## 5 Results
### Lava Flow Mapping  
Object-oriented change detection methods (Section 4.3) were used to analyze the lava flows of the different eruptive phases Karangetang I, Karangetang II and Krakatau I. Based on GVP reports [31] and NHI hotspot time series, the Karangetang I eruption event was temporally constrained and thus PlanetScope scenes from 19 July 2018 (pre-scene) and 4 May 2019 (post-scene) were selected for change analysis. Lava flow mapping was performed using the OBIA change detection based on vegetation change derived from the classified _dNDVI_ layer (Section 4.3, Figure 2). TanDEM-X DEM (12 m) was used for the derivation of the drainage system and subsequent identification of potential lava flows (Section 4.1). The derived lava area and the unaltered pre-scene are shown in Figure 5. Using the OBIA change detection methodology, the total area of the lava flow on the NNW flank of Karangetang was determined to be 0.49 km\({}^{2}\). The total length of the lava flow is 3425 m. At the widest point near the summit area, a width of 250 m was measured. In the region where the road overflow occurred, a width of 140 m was measured. Figure 6 shows the derived classes used to identify and model the potential lava outflow area. The Karangetang II eruption event was temporally constrained using the GVP reports [32,33] and NHI hotspot time series. OBIA change analysis was performed using PlanetScope scenes from 4 May 2019 (pre-scene) and 3 May 2020 (post-scene). Because the described volcanic activity was mainly confined to the vegetation-free area on the upper W flank of the Karangetang, the temporally aggregated NHI pixels were also included in the OBIA method in addition to the _dNDVI_. The combination of the NHI and _dNDVI_ areas formed the initial surface for the pixel-based region growing for mapping the lava surface (Section 4.3, Figure 2). Figure 4: Vegetation changes after the 22 December 2018 eruption at Krakatau (II) based on the _dNDVI_. Former vegetation areas in the E of Anak Krakatau were destroyed by the eruption. On Kecil island E of Anak Krakatau, there were thick ash and tephra deposits, as well as vegetation loss on the W coasts due to the tsunami wave. Figure 7 shows the pre- and post-scene of the W flank together with the derived lava area. During the Karangetang II eruptive phase, 0.66 km\({}^{2}\) was covered by lava. The lava  
Figure 5: Classification result of the lava flow on the NNW flank of Karangetang (I). (**a**) PlanetScope pre-scene (19 July 2018). (**b**) Classified lava flow area with the post-scene (4 May 2019) in the background. Figure 6: (**a**) Potential lava areas based on distance from the runoff model (derived from TanDEM-X DEM). All areas in the “potential lava area” class are within 45 m of the nearest point on the extracted drainage flow. (**b**) Change areas within vegetation (_dNDVI_) and change areas based on texture (GLCM) and brightness change. Areas in the change area class were classified based on GLCM and brightness thresholds. Based on the _dNDVI_ area, region-growing modelling is performed into the “change area”. Background: PlanetScope post-scene (4 May 2019). surface occupies almost the entire W flank of Karangetang and measures 880 m at its widest point. The resulting lava field is mainly concentrated in the non-vegetated area of the W flank. In the northern part of the W flank, lava also flowed into the vegetation area. The length of these lava deposits could be determined to be 1650 m. Figure 11 shows the NHI and _dNDVI_ areas, the potential lava areas (based on distance from the drainage system) and the change areas (based on texture and brightness changes). Based on temporal constraints using GVP reports [41; 42] and NHI hotspot time series, PlanetScope scenes on 14 March 2018 (pre) and 17 December 2018 (post) were selected for the Krakatau I eruption event. Since hardly any vegetated areas exist on Anak Krakatau, no corresponding _dNDVI_ changes could be included in the methodology.