areas, the lava flow could be well mapped in the vegetation area. Despite of the difficulty to spectrally differentiate newly deposited lava from the unvegetated background areas close to the crater, good results could be obtained using the OBIA method. During the Karangetang I eruptive phase, according to the GVP [31], the lava flowed over a length of up to 3.5 km and was about 160 m wide at the road crossing location. These results are in good agreement with the derived lengths (3.425 km) and widths (140 m at the road crossing) of the OBIA classification (Section 5.2). W and E of the lava flow, levees can be recognized (Figure 5b), which could be formed partly due to the channeling of the lava flow and the natural topography of the outflow stream [65]. The color deviation from the dark color, typical for basaltic-andesitic lava, in the upper part of the lava flow can possibly be explained by a renewed overprinting of brighter deposits from a pyroclastic flow or ash [21]. The small lava flow on the WNW flank of Karangetang was recognized almost exclusively based on vegetation changes (_dNDVI_) (Figure 6b). Similarly, no connection between the smaller lava flow in the WNW and the large lava flow in the NNW could be generated. Due to the thresholding used in generating the main flow networks from the DEM, this area was not included in the drainage system modelling. In Figure 9, the classified lava area was compared with a timeseries of multispectral false color imagery (Sentinel-2). Using this methodology, a qualitative assessment about the validity of the classification can be made.