# Changes in the Building Stock of Da Nang between 2015 and 2017
## 3 Data Description
### Spatial Aggregation  
The dataset <DaNang_buildings_hex250m_UTM49N.shp> aggregates the changes at a spatially regular grid, where each hexagon has a width of 250 m and encloses an area of 72 hectares. This allows us to identify patterns that are not related to administrative boundaries. As an example, Figure 8 shows the absolute differences in bungalows and villa-type buildings calculated by the following equations: t2017_3-t2015_3 and t2017_4-t2015_4. It shows that these building types and their changes are not drastically shaping the morphology of the city, as most of the hexagons have values close to zero. However, it also shows that most of the bungalows were demolished in the new urban development areas of Hoa Xuan (Cam Le district) and Hoa Quy (Ngu Hanh Son district) in the southwest of the city center, which is one of Da Nang's fastest urbanizing areas. Large masses of sand were piled to create new space for development at a confluence area of the Han River. In turn, most of the new villa-type buildings are constructed along the coastal area in the east to increase the number of holiday residences.  
The notable increase in buildings in Cam Le Cam Le is furthermore underlined by Figure 9, where two hotspots of new buildings (change type 2 as defined in Table 1) are visible: these are the wards of An Khe, Hoa Phat, and Hoa Minh in the north and the new construction areas in Hoa Xuan ward of Cam Le district enclosed by the Han River in the southwest. In particular, the increase in built-up areas in Cam Le was already confirmed in 2016 by [PERSON] and [PERSON] [17] and [PERSON] and [PERSON] [18].  
Figure 8: Absolute change in bungalow-type (left) and villa-type (right) buildings between 2015 and 2017.  
Figure 7: Relative increase in local (pc_t5, left) and modern (pc_t6, right) apartments between 2015 and 2017 per ward.  
This is especially alarming because these new residential areas are expected to lie below the tideline in 2050 [19] (Figure A3). In turn, most of the building changes (within the same type, change type 3 as defined in Table 1) occurred in the districts Hai Chau and Ngu Hanh Son, around the historic center of Da Nang. These are the parts of the city which are already extremely dense (Figure A2) and probably do not allow large increases in the building stock. Instead, additional parts or floors were added to the existing ones.  
To furthermore demonstrate the value of the presented dataset, we compared the number of demolitions displayed by the hexagonal aggregation to the demolitions automatically identified by [PERSON] et al. (2019), who derived digital surface models of the city from 2015 and 2017 to detect demolitions based on the threshold of the height difference [4]. However, they used hexagons with a diameter of 500 m, so both datasets were normalized by calculating the demolitions per square kilometer. As shown in Figure 10, the presented dataset not only offers finer spatial details, which make it more suitable to describe urban changes, but is also less prone to systematic overestimation, as it was reported by [PERSON] et al. (2019) [4].