## 3 Results
### Dynamic Detection of Changing Features: Urban Development around the WHS of Cyrene  
The UNESCO WHS of Cyrene in Libya is famous for the archaeological ruins of the Greek city, with the Acropolis, the Agora, and the Temple of Zeus being located north of the modern town of Shahat, while the necropolises extend across the cultural landscape. These suburban zones are currently the most vulnerable to threats to their conservation due to urban expansion of Shahat and the limited enforcement of planning regulations [21; 22]. [PERSON] et al. [5] have recently published a geospatial analysis of the cultural landscape changes around the Cyrene WHS that were observed since 1986 by combining the full VHR image catalogue available through Google Earth (2006-2017) with two Landsat scenes (1986, 2000) and one Sentinel-2 image (2017). From this analysis, the authors concluded that Shahat has expanded massively and activities affecting the preservation of the WHS and its immediate hinterland have accelerated over the past five years following the unstable situation of the country. The site-specific research question was whether we could use Sentinel-2 for its imaging capability at a high temporal frequency to log incidents of urban expansion as accurately as with ad hoc acquisitions. In the absence of Sentinel-2 images prior to mid-2015, we first re-examined the whole Google Earth dataset (Table A3 in Appendix A) to achieve a baseline knowledge scenario for the whole cultural landscape. Instead of classifying pixels according to 'urban' training samples as done in Reference [5], we ran a feature extraction analysis by searching for urban features (for example, new roads, new clusters of buildings, excavation sites) that typically appear when urban development is happening. As explained in Section 2, we compared the surface reflectance values between pairs of subsequent scenes (T\({}_{n-1}\) versus T\({}_{n}\), where n \(-\) 1 and n are the epochs of the two Sentinel-2 images composing each pair) and zoned clusters of pixels spatially distributed as urban block patterns. Using the above approach, we could track, both spatially and temporally, the urban expansion around Shahat through the appearance and, more importantly, the transformation of new town blocks. These appeared to be the basic units of new urban development. Their construction history can be summarised as per the example located east of Shahat that is reported in Figure 5. Initially, the topsoil is excavated to create the grid pattern of the new road network enclosing small lots of land (Figure 5a). The pavement of the roads is then laid down using more durable surface material (Figure 5b,c). In this case, a light brownish to whitish material is used, therefore, making the roads very distinctively visible from space (Figure 5c). Later, the land lots are populated by houses, concrete constructions and/or agricultural fields (Figure 5d,e). The block feature reported in Figure 5 is just one of the tens that we have extracted from the Google Earth (2009-2015; Figure 6a) and Sentinel-2 (2016-2017; Figure 6b) time series as they progressively appeared in the last five years (2012-2017) east, south, and west of Shahat. Interestingly, this type of feature was not present from 2006 to 2012. Since then, this feature seems to have spread across the cultural landscape surrounding the WHS of Cyrene, according to a spatial distribution and trends that could not be fully appreciated using only a supervised classification mixing spectral signatures of different urban features within the same 'urban' class. In 2012, the block features mostly appeared south of Shahat (purple polygons in Figure 6a). A year later similar features manifested east of the modern town, where archaeological records of the east necroolis are documented (green polygons in Figure 6a). In 2013, very limited incidents were observed west of Shahat, where the south necroolis was formerly located.