## 2 Materials and Methods
### Rationale
#### 2.1.2 Open Access and \"Tunable\" Data  
Satellite images that are made available according to an open access policy are obviously advantageous for purposes of archaeological studies and rescue archaeology, as well as for operational/commercial works with limited funding resources. In this research, we consider two main open access data sources: (1) Very High Resolution (VHR) imagery (\(\leq\)5 m) from commercial satellites made available through open platforms and (2) satellite imagery of high spatial resolution (5-30 m as per the definition followed in [62]) from institutional assets and initiatives such as the Copernicus Programme and its Sentinel missions managed by the European Space Agency (ESA). A substantial bulk of multi-temporal open access VHR satellite imagery is accessible through Google Earth and Bing Maps. As such, these visualization platforms are certainly valuable entry-level tools for assessing damage to cultural heritage [5]. However, their archives are limited to optical satellite imagery and both temporal availability and quality may vary from region to region. The more recent addition of Google Earth Engine (GEE), combining a multi-petabyte catalogue of satellite imagery and geospatial datasets with complex algorithms, potentially represents an advance for cultural heritage documentation and monitoring [15; 63; 64]. However, the requirement for understanding of coding in order to realize GEE's full potential, means that its use is restricted to a subset of archaeological remote sensing specialists. On the other hand, satellite constellations collecting images according to recurrent observations, at high temporal frequency, with global coverage, such as the Sentinel missions, provide a potential solution to the above limits, although the spatial resolution is definitely lower. More precisely, 10 m at best in the visible and near-infrared channels for Sentinel-2 multispectral optical image collections, and 20 m by 22 m range by azimuth resolution for high resolution Level-1 Ground Range Detected (GRD) products generated from Sentinel-1 SAR images collected in Interferometric Wide Swath Mode (IW; [65]). For the purposes of the proposed methodology, we focus on Sentinel-2 which is the Sentinel mission that has most rapidly attracted archaeologists' attention [62]. Instead, the use of Sentinel-1 SAR images in archaeology is still growing, given that these data require technical skills and software to be effectively exploited. Even if Sentinel-1 images can be accessed as GIS-ready GRD products, their interpretation and spatial analysis require some previous training. In this regard, it has been already acknowledged that further capacity building across archaeologists is needed to increase the user uptake of Sentinel-1 SAR data in archaeology [58; 66]. The archive of images produced by these satellites is available for free to everyone. Its application has already demonstrated considerable effectiveness in documenting different types of cultural heritage hazards such as flooding [67; 68], looting [4; 62] and urban sprawl [3; 69]. For the scope of the present research, it is worth mentioning that recent research has proved how Sentinel-1 and Sentinel-2 data can be effectively used for detection of archaeological mounds [70; 71]. Furthermore, Sentinel image collections are also accessible and can be processed in GEE. However, the above mentioned technical challenges still impede an effective use of this platform and its tools, hence strengthening the need to work with a remote sensing specialist. In this regard, the multidisciplinary collaboration between an archaeologist and a remote sensing specialist (see Section 2.1.1) is essential to effectively complement the use of open access data sources with other types of satellite imagery that can be collected and provided through different image collection mechanisms.