# Towards a Digital Twin Prototype of Alpine Glaciers: Proposal for a Possible Theoretical Framework
## 2 Glaciers Monitoring
### Global Services Providing Data Feeding Glacier DT  
As previously mentioned, EO data are nowadays widely used in many glacier-related applications. The way they can be directly exploited in glaciological applications has already been discussed in the previous section. However, one has also to remember that EO data are often at the basis of \"services\" offered by different providers that have therefore to be considered. It is authors' opinion that, looking at all the available services, the most desirable are the ones relying on data (both EO and in situ) that are continuously and regularly acquired (ensuring monitoring), uniformly pre-processed (ensuring comparability), free-of-charge (ensuring affordability) and, possibly, official. From this point of view, the EU invites researchers to capitalize on the already existing capabilities of Digital Twin Earth. Specifically, it is expected that whatever DT should profit from the data and capabilities of national and international institutions (e.g., European in situ data coordination by the EEA) and from European programs (e.g., Copernicus services and data). To achieve these instances, a global overview of existing potential service providers is mandatory. The six Copernicus Services--Atmosphere, Marine, Land, Climate Change, Security and Emergency--can certainly represent the starting point for this research. They, in fact, make available free access data coming from Sentinel's acquisitions and in situ measures and, in many cases, advanced ready-to-use products. The majority of Copernicus Services are managed thorough dedicated access points called Data and Information Access Services (DIAS, [52]). Other data and services related to environmental themes (hydrography, elevation, land cover, natural hazards, fires, etc.) and their evolution along time can be obtained through the INSPIRE Geoportal [53]. Geoports of the European Environment Agency (EEA, [54]) provides free data and services, as well. Data/services refer to a wide range of environmental topics: climate change, land use, water and marine environment, biodiversity-ecosystems, etc. One of the most important features of these archives is the availability of multi-temporal data covering time ranges that permit the analysis of ongoing trends of natural and anthropic phenomena. In the case of the glaciological applications, for example, EEA makes the European glaciers distribution map available, dated 2010 [55], and the map of avalanche events in the Alps, dated 1999 [56]. It is worth remembering that past data enable temporal comparisons and support forecasting providing elements for phenomena modelling and simulation, which represent an essential part in DT Earth. As a useful complement to glacier domain data/services, some auxiliary data can be obtained from the CORDA (COpernicus Reference Data Access) dashboard [57]. CORDA provides official reference data (i.e., orthoimages, administrative boundaries/units/toponyms, elevation data, land cover, hydrography, transport network, etc.) from the main European institutional bodies--EuroGeographic, EEA, and Eurostat--and from national and subnational authorities of member countries. As far as the Alpine Region is concerned, the HISTALP database [58] gathers many high-quality climate data time-series (temperature, air pressure, precipitation, etc.) from in situ observations since 1760. HISTALP provides and interesting overview about the temporal variation of the main climate variables along time at the climatic temporal scale. It relies on an initiative of the national weather services, led by the Austrian ZAMG (Zentralanstalt fur Meteorologie und Geodynamik). The APGD (Alpine Precipitation Grid Dataset) is, conversely, a high-resolution grid dataset of daily precipitation developed by MeteoSwiss and covering the Alpine Region [59] that well fits requirements of dynamic modelling that a DT should provide.