## 6 Discussion
### Advantages and Limitations of the Presented Lava Flow Mapping Approach
Figure 11 shows the results of the OBIA methodology for the Karangetang II event in a version with and without the inclusion of NHI hotspots. This comparison shows that for eruptions that produce volcanic deposits in unvegetated areas, significantly more accurate results can be obtained by incorporating the NHI pixels. Due to minor changes within the vegetation, the _dNDVI_ area only intersects a small portion of the drainage network. Accordingly, based on the distance analysis, the \"potential lava flow area\" class (Figure 2) does not represent the entire lava flow area. Thus, the NHI hotspot area contributes significantly to the improvement of the results, especially in the vegetation-free space. Figure 10: Results of the OBIA method with different drainage system models from TanDEM-X (**a**), Copernicus (**b**), SRTM (**c**), and ALOS PALSAR (**d**) DEMs for the Karangetang I eruptive phase. A threshold value for the extraction of the main outflows was set manually according to the resolution of the DEMs. The TanDEM-X discharge model most accurately derives the lava flow area in the unvegetated area. In the E of the unvegetated area, misclassifications are found in all results. Additionally, noticeable are the partially interrupted outflows in TanDEM-X, SRTM, and ALOS PALSAR DEMs, which are probably due to data errors. Based on the texture and brightness changes, objects that have undergone a change, but are located outside the volcanically overprinted _dNDVI_ or NHI areas can be identified. However, for the used classification variables (Table 2), the threshold values for class separation have to be manually adjusted for each application. A common problem with this is that the GLCM changes between pre- and post-scene are very small and do not robustly represent volcanic deposits in different study areas. Therefore, the optical derivation and subsequent definition of the respective thresholds proves to be difficult. By adding SAR amplitude information, the object-oriented classification of the alteration surfaces could be optimized. The deposits of volcanic hazards (e.g., lava flows, pyroclastic flows) change the surface roughness, which in turn leads to a change of the backscatter intensity of the radar signal [27; 76]. Changes at the surface due to volcanic activity also cause decorrelation of the interferometric coherence when comparing coherence data derived from pre-event and from co-event SAR image pairs [77]. In addition, the use of SAR data is highly recommended, especially in tropical regions which have a high probability of a cloudy sky [17]. A quantitative validation of the presented results is not an easy task. Reference data are scarce due to costly and sometimes hazardous surveys [20; 21; 24]. Accordingly, a classical accuracy assessment cannot be performed. One possibility is validation via visual derivation of the deposits from VHR satellite or aerial imagery. Usually, the deposits of volcanic hazards are very complex and difficult to interpret visually [21; 25], especially in unvegetated areas where there is little contrast between deposits and background. Accordingly, expert knowledge has to be available for this type of validation. Despite all this,  
Figure 11: Results of OBIA change detection methodology with and without NHI hotspots. The lava area derived without the incorporation of NHI pixels (in yellow) is significantly smaller and concentrated on the northern part of the W flank of the Karangetang. This is because in step 1 of the OBIA methodology, the areas of change based on the _dNDVI_ were intersected with the drainage network. Due to the only minor changes in vegetation, which were limited to the WNW outcropping lava tongue, only the area to the N could be detected as a potential lava area. Subsequent analysis of texture and brightness changes was limited to this area. The lava area with NHI hotspots (in red) is located further south on the W flank. This is due to the detection of hotspots in this area by the NHI, which were subsequently intersected with the drainage network.