## 3 Data  
A pyroclastic flow inundation map can be generated by field surveys from the crater rim to the furthest extent of the pyroclastic flow after a volcanic eruption. However, a field survey in an active volcano can be dangerous because of the exposure to hazardous gases and sudden activity. In contrast, remote sensing techniques are a useful tool for generating pyroclastic flow deposit maps, which provide a safe, cost-effective alternative to field mapping.  
A Landsat image is a picture of the surface of the Earth taken from outside the atmosphere at an altitude of approximately 818 km from the Earth's surface, on a scale of 1:250,000. Each Landsat image covers an area of 185 km\({}^{2}\) so that the aspect of a large object can be identified without exploring all of the surveyed areas [21]. Each color in the satellite image has a meaning; a color on the image indicates whether a reflection value corresponds to vegetation, aquatic bodies or a body of the Earth's surface rock [22]. Therefore, geological interpretation of the Landsat image is based on the difference between the reflection values [23].  
This study used satellite data from Landsat 7 ETM+ and Landsat 8. Landsat 7 ETM+ was used to see the changes of pyroclastic deposits from 2007 to 2012, while Landsat 8 was used to see changes in pyroclastic deposits from 2013 to 2017. However, the Landsat 7 ETM+ data suffer from the scan line corrector (SLC)-off phenomenon because of the failure of the SLC in the ETM+ instrument. These images can be restored by gap-filling the scan line using one-dimensional interpolation without any other supplementary data with the gap interpolation and filtering technique [24].  
The Landsat 7 and Landsat 8 image data from Mount Sinabung and Mount Merapi were taken from USGS earth explorer website [20] using RGB Bands 7, 4 and 2 to see the pyroclastic deposit in Landsat 7 and RGB Bands 7, 5 and 3 to see the pyroclastic deposits in Landsat 8 (Figures 2 and 3). Landsat image data are taken annually to analyze changes in pyroclastic deposit. Landsat images were selected based on their image quality. Several parameters were used to assess the quality of Landsat  
Figure 1: Map of Indonesia using Google Maps (**a**); location of Mt. Sinabung (**b**) and Merapi (**c**) on Sumatra and Java Islands from USGS earth explorer website [20].  
images, including sunlight and cloud cover in the image of the object of research. Based on these parameters, data selection is performed in different months and dates in each year.  
We select the Landsat image data representing data before eruption and after eruption to see the difference in pyroclastic deposit area of Mount Sinabung and Merapi. For Mount Sinabung, we chose Landsat image data taken 7 February 2007 as data before eruption, 17 November 2011 as data after the first eruption, 1 July 2016 as data after the second eruption and 28 July 2017 as current pyroclastic deposit data. For Mount Merapi, we chose Landsat image data taken 16 June 2007 as data before eruption, 10 May 2011 and 2 November 2014 as data after the big eruption in 2010 and 23 May 2017 as current pyroclastic deposit data.  
Figure 2: Landsat 7 (Bands 7, 4 and 2) and Landsat 8 (bands 7, 5 and 3) imagery data of Mount Sinabung taken in the year shown below each image. We can see the changes of pyroclastic deposits of Mount Sinabung from before eruption (**a**) 7 February 2007, after the first eruption on (**b**) 17 November 2011, after the second eruption on (**c**) 7 July 2016, and current pyroclastic deposits on (**d**) 28 July 2017, marked with grey color in the middle of the image.