# Towards a Digital Twin Prototype of Alpine Glaciers: Proposal for a Possible Theoretical Framework
## 2 Glaciers Monitoring
### The Role of Remote Sensing and Its Contribution to Glacier Inventories  
Satellite imagery and other remote sensing technologies can provide useful information about glaciers. Remote sensing plays a crucial role in the regular monitoring of the properties of glaciers thanks to a synoptic view, and to a repetitive and up-to-date coverage. Such properties are particularly useful for the monitoring of glaciers usually located in remote, inaccessible and inhospitable environments with limited field-based glaciological measurements--Until the early 1970s, aerial photography was the main remote sensing technique for glacier monitoring. The global glacier inventory, proposed in the late 1950s as a form of national glacier list, and known as World Glacier Inventory (WGI), was primarily based during the 1970s on aerial photographs and maps [13; 14]. As the original inventory method was too time-consuming and inapplicable for some remote areas, a new method was developed and introduced in the early 1980s by means of satellite imagery [15; 16] and Digital Elevation Models (DEMs) using Geographic Information Systems (GIS) [17]. The launch of the space-borne sensor Landsat Multispectral Scanner (MSS) in 1972, with a spatial resolution of 60 m, has indeed guaranteed continuity with respect to the previous ordinary adopted aerial data in the historical analysis of glacier extension. The subsequent generation of medium and high-spatial-resolution satellites provided, as a final result, a rich dataset of images over several decades, thus ensuring a robust means for change evaluation and trend analysis. Figure 3: (**a**) Distribution of the global reference glaciers (47) with more than 30 continued observation years for the time-period 1949/50–2021/22. Base map Source: National Geographic World Map from ESRI, USGS, Garmin, NOAA. (**b**) Annual mass balance of reference glaciers. Annual mass change values are given on the \(y\)-axis in tonnes per square meter (1000 kg m\({}^{-2}\)). Green and yellow bars represent negative and positive variations, respectively. Data Source: WGMS (2021, updated and earlier reports). In fact, in the 1990s, several studies applied Landsat Thematic Mapper (TM) imagery for mapping glacier extension, change assessment and glacier characterization thanks to the improved spatial resolution (i.e., 30 m) and the availability of a shortwave infrared band, able to discriminate snow from clouds as well as glacier coverage from the ground [17; 18; 19]. During these years, a large-scale inventory of the world glaciers started with the Global Ice Measurements from Space (GLIMS) initiative promoted by NASA [20] also thanks to the development of automatic glacier mapping algorithms and GIS-based data processing. GLIMS is committed to monitoring glaciers around the world by primarily using data from optical satellite instruments such as the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER); it covers about the 58% of global glaciated areas, recording a wide range of attributes. GLIMS feeds and maintains a geospatial database accessible via the internet, featuring interactive maps and providing an interoperability standard web mapping service [21]. As far as optical data are concerned, the ASTER sensor has been used for a large number of glaciological applications all around the world [22; 23; 24; 25]. This is mainly due to its high spatial resolution (i.e., VNIR Ground Sampling Distance--GSD = 15 m) and to its capability of acquiring stereoscopic images useful to derive global DEMs. Alpine glaciers, specifically, have been, and are, heavily monitored through images from ASTER and Landsat/TM/ETM+/OLI data, whose spatial, spectral and temporal resolutions proved to be consistent with the extraction of quantitative indicators of both seasonal and longer-term changes of glaciers [26; 27; 28; 29]. At the European level, the Copernicus program has significantly improved the capabilities of monitoring glacier extension in the Alpine Region.